> For the complete documentation index, see [llms.txt](https://a7246c5516ab4c80cdfe21ca2be3e40c.gitbook.io/nodejs-docs/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://a7246c5516ab4c80cdfe21ca2be3e40c.gitbook.io/nodejs-docs/webcrypto.md).

# webcrypto

### Web Streams API

The Web Streams API allows you to handle data that is not available all at once, such as data coming from a server over the network.

**How It Works:**

Imagine you're watching a video on the internet. The video doesn't load all at once. Instead, it comes in small chunks, or "chunks", as you watch. The Web Streams API lets you handle these chunks as a "stream" of data.

**Real-World Applications:**

* **Video Streaming:** Handle video data that comes in chunks, allowing you to watch the video without waiting for the entire thing to download.
* **Audio Streaming:** Handle audio data that comes in chunks, allowing you to listen to music or podcasts without waiting for the entire track to download.
* **Data Transfer:** Handle large files or datasets that are too big to send or receive all at once.

**Example:**

Here's a simplified example of receiving data from a stream:

```javascript
const createReadableStream = require("stream").Readable;

// Create a stream that will emit chunks of data
const readableStream = createReadableStream();

// Handle the chunks of data
readableStream.on("data", function (chunk) {
  console.log(chunk.toString());
});

// Push chunks of data into the stream
readableStream.push("Hello");
readableStream.push("World");
readableStream.push(null); // Signal the end of the stream
```

In this example, the `readableStream` emits chunks of data, each chunk containing a part of the "Hello World" message. The `data` event handler prints each chunk to the console.

***

**What are Web Streams?**

Web Streams are like a conveyor belt that moves data from one place to another. This data can come from files, the network, or even other streams.

**Types of Web Streams:**

There are three main types of Web Streams:

* **ReadableStream:** This stream can read data. Imagine a faucet that pours water into a cup.
* **WritableStream:** This stream can write data. Imagine a sink that drains water out of a cup.
* **TransformStream:** This stream can transform data. Imagine a filter that cleans water before it goes into a cup.

**How to Use Web Streams:**

To use a Web Stream, you can create a `ReadableStream` to read data from a source, a `WritableStream` to write data to a destination, or a `TransformStream` to transform data before writing it to a destination.

Here's an example of how to read data from a file using a `ReadableStream`:

```javascript
const file = "my-file.txt";
const stream = fs.createReadStream(file);

stream.on("data", function (chunk) {
  // Process the chunk of data
});

stream.on("end", function () {
  // All data has been processed
});
```

And here's an example of how to write data to a file using a `WritableStream`:

```javascript
const file = "my-file.txt";
const stream = fs.createWriteStream(file);

stream.write("Hello, world!");

stream.end();
```

**Real-World Applications:**

Web Streams are used in a variety of real-world applications, such as:

* Streaming video and audio
* Loading large files from the network
* Processing data in real-time
* Creating custom data pipelines

***

#### Example ReadableStream

**Explanation:**

A `ReadableStream` is a source of data that you can read from. In this example, we create a stream that pushes the current time every second.

**Code:**

```
import { ReadableStream } from 'node:stream/web';
import { setInterval as every } from 'node:timers/promises';
import { performance } from 'node:perf_hooks';

const SECOND = 1000;

const stream = new ReadableStream({
  async start(controller) {
    for await (const _ of every(SECOND)) controller.enqueue(performance.now());
  },
});

for await (const value of stream) console.log(value);
```

**Simplified Explanation:**

We create a stream using the `ReadableStream` constructor. The `start` method is called when the stream starts to read data. Inside the `start` method, we use the `every` function to create a loop that runs every second. In each loop, we add the current time to the stream using the `enqueue` method.

The `for await` loop is used to read data from the stream. It will wait until data is available and then log it to the console.

**Real-World Example:**

A real-world example of a readable stream is a file stream. When you read a file, the file system creates a readable stream that you can read from.

**Potential Applications:**

Readable streams can be used in a variety of applications, such as:

* Reading files
* Reading data from a network connection
* Reading data from a database
* Displaying real-time data in a web application

***

### Node.js Web Cryptography API

The Web Cryptography API provides a standardized way to perform cryptographic operations in a web browser or Node.js environment. It offers a set of functions that enable developers to securely encrypt and decrypt data, generate and verify signatures, and perform other cryptographic tasks.

#### Topics:

**1. Key Generation**

* **Purpose:** Creating unique keys for encrypting, decrypting, and signing data.
* **Example:**

```js
crypto.subtle.generateKey(
  {
    name: "AES-CBC",
    length: 256,
  },
  true,
  ["encrypt", "decrypt"]
);
```

**2. Data Encryption and Decryption**

* **Purpose:** Securely encrypting or decrypting data using symmetric or asymmetric algorithms.
* **Example:**

```js
const encryptedData = await crypto.subtle.encrypt(
  {
    name: "AES-CBC",
    iv: initializationVector,
  },
  key,
  plaintext
);
```

**3. Signing and Verification**

* **Purpose:** Creating digital signatures for verifying the authenticity of data and messages.
* **Example:**

```js
const signature = await crypto.subtle.sign(
  {
    name: "RSA-SHA256",
  },
  privateKey,
  message
);
```

**4. Hashing**

* **Purpose:** Creating one-way hash functions for checking data integrity and identifying duplicate data.
* **Example:**

```js
const hash = await crypto.subtle.digest(
  {
    name: "SHA-256",
  },
  data
);
```

**5. Random Value Generation**

* **Purpose:** Generating cryptographically secure random numbers for various security-related tasks.
* **Example:**

```js
const randomValues = await crypto.getRandomValues(new Uint8Array(16));
```

#### Real-World Applications:

* **Secure Communication:** Encrypting and decrypting messages in chat applications and email systems.
* **Data Protection:** Securing sensitive data stored in databases or cloud services.
* **Digital Signatures:** Verifying the authenticity of contracts, documents, and software updates.
* **Password Hashing:** Storing passwords securely in a database by hashing them, making it difficult for attackers to crack.
* **Blockchain Applications:** Generating and verifying cryptocurrencies, securing smart contracts, and maintaining network integrity.

***

#### `ReadableStream`

* The `ReadableStream` interface of the Web Crypto module provides a standard way to read data from a `CryptoOperation`.
* It represents a stream of data that can be read from.
* It allows you to get data out of a `CryptoOperation`, such as the result of an encryption, decryption, signing, or verification operation.

#### How to use `ReadableStream`

To use `ReadableStream`, you first need to create a `CryptoOperation` object. You can do this by calling the `crypto.subtle` method and passing in the appropriate operation name and algorithm. Once you have a `CryptoOperation` object, you can call the `read()` method to get the data out of the operation. The `read()` method returns a `Promise` that resolves to a `Uint8Array` containing the data.

Here is an example of how to use `ReadableStream` to read the result of an encryption operation:

```
const key = await crypto.subtle.generateKey({
  name: 'AES-CBC',
  length: 256
});

const iv = crypto.getRandomValues(new Uint8Array(16));

const data = 'Hello, world!';

const encryptedData = await crypto.subtle.encrypt({
  name: 'AES-CBC',
  iv,
  key
}, key, data);

const readableStream = encryptedData.readable;

const decryptedData = await readableStream.read();

console.log(decryptedData); // 'Hello, world!'
```

#### Real-world applications

`ReadableStream` can be used in a variety of real-world applications, such as:

* Encrypting and decrypting data for secure storage or transmission.
* Signing and verifying data to ensure its integrity.
* Generating random data for use in cryptography.

#### Potential applications

The `ReadableStream` interface can be used in a variety of potential applications, including:

* **Encrypting and decrypting data:** `ReadableStream` can be used to read the result of an encryption or decryption operation. This can be useful for securely storing or transmitting data.
* **Signing and verifying data:** `ReadableStream` can be used to read the result of a signing or verification operation. This can be useful for ensuring the integrity of data.
* **Generating random data:** `ReadableStream` can be used to read the result of a random data generation operation. This can be useful for generating random data for use in cryptography.

***

**`new ReadableStream([underlyingSource [, strategy]])`**

This function creates a new **ReadableStream** object.

#### Parameters

* **`underlyingSource`** (Object): The underlying source of data for the stream. This object must have the following properties:
  * **`start`** (Function): A function that is called when the stream is created. This function can be used to initialize the stream and start reading data.
  * **`pull`** (Function): A function that is called when the stream's internal queue is not full. This function can be used to read data from the underlying source and add it to the queue.
  * **`cancel`** (Function): A function that is called when the stream is canceled. This function can be used to clean up any resources that were allocated by the stream.
* **`strategy`** (Object): An optional object that can be used to configure the stream's behavior. This object can have the following properties:
  * **`highWaterMark`** (number): The maximum size of the stream's internal queue. When the queue reaches this size, the stream will stop reading data from the underlying source until the queue has been drained.
  * **`size`** (Function): A function that is used to calculate the size of each chunk of data. This function is used to determine how much data can be added to the stream's internal queue before the queue reaches its high water mark.

#### Usage

The following code shows how to create a new `ReadableStream` object:

```
const stream = new ReadableStream({
  start(controller) {
    // This function is called when the stream is created.
    // It can be used to initialize the stream and start reading data.

    // In this example we're going to read data from a file.
    const file = fs.createReadStream('file.txt');

    // We can use the `readable.pipe()` method to pipe the data from the file
    // into the stream.
    file.pipe(controller);
  },

  pull(controller) {
    // This function is called when the stream's internal queue is not full.
    // It can be used to read data from the underlying source and add it to the queue.

    // In this example we're going to continue reading data from the file
    // until there is no more data to read.
    const chunk = file.read();

    // If there is no more data to read, we can close the stream.
    if (chunk === null) {
      controller.close();
      return;
    }

    // Otherwise, we can add the data to the queue.
    controller.enqueue(chunk);
  },

  cancel(reason) {
    // This function is called when the stream is canceled.
    // It can be used to clean up any resources that were allocated by the stream.

    // In this example we're going to close the file.
    file.close();
  }
});

// We can use the `stream.getReader()` method to get a reader for the stream.
const reader = stream.getReader();

// We can then use the `reader.read()` method to read data from the stream.
reader.read().then(({ value, done }) => {
  // The `value` property contains the next chunk of data from the stream.
  // The `done` property is a boolean that indicates whether or not there is any
  // more data to read.

  if (done) {
    // If there is no more data to read, we can close the reader.
    reader.close();
    return;
  }

  // Otherwise, we can continue reading data from the stream.
  reader.read().then(({ value, done }) => {
    // ...
  });
});
```

#### Real World Applications

Readable streams can be used in a variety of real-world applications, such as:

* Streaming data from a server to a client
* Reading data from a file or other source
* Piping data between different processes or programs

#### Potential Applications

Here are some potential applications for readable streams:

* **Streaming video:** A readable stream can be used to stream video data from a server to a client. This can be used to provide live video streaming or to allow users to download videos on demand.
* **File downloads:** A readable stream can be used to download files from a server. This can be used to allow users to download files from a website or to transfer files between different computers.
* **Data processing:** A readable stream can be used to process data from a variety of sources. This can be used to perform data analysis, data mining, or other data-intensive tasks.

***

**Topic: `readableStream.locked` property**

**Simplified Explanation:**

The `readableStream.locked` property tells you if someone is currently reading data from the stream. It's like a "busy" sign for the stream.

**Detailed Explanation:**

A `ReadableStream` is like a water pipe that constantly flows data. If someone opens a faucet (a reader) and starts using the water, the stream becomes "locked." This prevents other faucets from opening and using the water simultaneously.

The `readableStream.locked` property tracks this status. When it's `false`, no one is reading from the stream. When it's `true`, someone is consuming its data.

**Code Snippet:**

```
const readableStream = new ReadableStream();

// Check if someone is reading from the stream
const isLocked = readableStream.locked;
```

**Real-World Example:**

A media player app may use a `ReadableStream` to handle a video file. When the user presses "play," the stream becomes locked, preventing other apps from accessing the video data while it's being played.

**Applications:**

* Managing concurrent data access
* Preventing data corruption in multi-threaded environments
* Enforcing data sequencing in streaming protocols

***

The `readableStream.cancel([reason])` method cancels a readable stream. This means that the stream will stop emitting data and will eventually close.

The `reason` argument is optional and specifies the reason for canceling the stream. This reason will be passed to any listeners of the `close` event on the stream.

Here is a simplified example of how to use the `readableStream.cancel([reason])` method:

```typescript
const readableStream = new ReadableStream({
  start(controller) {
    // Start the stream.
  },
  pull(controller) {
    // Read data from the stream.
  },
  cancel(reason) {
    // Cancel the stream.
  },
});

readableStream.cancel("The user canceled the stream.");
```

In this example, the `readableStream.cancel("The user canceled the stream.")` method is called to cancel the stream. The `reason` argument is specified as `"The user canceled the stream."`. This reason will be passed to any listeners of the `close` event on the stream.

The `readableStream.cancel([reason])` method can be used to cancel a stream for any reason. This could be because the user canceled the stream, because an error occurred, or because the stream is no longer needed.

Here are some potential applications for the `readableStream.cancel([reason])` method:

* **User cancels the stream:** The user can cancel the stream by calling the `readableStream.cancel([reason])` method. This could be because the user no longer needs the data from the stream, or because the user encountered an error.
* **Error occurs:** If an error occurs while reading from the stream, the stream will be automatically canceled. The `reason` for the cancelation will be the error that occurred.
* **Stream is no longer needed:** If the stream is no longer needed, it can be canceled by calling the `readableStream.cancel([reason])` method. This could be because the data from the stream has been processed, or because the stream is no longer being used.

***

#### `readableStream.getReader([options])`

* **Purpose**: Causes the `readableStream.locked` to be `true`.
* **Parameters**:
  * `options`:
    * `mode`: Specifies the mode of the reader. Can be `'byob'` or undefined. If `'byob'`, the reader will be a `ReadableStreamBYOBReader`, otherwise it will be a `ReadableStreamDefaultReader`.
* **Return Value**: Returns a `ReadableStreamDefaultReader` or `ReadableStreamBYOBReader`, depending on the value of the `mode` option.

**Explanation**:

The `getReader()` method creates a reader for the readable stream. Readers provide a way to read data from the stream in chunks. By default, the reader will be a `ReadableStreamDefaultReader`. However, you can specify the `mode` option to create a `ReadableStreamBYOBReader`. `ReadableStreamBYOBReader`s are more efficient for reading large amounts of data, as they allow you to provide your own buffer for the data to be read into.

**Real-World Example**:

The following code snippet shows how to create a `ReadableStreamDefaultReader` and use it to read data from a readable stream:

```js
const stream = new ReadableStream();
const reader = stream.getReader();

reader.read().then((result) => {
  const { done, value } = result;

  if (done) {
    console.log("No more data to read.");
  } else {
    console.log(`Read ${value.length} bytes of data.`);
  }
});
```

**Potential Applications**:

Readable streams and readers are used in a variety of applications, including:

* **Networking**: Reading data from a network connection.
* **File I/O**: Reading data from a file.
* **Data processing**: Reading data from a data source and processing it.

***

**ReadableStream.pipeThrough Method**

The `pipeThrough` method connects a readable stream to a pair of readable and writable streams provided in the `transform` argument. The data from the readable stream is written into the writable stream, potentially transformed, and then pushed to the readable stream. The transformed readable stream is returned.

**Parameters:**

* `transform`: An object with the following properties:
  * `readable`: The readable stream to which the writable stream will push the potentially modified data.
  * `writable`: The writable stream to which the readable stream's data will be written.
* `options`: An optional object with the following properties:
  * `preventAbort`: When `true`, errors in the readable stream will not cause the writable stream to be aborted.
  * `preventCancel`: When `true`, errors in the writable stream do not cause the readable stream to be canceled.
  * `preventClose`: When `true`, closing the readable stream does not cause the writable stream to be closed.
  * `signal`: An AbortSignal that can be used to cancel the data transfer.

**Return Value:**

* The transformed readable stream.

**Example:**

```javascript
const readableStream = new ReadableStream({
  start(controller) {
    controller.enqueue("a");
  },
});

const transformStream = new TransformStream({
  transform(chunk, controller) {
    controller.enqueue(chunk.toUpperCase());
  },
});

const transformedStream = readableStream.pipeThrough(transformStream);

for await (const chunk of transformedStream) {
  console.log(chunk); // Prints: 'A'
}
```

**Real-World Applications:**

* Preprocessing data before writing it to a database.
* Filtering data from a web API and sending it to a visualization component.
* Transforming data from one format to another.
* Chaining multiple data transformations together.

***

#### What is `pipeTo()`, and how does it work?

`pipeTo()` is a method that allows you to connect two streams together. A stream is a sequence of data, and it can be either readable or writable. A readable stream can be read from, and a writable stream can be written to.

When you call `pipeTo()`, you are connecting the readable stream to the writable stream. This means that any data that is read from the readable stream will be automatically written to the writable stream.

The `pipeTo()` method takes two arguments: the destination stream and an options object. The destination stream is the writable stream that you want to connect to. The options object can be used to specify how the data is transferred between the streams.

For example, you can use the `preventAbort` option to prevent the destination stream from being aborted if the readable stream encounters an error. You can also use the `preventCancel` option to prevent the readable stream from being canceled if the destination stream encounters an error.

#### Real-world example

Here is a real-world example of how you can use `pipeTo()` to copy the contents of one file to another file:

```javascript
const fs = require("fs");

const readableStream = fs.createReadStream("input.txt");
const writableStream = fs.createWriteStream("output.txt");

readableStream.pipeTo(writableStream);
```

In this example, the `readableStream` is the stream that is connected to the file `input.txt`. The `writableStream` is the stream that is connected to the file `output.txt`. When the `pipeTo()` method is called, the data from the `input.txt` file will be automatically copied to the `output.txt` file.

#### Potential applications

`pipeTo()` can be used in a variety of applications, such as:

* Copying files
* Transforming data
* Filtering data
* Merging data streams

#### Conclusion

`pipeTo()` is a powerful method that can be used to connect two streams together. It is a simple and efficient way to transfer data between streams.

***

### `readableStream.tee()`

The `tee()` method in Node.js's `stream` module returns a pair of new `ReadableStream` instances that will receive the same data as the original `readableStream`. This allows you to create multiple readers for the same data source.

#### Syntax

```typescript
tee(): [ReadableStream, ReadableStream];
```

#### Return value

The `tee()` method returns an array of two `ReadableStream` instances. The first stream will receive the same data as the original `readableStream`, and the second stream will receive a copy of the data.

#### Usage

The following code sample shows you how to use the `tee()` method:

```typescript
const { Readable } = require("stream");

const readable = new Readable({
  read() {},
});

const [stream1, stream2] = readable.tee();

stream1.on("data", (chunk) => {
  console.log(`Stream 1 received data: ${chunk.toString()}`);
});

stream2.on("data", (chunk) => {
  console.log(`Stream 2 received data: ${chunk.toString()}`);
});

readable.push("Hello world");
```

In this example, the `readable` stream is a simple readable stream that emits the string 'Hello world'. The `tee()` method is called on the `readable` stream, and the resulting array contains two new `ReadableStream` instances, `stream1` and `stream2`.

The `data` event is listened for on both `stream1` and `stream2`. When the `readable` stream emits the string 'Hello world', both `stream1` and `stream2` will receive the data and log it to the console.

#### Potential applications

The `tee()` method can be used in a variety of applications, such as:

* **Creating multiple readers for the same data source.** This can be useful for scenarios where you need to process the same data in different ways.
* **Creating a copy of a data stream.** This can be useful for scenarios where you need to store a copy of the data for later use.
* **Buffering data before processing.** This can be useful for scenarios where you need to wait until you have a certain amount of data before processing it.

***

### `readableStream.values([options])`

This method allows you to consume the data from a `ReadableStream` using an asynchronous iterator. It returns an async iterator that you can use in a `for await...of` loop to iterate over the data in the stream.

**Options**

The `options` parameter is an optional object that can contain the following properties:

* `preventCancel` {boolean}: When `true`, prevents the `ReadableStream` from being closed when the async iterator abruptly terminates. **Default**: `false`.

**Usage**

To use the `readableStream.values()` method, you can use the following syntax:

```javascript
for await (const chunk of readableStream.values()) {
  // Do something with the chunk of data
}
```

In the above example, the `for await...of` loop will iterate over the data in the `readableStream` and assign each chunk of data to the `chunk` variable. You can then use the `chunk` variable to do whatever you want with the data.

**Example**

The following example shows how to use the `readableStream.values()` method to consume the data from a `ReadableStream` that is created from a file:

```javascript
const fs = require("fs");
const { ReadableStream } = require("stream");

const fileStream = fs.createReadStream("somefile.txt");
const readableStream = new ReadableStream({
  read() {
    // Read data from the file and push it into the stream
  },
});

for await (const chunk of readableStream.values()) {
  console.log(chunk.toString());
}
```

**Real-World Applications**

The `readableStream.values()` method can be used in a variety of real-world applications, such as:

* Consuming data from a file or network stream
* Iterating over the results of a database query
* Processing data in a pipeline
* Creating a custom data source for a web application

***

**Async Iteration with ReadableStream**

**What is Async Iteration?**

Async iteration is a way of consuming a stream of data asynchronously, meaning you don't have to wait for the entire stream to be available before you can start processing it.

**What is a ReadableStream?**

A ReadableStream represents a stream of data that can be read from. It's like a pipe that you can get data chunks from.

**How to Use Async Iteration with ReadableStream**

You can use the `for await` syntax to iterate over a ReadableStream:

```javascript
const stream = new ReadableStream();

for await (const chunk of stream) {
  console.log(chunk);
}
```

This code will print each chunk of data from the stream as it becomes available. The `for await` syntax will automatically wait for the next chunk to be available before continuing.

**Preventing Automatic Closing**

By default, if you exit the async iteration early (e.g., with a `break` or `return` statement), the ReadableStream will be closed. To prevent this, you can use the `readableStream.values()` method to acquire the async iterator and set the `preventCancel` option to `true`:

```javascript
const stream = new ReadableStream();
const asyncIterator = stream.values({ preventCancel: true });

for await (const chunk of asyncIterator) {
  console.log(chunk);

  // Exit the iteration early but don't close the stream
  if (condition) {
    break;
  }
}
```

**Real-World Applications**

Async iteration with ReadableStream can be used in many real-world applications:

* **Streaming video:** You can use a ReadableStream to stream video data from a server to a client, allowing the client to start playing the video immediately without having to wait for the entire file to download.
* **Real-time data processing:** You can use a ReadableStream to process data in real time, such as processing sensor data or financial data.
* **Chat applications:** You can use a ReadableStream to receive new messages from a chat server, allowing you to display them in a chat window as they arrive.

**Improved Code Snippet**

Here's an improved version of the code snippet from the documentation:

```javascript
import { Buffer } from "node:buffer";

const stream = new ReadableStream(getSomeSource());

for await (const chunk of stream) {
  console.log(Buffer.from(chunk).toString());
}
```

This code snippet uses the `Buffer` class from the `node:buffer` module to convert the chunk of data to a string before logging it.

***

#### Transferring with `postMessage()`

**Simplified Explanation:**

You can use `postMessage()` with a `MessagePort` to send a {ReadableStream} instance to another part of your program.

**Detailed Explanation:**

1. Create a new {ReadableStream} instance.
2. Create a new `MessageChannel`. This will give you two `MessagePort` objects: `port1` and `port2`.
3. Add a message listener to `port1`. When a message is received, it will contain the {ReadableStream} instance.
4. Use `port2` to send the {ReadableStream} instance to the other part of your program.

**Example:**

```
const stream = new ReadableStream({
  start(controller) {
    controller.enqueue('Hello');
    controller.enqueue('World');
    controller.close();
  }
});

const { port1, port2 } = new MessageChannel();

port1.onmessage = (event) => {
  event.data.getReader().read().then(chunk => {
    console.log(chunk);
  });
};

port2.postMessage(stream);
```

**Output:**

```
{ value: 'Hello', done: false }
{ value: 'World', done: false }
{ value: undefined, done: true }
```

**Real-World Application:**

This technique can be used to transfer data between different parts of a web application, such as between a parent window and an iframe.

***

**`ReadableStream.from(iterable)`**

**What it is:**

* A method that creates a readable stream from an iterable object.
* You can use it to convert any existing iterable, like an array or a generator function, into a readable stream.

**How it works:**

* The `iterable` object must implement the `Symbol.asyncIterator` or `Symbol.iterator` protocol.
* The method will create a readable stream that emits chunks of data from the iterable object.
* The stream will end automatically when the iterable object is exhausted.

**Code Example:**

```javascript
// Create an iterable object
const iterable = ["a", "b", "c"];

// Create a readable stream from the iterable
const stream = ReadableStream.from(iterable);

// Read the chunks from the stream
stream.pipeThrough(new TransformStream()).pipeTo(new WritableStream());
```

**Real-World Application:**

* You can use this method to create readable streams from any existing iterable data structure.
* For example, you could use it to stream data from a database query or a file system.

***

### Simplified Explanation of `ReadableStreamDefaultReader`

**ReadableStreamDefaultReader** is a special reader that comes with every `ReadableStream`. It's like a magic box that lets you access the data flowing through the stream.

**Opaque Values:**

The data that flows through a stream is like a secret message. The `ReadableStreamDefaultReader` treats these messages as if they were sealed boxes without knowing what's inside. This makes it possible to work with any type of data, like numbers, strings, or even objects.

**How it Works:**

To use the `ReadableStreamDefaultReader`, you first need to create it by calling `readableStream.getReader()`. This gives you a reader object that you can use to read the data from the stream.

To read data, you call `reader.read()`. This returns a `ReadableStreamDefaultReadResult` object, which contains a `value` property that holds the next chunk of data from the stream.

**Real-World Applications:**

Readable streams are used in many different applications, such as:

* Reading data from files
* Receiving data over a network
* Processing data in web workers

In each of these cases, the `ReadableStreamDefaultReader` plays a crucial role in accessing the data flowing through the stream.

**Example Implementation:**

```javascript
// Create a readable stream
const readableStream = new ReadableStream();

// Get the default reader
const reader = readableStream.getReader();

// Read data from the stream
reader.read().then((result) => {
  // Do something with the data in `result.value`
});
```

In this example, the `reader` object can be used to read data from the `readableStream` until it's empty. The `read()` method returns a promise that resolves when new data is available.

***

#### `new ReadableStreamDefaultReader(stream)`

* `stream` {ReadableStream}

Creates a new `ReadableStreamDefaultReader` that is locked to the given `ReadableStream`.

**Parameters**

* `stream`: The `ReadableStream` to which the reader will be locked.

**Return Value**

A new `ReadableStreamDefaultReader` instance.

**Example**

```
// Create a readable stream.
const stream = new ReadableStream({
  start(controller) {
    // The start method is called when the stream is first opened.
    // It is responsible for generating and pushing data into the stream.
    controller.enqueue('Hello');
    controller.enqueue('World');
    controller.close();
  }
});

// Create a reader for the stream.
const reader = stream.getReader();

// Read data from the stream.
reader.read().then((result) => {
  if (result.done) {
    // The stream has been closed.
  } else {
    // The stream has data available.
    console.log(result.value); // 'Hello'
  }
});
```

**Applications**

Readable streams are used in a variety of applications, including:

* **File I/O**: Reading and writing files.
* **Networking**: Sending and receiving data over a network.
* **Data processing**: Transforming and filtering data.
* **Audio and video**: Streaming audio and video content.

***

**readableStreamDefaultReader.cancel()**

The `readableStreamDefaultReader.cancel()` method in `webcrypto` cancels the readable stream and returns a promise that is fulfilled when the underlying stream has been canceled.

**Syntax:**

```typescript
cancel([reason]): Promise<undefined>;
```

**Parameters:**

* `reason` (optional): A reason for canceling the stream.

**Return Value:**

A promise that is fulfilled when the stream has been canceled.

**Example:**

```typescript
const stream = new ReadableStream({
  start(controller) {
    controller.enqueue("Hello");
    controller.enqueue("World");
    controller.close();
  },
});

const reader = stream.getReader();

reader.cancel().then(() => {
  console.log("Stream canceled");
});
```

**Output:**

```
Stream canceled
```

**Applications:**

* Canceling a stream that is no longer needed.
* Canceling a stream due to an error.

***

### `readableStreamDefaultReader.closed`

#### Description

The `closed` property of the `ReadableStreamDefaultReader` interface returns a promise that is fulfilled with `undefined` when the associated {ReadableStream} is closed or rejected if the stream errors or the reader's lock is released before the stream finishes closing.

#### Syntax

```typescript
readonly closed: Promise<undefined>;
```

#### Real-World Example

```typescript
const reader = readableStream.getReader();
reader.closed.then(() => {
  // The stream has been closed.
});
```

***

### `readableStreamDefaultReader.read()`

This method lets you read the next chunk of data from a `ReadableStream`. It returns a `Promise` that resolves with an object containing a `value` (the data) and a `done` flag (indicating whether there's more data to read).

#### Example:

```js
const reader = stream.getReader();

reader.read().then(({ value, done }) => {
  if (done) {
    // No more data to read
  } else {
    // Process the data in `value`
    // Continue reading the stream
    reader.read().then(...)
  }
});
```

#### Applications:

This method is useful for scenarios where you want to process data from a stream incrementally as it becomes available. For example, you could use it to:

* Read data from a file in chunks
* Read data from a network stream
* Process a stream of data from a sensor or device

***

### `readableStreamDefaultReader.releaseLock()`

#### Explanation:

Imagine you have a tap (Readable stream) and a glass of water (ReadableStreamDefaultReader). You want to take a sip, so you turn the tap on (acquire lock). Once you're done drinking, you need to release the glass so that others can use the tap (release lock).

#### Code Snippet:

```
const myStream = new ReadableStream();
const myReader = myStream.getReader();

// Acquire the lock
myReader.acquireLock();

// Do something with the stream

// Release the lock
myReader.releaseLock();

```

#### Real-world Implementation:

In real-world applications, this method is useful when you want to pause or stop a stream, release memory, or allow others to access the stream.

#### Potential Applications:

* Data streaming: Pausing or stopping data transfer to save bandwidth.
* Background tasks: Releasing locks on streams that are not actively being used.
* Parallel processing: Allowing multiple processes to access a stream without conflicts.

***

**ReadableStreamBYOBReader**

The `ReadableStreamBYOBReader` is like a special way to read data from a stream that contains bytes, like a file or a network connection. Instead of using its own buffer to store the data, it lets you bring your own buffer and fill it with the data you want to read. This can be more efficient because it avoids copying the data back and forth between buffers.

**Example:**

Imagine you have a file that you want to read. Instead of using the `readFileSync` method, which would read the entire file into memory at once, you can use `ReadableStreamBYOBReader`. This way, you can read the file in chunks, bringing your own buffer to store each chunk. This is especially useful if the file is large and you don't want to use up all of your memory to read it all at once.

```javascript
const fs = require("fs");

const stream = fs.createReadStream("large_file.txt", {
  highWaterMark: 1024, // Read in chunks of 1024 bytes
  encoding: "utf8", // Decode the data as text
});

const reader = stream.getReader({ mode: "byob" });

const chunks = [];
let result;
do {
  result = await reader.read(Buffer.alloc(1024));
  if (result.value !== undefined) chunks.push(Buffer.from(result.value));
} while (!result.done);

const data = Buffer.concat(chunks).toString();

console.log(data);
```

In this example, the `createReadStream` method creates a stream that reads from the `large_file.txt` file. The `highWaterMark` option specifies the size of the chunks that will be read at a time. The `encoding` option specifies that the data should be decoded as text.

The `getReader` method creates a reader for the stream. The `mode` option specifies that we want to use the BYOB (bring your own buffer) mode.

The `read` method reads data from the stream into the provided buffer. In this example, we create a new buffer of size 1024 bytes each time we call `read`.

The loop continues reading data from the stream until we reach the end of the file (`result.done` becomes `true`).

Finally, we concatenate all the chunks into a single buffer and convert it to a string.

**Applications:**

The `ReadableStreamBYOBReader` can be used in any situation where you need to read byte-oriented data efficiently, such as:

* Reading large files
* Streaming data from a network connection
* Processing binary data

***

**Simplified Explanation:**

Imagine you have a stream of water flowing through a pipe. A `ReadableStreamBYOBReader` is like a special cup you can use to drink from the stream. It's "bring your own bottle" because it doesn't come with a cup, but it will fill any cup you give it.

**Technical Details:**

* **stream:** The stream of data you want to read from.

**Real-World Example:**

You can use a `ReadableStreamBYOBReader` to read a video stream from a server. The server sends the video data as a stream, and you can use the reader to fill a video player with the data.

**Code Example:**

```
const stream = new ReadableStream();
const reader = new ReadableStreamBYOBReader(stream);

reader.read().then(({value}) => {
  // Here, 'value' is a chunk of the video data.
});
```

**Potential Applications:**

* Video streaming
* File downloads
* Real-time data processing

***

#### What is `readableStreamBYOBReader.cancel([reason])`?

`readableStreamBYOBReader.cancel([reason])` is a method that cancels a readable stream, meaning it will stop reading data from the underlying source.

#### How does it work?

When you call `readableStreamBYOBReader.cancel([reason])`, the stream will stop reading data and the promise returned by the method will be fulfilled.

The `reason` parameter is optional and can be used to provide a reason for canceling the stream.

#### Why would I want to cancel a readable stream?

There are a few reasons why you might want to cancel a readable stream:

* You are no longer interested in the data being read from the stream.
* The stream is producing too much data and you need to stop it from overwhelming your application.
* There was an error reading from the stream and you want to stop the stream from continuing.

#### Example

The following code shows how to use `readableStreamBYOBReader.cancel([reason])` to cancel a readable stream:

```js
const fs = require("fs");

const readableStream = fs.createReadStream("file.txt");

readableStream.on("data", (data) => {
  // Do something with the data.
});

// Cancel the stream after 10 seconds.
setTimeout(() => {
  readableStream.cancel("Timeout");
}, 10000);
```

In this example, the readable stream will be canceled after 10 seconds. The `reason` parameter is set to `Timeout` to indicate that the stream was canceled because it timed out.

#### Real-world applications

`readableStreamBYOBReader.cancel([reason])` can be used in a variety of real-world applications, such as:

* Stopping a stream when the user navigates away from a page.
* Limiting the amount of data that is read from a stream.
* Stopping a stream when an error occurs.

***

#### `readableStreamBYOBReader.closed`

* Type: Boolean
* Returns `true` if the associated {ReadableStream} is closed and `false` otherwise.

**Real-world example:**

```javascript
const reader = stream.getReader();

reader.closed.then(() => {
  // The stream has been closed.
});

stream.cancel();

reader.closed.then((reason) => {
  // The stream was canceled.
});
```

**Potential applications:**

* Waiting for a stream to close before performing an operation.
* Determining if a stream has been canceled.

***

**Simplified Explanation:**

**`readableStreamBYOBReader.read()`** is a method that allows you to read data from a readable stream in chunks. Instead of returning the data directly, it returns a promise that resolves when the data is available.

**Parameters:**

* **`view`**: A buffer, typed array, or data view where the data will be stored.
* **`options`**: An optional object with a `min` property to specify the minimum number of elements to wait for before fulfilling the promise.

**Return Value:**

The promise resolves with an object containing:

* **`value`**: The requested data.
* **`done`**: A boolean indicating if there is no more data to read.

\**Example:*

```javascript
const readableStream = fs.createReadStream("my-file.txt");
const reader = readableStream.getReader();

reader.read().then(({ value, done }) => {
  if (!done) {
    // Process the data in `value`
  }
});
```

**Applications in Real World:**

* **Streaming large files over the network:** Instead of loading the entire file into memory, you can use `readableStreamBYOBReader.read()` to retrieve data in chunks as needed.
* **Real-time data processing:** When data is constantly being generated, `readableStreamBYOBReader.read()` allows you to process it incrementally without having to wait for the entire dataset.
* **Load balancing:** By using multiple readers to read from the same stream, you can distribute the load and improve performance.

***

#### `readableStreamBYOBReader.releaseLock()`

**Explanation:**

When you read data from a stream using Node.js's `ReadableStreamBYOBReader`, the reader locks the stream to prevent other readers from accessing it. However, if you're done reading from the stream, you need to release the lock so that other readers can use it.

**Simplified Analogy:**

Imagine a library with a book. When you borrow the book, you're "locking" it so that no one else can read it until you return it. When you're done reading it, you need to put it back on the shelf so that others can borrow it.

**Code Example:**

```javascript
const reader = stream.getReader();

// Read some data from the stream
reader.read().then((result) => {
  // Process the data

  // When you're done reading, release the lock
  reader.releaseLock();
});
```

**Real-World Application:**

Readable streams are used to read data from sources such as files, network sockets, and databases. Releasing the lock ensures that other readers can access the stream when you're finished. This is important for efficient data processing, especially in multithreaded environments where multiple readers may need to access the same stream concurrently.

***

**ReadableStreamDefaultController**

Every readable stream in Node.js has a controller that manages its internal queue of data. The `ReadableStreamDefaultController` is the default controller for streams that are not byte-oriented, such as streams of objects or strings.

**Simplified Explanation:**

Imagine a stream as a water pipe. Data flows through the pipe, and the controller is like a faucet that controls the flow. The `ReadableStreamDefaultController` is a special kind of faucet that works best for non-byte-oriented data.

**Real-World Implementation:**

Here's an example of creating a readable stream with a `ReadableStreamDefaultController`:

```javascript
const {
  ReadableStream,
  ReadableStreamDefaultController,
} = require("stream/web");

const controller = new ReadableStreamDefaultController();
const stream = new ReadableStream({
  start(controller) {
    // This function is called when the stream is started.
    // Here, we push some data into the queue.
    controller.enqueue("Hello");
    controller.enqueue("World");
  },
  pull(controller) {
    // This function is called when the stream wants more data.
    // Here, we check if there's more data in the queue and push it.
    if (!controller.desiredSize) {
      return;
    }

    while (controller.desiredSize > 0) {
      const chunk = controller.queue.shift();
      if (!chunk) {
        break;
      }
      controller.enqueue(chunk);
    }
  },
  cancel(controller) {
    // This function is called when the stream is canceled.
    // Here, we close the queue.
    controller.queue.close();
  },
});

stream.on("data", (chunk) => {
  console.log(`Received chunk: ${chunk}`);
});

stream.on("close", () => {
  console.log("Stream closed.");
});

// Start the stream.
stream.start();

// Cancel the stream after 5 seconds.
setTimeout(() => {
  stream.cancel();
}, 5000);
```

**Applications:**

The `ReadableStreamDefaultController` can be used in a variety of applications, including:

* Reading data from a file or network stream
* Parsing data from a JSON or XML file
* Transforming data from one format to another

***

In Node.js, a ReadableStream is an object that represents a stream of data that can be read from. A ReadableStreamDefaultController is an object that provides methods to control the flow of data from a ReadableStream. The `close()` method on a ReadableStreamDefaultController closes the ReadableStream, which means that no more data will be read from the stream.

Here is an example of how to use the `close()` method on a ReadableStreamDefaultController:

```
const { Readable } = require('stream');

const readableStream = new Readable();
const readableStreamController = readableStream.controller;

// Close the readable stream
readableStreamController.close();
```

After calling `close()` on the ReadableStreamDefaultController, the ReadableStream will emit a `close` event. This event can be used to perform any necessary cleanup operations, such as closing any open files or resources.

Here is a real-world example of how the `close()` method can be used in a Node.js application:

```
// Create a readable stream from a file
const fs = require('fs');
const readableStream = fs.createReadStream('file.txt');

// Create a ReadableStreamDefaultController for the readable stream
const readableStreamController = readableStream.controller;

// Listen for the `close` event on the readable stream
readableStream.on('close', () => {
  // Perform any necessary cleanup operations
  console.log('The readable stream has been closed.');
});

// Close the readable stream
readableStreamController.close();
```

In this example, we create a readable stream from a file and then create a ReadableStreamDefaultController for the stream. We then listen for the `close` event on the readable stream and perform any necessary cleanup operations when the event is emitted. Finally, we call the `close()` method on the ReadableStreamDefaultController to close the readable stream.

***

#### **`readableStreamDefaultController.desiredSize`**

* **Type:** `number`
* **Usage:**

  The `desiredSize` property of the `ReadableStreamDefaultController` interface represents the amount of data that the underlying source is expected to produce before the stream becomes readable again.

  In other words, it's the amount of data that the stream needs to fill its internal buffer before it can start emitting `'data'` events.
* **Example:**

  The following code uses the `desiredSize` property to log the amount of data that the stream wants to receive before it becomes readable again:

  ```typescript
  const stream = new ReadableStream({
    start(controller) {
      // ...
      console.log(`Desired size: ${controller.desiredSize}`);
    },
  });
  ```
* **Applications:**

  The `desiredSize` property can be used to:

  * Optimize the performance of the stream by prefetching data in advance.
  * Prevent the stream from becoming overwhelmed by data.
  * Control the flow of data through the stream.

***

#### `readableStreamDefaultController.enqueue([chunk])`

* `chunk` {any}

Appends a new chunk of data to the {ReadableStream}'s queue.

This method enqueues data received from a readable stream. You will call this method from the transform function of a transform stream or the write method of a writable stream.

**Syntax:**

```typescript
readableStreamDefaultController.enqueue([chunk]);
```

**Parameters:**

* `chunk`: (optional) The chunk of data to be enqueued in the stream.

**Return value:**

* None

**Example:**

```typescript
// Transform stream reads an input file, converts all lowercase characters to uppercase, and then writes the result to an output file.

const { Transform, pipeline } = require("stream");
// Create a transform stream
const myTransform = new Transform({
  transform(chunk, encoding, callback) {
    // Get the chunk of data from the input file
    const transformedChunk = chunk.toString().toUpperCase();
    // Enqueue the transformed chunk in the output stream
    this.push(transformedChunk);
    // Call the callback to indicate that the transformation is complete
    callback();
  },
});

// Create a pipeline to connect the input file, the transform stream, and the output file
pipeline(
  fs.createReadStream("input.txt"),
  myTransform,
  fs.createWriteStream("output.txt"),
  (error) => {
    if (error) {
      console.error("Error occurred in the pipeline:", error);
    } else {
      console.log("File transformed successfully");
    }
  }
);
```

**Applications:**

* Data processing and transformation
* File reading and writing
* Network communication

***

#### `ReadableStreamDefaultController.error([error])`

The `ReadableStreamDefaultController.error([error])` method signals an error that causes the {ReadableStream} to error and close.

**Syntax**

```
readableStreamDefaultController.error([error])
```

**Parameters**

* `error` {any}
  * The error to signal.

**Throws**

* A TypeError if the stream is locked.
* An Error if the stream is not in a valid state to error.

**Example**

```js
const stream = new ReadableStream({
  start(controller) {
    controller.push("hello");
    controller.push("world");
    controller.error(new Error("Something went wrong!"));
  },
});
stream.on("error", (err) => {
  console.error(err);
});
```

**Output**

```
Error: Something went wrong!
```

#### Real World Applications

The `ReadableStreamDefaultController.error()` method can be used to signal errors that occur while the stream is being read. This can be useful for handling errors that occur when reading from a file or a network connection.

#### Potential Applications

* Error handling in file reading
* Error handling in network connections
* Error handling in data processing pipelines

***

**Simplified Explanation:**

A `ReadableByteStreamController` is like a traffic controller for a stream of bytes in a `ReadableStream`. It decides when to let bytes through the stream and when to stop or slow down the flow.

**Detailed Explanation:**

A `ReadableByteStreamController` is a JavaScript object that manages the internal state of a `ReadableStream` that handles bytes. It controls the flow of bytes from the stream's source (the producer) to its destination (the consumer).

The `ReadableByteStreamController` has several responsibilities:

* **Enqueue Byte Chunks:** It receives chunks of bytes from the producer and adds them to an internal queue.
* **Control Flow:** It can control the rate at which bytes are read from the queue, using methods like `enqueue` and `close`.
* **Handle Backpressure:** When the consumer is slow in reading bytes, the controller can stop or slow down the flow of bytes to prevent the stream from overflowing.
* **Error Handling:** It can handle and report errors that occur during the reading process.

**Real-World Example:**

Imagine a pipeline carrying water from a reservoir to your house. The `ReadableByteStreamController` would be like the gatekeeper at the start of the pipeline. It would control the flow of water to ensure that you have a steady supply, but not too much that it overflows.

**Code Example:**

```javascript
// Create a ReadableStream and its controller
const readableStream = new ReadableStream({
  start(controller) {
    // Controller methods to enqueue bytes and control flow
    controller.enqueue(Uint8Array.from([1, 2, 3]));
    controller.close();
  },
});

// Create a reader to consume the bytes from the stream
const reader = readableStream.getReader();
reader.read().then(({ done, value }) => {
  if (!done) {
    // Process the bytes in the value Uint8Array
  }
});
```

**Potential Applications:**

* **Streaming media:** Controlling the flow of video or audio data from a server to a video player.
* **File downloads:** Regulating the transfer rate of a file being downloaded from the internet.
* **Data pipelines:** Managing the flow of data between different processing stages in a data pipeline.

***

### `readableByteStreamController.byobRequest`

The `readableByteStreamController.byobRequest` property in `webcrypto` is to obtain a BYOB request object, which allows you to write bytes to that chunk in an arbitrary way. This can be useful, for instance, if you want to specify the exact source and destination buffer views and offsets.

* `value` : {ReadableStreamBYOBRequest}

***

### `readableByteStreamController.close()`

**Simplified Explanation:**

`readableByteStreamController.close()` is a method that tells the stream that there is no more data to send.

**Real World Example:**

Imagine you have a pipe that carries water. You start pouring water into the pipe, and the water flows out the other end. When you finish pouring water, you close the tap at the source of the pipe. Closing the tap is similar to calling `readableByteStreamController.close()`. It signals that there is no more water to flow through the pipe.

**Code Example:**

```javascript
// Create a readable stream controller
const { ReadableByteStreamController } = require("stream/web");
const controller = new ReadableByteStreamController();

// Create a readable stream using the controller
const stream = new ReadableStream({
  start(controller) {
    // Send some data
    controller.enqueue(new Uint8Array([1, 2, 3]));

    // Close the stream when done
    controller.close();
  },
});

// Listen for data on the stream
stream.on("data", (chunk) => {
  console.log(chunk); // Prints [1, 2, 3]
});
```

**Potential Applications:**

`readableByteStreamController.close()` is useful in situations where you need to control the flow of data in a stream. For example:

* **File upload:** You can close the stream when the file upload is complete to indicate that there is no more data to send.
* **Data streaming:** You can close the stream when the data source is exhausted to prevent the stream from receiving more data.

***

#### readableByteStreamController.desiredSize

The `readableByteStreamController.desiredSize` property in Node.js returns the amount of data remaining to fill the `ReadableStream`'s queue.

**Syntax:**

```
const desiredSize = readableByteStreamController.desiredSize;
```

**Return value:**

A number representing the amount of data remaining to fill the `ReadableStream`'s queue.

**Example:**

```javascript
const { ReadableStream, ReadableByteStreamController } = require("stream/web");

const readableStream = new ReadableStream({
  start(controller) {
    // The desired size is initially 0.
    console.log(controller.desiredSize); // 0

    // Enqueue some data.
    controller.enqueue("Hello");

    // The desired size is now the amount of data remaining to fill the queue.
    console.log(controller.desiredSize); // 10
  },
});

readableStream.on("data", (chunk) => {
  console.log(`Received data: ${chunk}`);
});

// Output:
// 0
// Received data: Hello
```

**Real-world applications:**

The `readableByteStreamController.desiredSize` property can be used to control the flow of data through a `ReadableStream`. For example, it can be used to:

* Throttle the amount of data that is enqueued at a time.
* Backpressure the source of the data if the queue is full.

**Potential applications:**

* **Audio/video streaming:** The desired size can be used to control the amount of data that is buffered before it is played back. This can help to prevent buffering and ensure a smooth playback experience.
* **Data ingestion:** The desired size can be used to control the rate at which data is ingested into a database or other data store. This can help to prevent overload and ensure that the data store can keep up with the incoming data.

***

#### enqueue()

The `enqueue()` function is used to append a chunk of data to the `ReadableStream`'s internal queue. A `ReadableStream` is a stream that can be read from, and the `enqueue()` function adds data to the stream that can be read later.

The `chunk` parameter is the data that is being added to the stream. It can be a Buffer, a TypedArray, or a DataView.

Here is an example of how to use the `enqueue()` function:

```
const readableStream = new ReadableStream();

// Create a chunk of data
const chunk = Buffer.from('Hello world');

// Enqueue the chunk of data to the stream
readableStream.enqueue(chunk);

// Read the data from the stream
readableStream.read().then((result) => {
  console.log(result.data.toString()); // Output: Hello world
});
```

In this example, we create a `ReadableStream` and enqueue a chunk of data to the stream. We then read the data from the stream and print it to the console.

#### Potential Applications

The `enqueue()` function can be used in a variety of applications, including:

* Streaming large files to a client
* Sending data to a server in chunks
* Buffering data for later processing

***

**Simplified Explanation:**

The `error` method is used to indicate that an error has occurred in the readable byte stream controller. This will cause the stream to close and any data that hasn't been read yet will be discarded.

**Real-World Example:**

Imagine you have a stream of data that you're reading from a file. While reading the file, you encounter a corrupted section of data. To handle this error, you would use the `error` method to signal the error and close the stream. This would prevent any further data from being read from the corrupted section.

**Code Implementation:**

```javascript
const { ReadableStream } = require("stream");

const readableStream = new ReadableStream();

// Simulate an error occurring while reading the stream
setTimeout(() => {
  readableStream.controller.error(
    new Error("Error occurred while reading stream")
  );
}, 2000);

// Create a reader to read data from the stream
const reader = readableStream.getReader();

// Read data from the stream until an error occurs
reader
  .read()
  .then(({ done, value }) => {
    if (done) {
      console.log("Stream closed due to error");
    } else {
      console.log(`Data read: ${value}`);

      // Continue reading until an error occurs
      return reader.read();
    }
  })
  .catch((error) => {
    console.error("Error occurred while reading stream:", error);
  });
```

**Potential Applications:**

The `error` method can be used in a variety of applications, including:

* Handling errors while reading data from a file or network
* Preventing corrupted data from being propagated downstream
* Gracefully closing a stream when an error occurs

***

**ReadableStreamBYOBRequest**

Imagine you have a water pipe that can send water to your house. If you want to get water, you can use a bucket and place it under the pipe to collect the water. The `ReadableStreamBYOBRequest` is like the bucket that you put under the pipe to collect the data that the stream is sending.

**How it works:**

1. When you want to read data from a stream, you create a `ReadableStreamBYOBReader` object. This object is like a person that manages the bucket.
2. The `ReadableStreamBYOBReader` object will create a `ReadableStreamBYOBRequest` object, which is the bucket.
3. The `ReadableStreamController` object, which is like the person controlling the water pipe, will fill the bucket with data.
4. Once the bucket is filled, the `ReadableStreamBYOBReader` object will tell the `ReadableStreamController` object that the bucket is full and ready to be used.

**Real-world example:**

Imagine you are downloading a large file from the internet. The browser will use a `ReadableStreamBYOBRequest` object to download the file in chunks. Each chunk of data will be stored in the bucket. Once the bucket is full, the browser will tell the server to send more data.

**Applications:**

* Streaming large files
* Processing data in real time
* Creating custom data pipelines

**Code example:**

```javascript
const reader = stream.getReader({ mode: "byob" });
const request = reader.read();

// The bucket is empty, so we wait for the stream to fill it.
await request.promise;
```

**Potential applications:**

* Downloading files in chunks
* Processing data on the fly
* Creating custom data pipelines

***

### `readableStreamBYOBRequest.respond(bytesWritten)`

When we use `Crypto.subtle.encrypt()` or `Crypto.subtle.decrypt()` with the option `{ mode: 'decrypt' }` we can pass an array buffer to `view` which the result will be written to.

This method signals to the algorithm that `bytesWritten` number of bytes have been written to the buffer.

**Example:**

```
const { subtle } = require('crypto');
const { PassThrough } = require('stream');

const readableStream = new PassThrough();

subtle.encrypt(
  {
    name: 'AES-CBC',
    iv: Buffer.from('0123456789abcdef', 'hex')
  },
  subtle.importKey('raw', keyArrayBuffer, { name: 'AES-CBC' }, false, ['encrypt']),
  readableStream
)

.then(({ result }) => {
  let totalBytesWritten = 0;
  result.on('data', (chunk) => {
    totalBytesWritten += chunk.byteLength;
  });

  // When this event fires, `totalBytesWritten` is the number of bytes the
  // stream has written to the buffer.
  result.on('close', () => {
    console.log(`${totalBytesWritten} bytes written to the buffer`);
  });

  // Call respond(bytesWritten) after writing to the view.
  result.write(Buffer.from('Hello world!'));
  result.respond(Buffer.from('Hello world!').length);
});
```

**Real-world applications:**

* Securely encrypting and decrypting data in streaming applications.
* Encrypting data in chunks for more efficient processing.
* Decrypting data in chunks for faster response times.

***

**Topic:** `readableStreamBYOBRequest.respondWithNewView(view)`

**Explanation:**

Imagine you have a long video file that you want to read and process chunk by chunk. Instead of loading the entire file into memory, you can use a `ReadableStream` to read the file in smaller pieces.

`ReadableStreamBYOBRequest` is a special type of `ReadableStream` that allows you to provide your own buffer (`Buffer`, `TypedArray`, or `DataView`) where the data can be written. This is useful if you want to avoid copying data unnecessarily or if you need to perform operations on the data as it is being read.

The `respondWithNewView` method signals that you have finished writing to the provided buffer and that the request has been fulfilled.

**Code Example:**

```javascript
const crypto = require("crypto");
const { Readable } = require("stream");

const readableStream = new Readable();
readableStream.push("Hello");
readableStream.push("World");
readableStream.push(null); // Signal end of stream

const request = readableStream.pipeThrough(crypto.createHash("sha256"));
const buffer = Buffer.alloc(32);

request.on("data", (chunk) => {
  // Write the data to the buffer
  buffer.write(chunk);
});

request.on("end", () => {
  // Signal that we're done writing to the buffer
  request.respondWithNewView(buffer);
});
```

**Potential Applications:**

* **Data processing:** Reading and processing large files without loading them entirely into memory.
* **Streaming:** Sending or receiving data as a continuous stream without buffering the entire content.
* **Security:** Hashing data in a streaming fashion without storing the entire data in memory.

***

#### `readableStreamBYOBRequest.view`

The `view` property of the `readableStreamBYOBRequest` interface is a Buffer or TypedArray that can be used to hold the output data from the request.

The `view` property is optional. If it is not provided, the request will create a new Buffer or TypedArray to hold the output data.

The `view` property can be used to improve performance by avoiding unnecessary copying of data.

**Example:**

```typescript
const readableStreamBYOBRequest = crypto.subtle.encrypt(
  { name: "AES-CBC" },
  key,
  iv
);

readableStreamBYOBRequest.on("data", (data) => {
  // Process the data.
});

readableStreamBYOBRequest.on("end", () => {
  // All of the data has been processed.
});

const output = readableStreamBYOBRequest.view;
```

***

**What is a WritableStream?**

A WritableStream is like a pipe that you can write data into. It's a destination for data that comes from a stream.

**How to use a WritableStream:**

To use a WritableStream, you first need to create one. You can do this using the `WritableStream()` constructor.

```js
const stream = new WritableStream({
  write(chunk) {
    console.log(chunk);
  },
});
```

The `write()` function is called every time data is written to the stream. In the example above, the `console.log()` function is used to print the data to the console.

**Getting a Writer for a WritableStream:**

Once you have created a WritableStream, you can get a writer for it using the `getWriter()` method. The writer is what you use to write data to the stream.

```js
const writer = stream.getWriter();

await writer.write("Hello World");
```

The `write()` method takes a single argument, which is the data that you want to write to the stream. The `await` keyword is used to wait for the write operation to complete.

**Real-world use case:**

WritableStreams are used in a variety of applications, such as:

* Logging
* Caching
* Data processing
* Networking

**Improved code example:**

Here is an improved version of the code example above:

```js
const { WritableStream } = require("stream/web");

const stream = new WritableStream({
  write(chunk) {
    console.log(chunk.toString());
  },
});

const writer = stream.getWriter();

writer.write("Hello World");

// Close the stream when you are finished writing to it.
writer.close();
```

This example uses the `toString()` method to convert the data to a string before printing it to the console. The `close()` method is used to close the stream when you are finished writing to it.

***

**WritableStream**

A WritableStream represents a writable stream of data. It is an abstraction provided by the Web Streams API that allows you to write data to a destination in a controlled manner.

**UnderlyingSink**

The underlyingSink is the object that will actually receive the data written to the WritableStream. It must implement the following methods:

* **start(controller)**: This method is called when the WritableStream is created. The controller parameter is a WritableStreamDefaultController object that provides methods for controlling the flow of data.
* **write(chunk, controller)**: This method is called when a chunk of data is written to the WritableStream. The chunk parameter is the data to be written, and the controller parameter is a WritableStreamDefaultController object that provides methods for controlling the flow of data.
* **close()**: This method is called when the WritableStream is closed.
* **abort(reason)**: This method is called to abruptly close the WritableStream.

**Strategy**

The strategy object is used to configure the behavior of the WritableStream. It can contain the following properties:

* **highWaterMark**: The maximum internal queue size before backpressure is applied.
* **size(chunk)**: A user-defined function used to identify the size of each chunk of data.

**Real-World Implementation**

Here is a simple example of how to use a WritableStream to write data to a file:

```js
const { WritableStream } = require("stream");

const fs = require("fs");

const writableStream = new WritableStream({
  underlyingSink: {
    start: (controller) => {
      // Open the file for writing.
      const file = fs.openSync("file.txt", "w");

      // Create a WritableStreamDefaultController object.
      const controller = new WritableStreamDefaultController();

      // Set the controller's sink to the file object.
      controller.sink = file;

      // Start the stream.
      controller.start();
    },
    write: (chunk, controller) => {
      // Write the chunk to the file.
      fs.writeSync(controller.sink, chunk);

      // Signal that the write was successful.
      controller.write();
    },
    close: () => {
      // Close the file.
      fs.closeSync(controller.sink);
    },
    abort: (reason) => {
      // Abort the stream.
      controller.abort(reason);
    },
  },
});

// Write some data to the stream.
writableStream.write("Hello, world!");

// Close the stream.
writableStream.close();
```

**Potential Applications**

WritableStreams can be used in a variety of applications, including:

* Writing data to a file
* Writing data to a network socket
* Writing data to a database
* Writing data to a cloud storage service

***

**What is `writableStream.abort`?**

Imagine you have a water pipe with running water. `WritableStream` is like a way to control the flow of water in the pipe. You can write (add) water to the pipe, and the water will flow through.

`writableStream.abort` is like suddenly turning off the main valve that controls the water flow. It abruptly stops the flow of water in the pipe.

**What happens when you abort?**

When you abort the `WritableStream`, it cancels any pending writes. These are like little packets of water that are waiting to be added to the pipe. The promises associated with these writes will be rejected, meaning they will fail.

**Code example:**

```javascript
const writableStream = createWritableStream();

// Add some data to the stream
writableStream.write("Hello");
writableStream.write("World");

// Abort the stream, canceling any further writes
writableStream.abort();
```

**Real-world applications:**

Aborting a `WritableStream` can be useful in situations where you need to stop the flow of data suddenly. For example, you might want to abort the stream if the user cancels a file upload or if you encounter an error while writing to the stream.

**Simplified explanation:**

Think of it like this: You're writing a story on a computer. `WritableStream` is like the pen you're using to write. `writableStream.abort` is like suddenly taking the pen away from yourself and throwing it away. It stops you from writing any more of the story.

***

### `writableStream.close()`

**Simplified Explanation:**

When you have finished writing data to a stream, you use `close` to tell the stream that you're done, and it can wrap up any remaining work.

**Detailed Explanation:**

A stream is like a water pipe that you can use to send data from one place to another. Writing data to a stream is like pouring water into the pipe. When you have poured all the water you want, you close the pipe to stop the flow.

Closing a `WritableStream` is the same idea. It tells the stream that you're done sending data, and it can finish any processing or cleanup it needs to do. The stream will then return a promise that will be resolved when the closing process is complete.

**Code Snippet:**

```
const {WritableStream} = require('stream');

const stream = new WritableStream();

stream.write('Hello, world!');

stream.close()
  .then(() => {
    console.log('Stream closed successfully.');
  });
```

**Real-World Example:**

Here's a real-world example of how you might use `writableStream.close()`:

* You have a program that reads data from a file and writes it to a database. When the program has finished reading the file, it closes the stream to the database to save any remaining changes.
* You have a program that generates a CSV file from a database. When the program has finished generating the file, it closes the stream to the CSV file to save the data.

**Potential Applications:**

* Closing streams is important for ensuring that data is properly processed and saved.
* Closing streams helps prevent resource leaks, which can slow down your program or even crash it.

***

**`writableStream.getWriter()`**

This method creates and returns a new **writer** that can be used to write data into a **writable stream**. A **writer** is an object that represents a writable endpoint in a **stream**. It provides methods to write data to the stream and to close the stream when finished.

**Creating a Writer:**

```
const writableStream = getWritableStream();
const writer = writableStream.getWriter();
```

**Writing Data to the Stream:**

The `writer` object provides two methods to write data to the stream:

* `write(data)`: Writes the specified `data` to the stream.
* `write(data, options)`: Writes the specified `data` to the stream with the given `options`. The `options` parameter can be used to specify additional options, such as whether to write the data as a string or an array buffer.

**Closing the Stream:**

When finished writing data to the stream, it's important to close the stream to release any resources associated with it. This can be done by calling the `close()` method on the `writer` object:

```
writer.close().then(() => {
  // Stream closed successfully
});
```

**Real-World Example:**

A common use case for `writableStream.getWriter()` is to create a **file writer** for writing data to a file. Here's an example:

```javascript
import { createWriteStream } from "fs";

const writableStream = createWriteStream("output.txt");
const writer = writableStream.getWriter();

// Write some data to the file
writer.write("Hello, world!\n");

// Close the stream
writer.close().then(() => {
  console.log("File written successfully");
});
```

In this example, we create a **writable stream** for writing to the `output.txt` file, then create a **writer** for the stream. We use the `write()` method to write some data to the file, then close the stream when finished.

***

**`writableStream.locked` Property**

**Plain English Explanation:**

The `writableStream.locked` property tells us if there is someone currently writing (or sending) data to this stream. It acts like a "door lock" to make sure that only one person can write at a time.

**Detailed Explanation:**

* When `writableStream.locked` is `false`, the stream is open for writing.
* When `writableStream.locked` is `true`, the stream is locked because someone is currently writing to it. No one else can write until the current writer is done.

**Code Snippet:**

```javascript
const stream = new WritableStream();

if (stream.locked) {
  // The stream is currently locked, so we can't write to it.
} else {
  // The stream is open for writing, so we can write to it.
}
```

**Real-World Example:**

Imagine you have a group of friends who want to write a story together. You create a shared document and decide that only one person can write at a time to avoid confusion. You use a "lock" system to ensure that only one person has access to the document at a time.

The `writableStream.locked` property works in a similar way. It helps ensure that only one person can write to the stream (send data) at a time to prevent data corruption.

**Potential Applications:**

* **Preventing data corruption:** Locking the stream prevents multiple writers from sending data at the same time, reducing the risk of errors and data loss.
* **Orderly data transfer:** Ensuring that only one writer can access the stream at a time helps maintain the order of data being sent, making it easier to process and understand.
* **Resource management:** Limiting the number of simultaneous writers can help manage system resources, such as memory and processing power, by preventing overload.

***

**Transferring data with postMessage()**

**Explanation:** postMessage() is a function that allows you to send messages and data between different windows or workers in a web application. In this case, we use it to transfer a WritableStream instance from one window to another.

**Simplified Explanation:** Imagine you have two boxes, one in each window. You can send the contents of one box to the other box using a message. In this example, we're sending a WritableStream, which is like a sink where you can pour data into.

**Code Snippet:**

```js
// Create a new WritableStream and a MessageChannel
const writableStream = new WritableStream();
const { port1, port2 } = new MessageChannel();

// Listen for messages on port1
port1.addEventListener("message", ({ data }) => {
  // Get the writer for the WritableStream and write some data to it
  data.getWriter().write("hello");
});

// Post the WritableStream to port2
port2.postMessage(writableStream, [writableStream]);
```

**Real-World Application:**

One potential application is to send data from a background task to a foreground window. For example, you could have a long-running computation in a web worker, and when it's finished, you could send the result to the main window for display.

***

#### Class: `WritableStreamDefaultWriter`

**Introduction:**

`WritableStreamDefaultWriter` is a built-in class in Node.js's `webcrypto` module that allows you to write data to a `WritableStream` object. It provides a convenient way to write chunks of data to a stream and manage the underlying buffer.

**Simplified Explanation:**

A `WritableStreamDefaultWriter` is like a mailman who delivers letters to a mailbox (`WritableStream`). The mailman (writer) takes a letter (data), writes it on paper (into the buffer), and places it in the mailbox. This process continues until the mailman delivers all the letters or the mailbox is full.

**Explanation of Topics:**

**1. Writing Data:**

* **write(data):** Writes a chunk of data to the buffer. The data can be anything, such as a string, a buffer, or an array.
* **writeThrough(data):** Writes data directly to the stream without buffering it. This is useful when you need to send data immediately.

**2. Managing the Buffer:**

* **getBufferedAmount():** Returns the amount of data currently buffered.
* **releaseLock():** Releases the lock on the buffer after all data has been written.
* **close():** Closes the writer and flushes any remaining data to the stream.

**Real-World Application:**

`WritableStreamDefaultWriter` is useful in any situation where you need to write data to a stream. For example:

* Streaming large files to a server
* Writing to a database log file
* Sending data between multiple clients and servers

**Example Code:**

```javascript
// Create a WritableStream and its Writer
const stream = new WritableStream();
const writer = stream.getWriter();

// Write some data to the stream
writer.write("Hello, world!\n");
writer.write("This is a test.\n");

// Release the buffer lock
writer.releaseLock();

// Close the writer
writer.close();
```

This code creates a `WritableStream` and its `Writer`. Then, it writes some data to the stream and releases the buffer lock. Finally, it closes the writer.

***

**Simplified Explanation:**

Imagine you have a water faucet (`WritableStream`) and a cup (`WritableStreamDefaultWriter`). The `new WritableStreamDefaultWriter(stream)` method connects the cup to the faucet, allowing you to fill it with water.

**Detailed Explanation:**

* **WritableStream:** A "pipe" that allows data to be written to it.
* **WritableStreamDefaultWriter:** A "faucet" that allows you to write data to a `WritableStream`.
* **Locking:** Once created, the writer is "locked" to the stream, meaning it can't be used with any other streams.
* **Purpose:** The writer provides a convenient way to write data to the stream.

**Code Snippet:**

```javascript
const writableStream = new WritableStream();
const writer = new WritableStreamDefaultWriter(writableStream);

writer.write("Hello, world!"); // Write data to the stream
```

**Real-World Examples:**

* **Logging:** Using a `WritableStream` and writer to log data to a file.
* **Data Transfer:** Transferring data between two processes using `WritableStream`s and writers.
* **Data Serialization:** Converting data into a format that can be written to a `WritableStream`.

**Potential Applications:**

* **Real-time data streaming:** Sending data from a server to a client as it is generated.
* **File writing:** Writing data to a file or database.
* **Data analysis:** Processing and analyzing large datasets in real time.

***

**Topic: `writableStreamDefaultWriter.abort()` method**

**Simplified Explanation:**

Imagine you have a conveyor belt with items to be assembled. The `writableStreamDefaultWriter.abort()` method is like hitting an emergency stop button. It abruptly stops the conveyor belt, canceling all remaining items in line.

**Detailed Explanation:**

The `writableStreamDefaultWriter.abort()` method is used to terminate a `WritableStream` prematurely. This means that all pending write operations (the items on the conveyor belt) are canceled.

The `reason` parameter is an optional argument that can be provided to specify why the stream was aborted. This information can be useful for debugging purposes.

**Usage:**

```javascript
const writableStream = new WritableStream();
const writer = writableStream.getWriter();

// Queue some writes
writer.write("Hello");
writer.write("World");

// Abort the stream
writer.abort(new Error("Oops!"));
```

**Real-World Applications:**

* **Error handling:** If an error occurs during a write operation, the stream can be aborted to prevent further data loss.
* **Resource cleanup:** When a stream is no longer needed, it can be aborted to release any associated resources.
* **Canceling pending writes:** If the data being written is no longer relevant or needs to be replaced, the stream can be aborted to cancel the pending writes.

**Improved Code Snippet:**

The following code snippet demonstrates how to handle errors that occur during writing to a stream:

```javascript
const writableStream = new WritableStream();
const writer = writableStream.getWriter();

writer.write("Hello");
writer.write("World");

writer.onwrite = () => {
  // Write successful
};

writer.onwriteerror = (error) => {
  // Handle error and abort stream
  writer.abort(error);
};
```

***

**`WritableStreamDefaultWriter.close()` method in `webcrypto`**

The `writableStreamDefaultWriter.close()` method in `webcrypto` closes the `WritableStream` when no additional writes are expected.

**Syntax:**

```typescript
close(): Promise<undefined>;
```

**Return value:**

A promise fulfilled with `undefined`.

**Potential Applications:**

Closing the `WritableStream` when no further writes are required helps to clean up resources and ensure that the stream is properly terminated. This can be useful in scenarios where the stream is used for tasks such as writing data to a file or sending data over a network connection.

**Real-World Code Implementation:**

```typescript
const writableStream = new WritableStream();
const writer = writableStream.getWriter();

writer.write("Hello, world!");

// Close the stream when finished writing.
writer.close();
```

***

**WritableStreamDefaultWriter.closed**

The `WritableStreamDefaultWriter.closed` property is a Promise that is fulfilled with `undefined` when the associated {WritableStream} is closed. If the stream errors or the writer's lock is released before the stream finishes closing, the Promise is rejected.

**Example:**

```javascript
const stream = new WritableStream();
const writer = stream.getWriter();

writer.write("Hello");
writer.close();

writer.closed.then(() => {
  // The stream has been closed.
});
```

**Real-world applications:**

* Ensuring that data is not written to a stream after it has been closed.
* Waiting for a stream to close before releasing resources.
* Coordinating the closing of multiple streams.

***

**WritableStreamDefaultWriter.desiredSize**

**Type**: Number

**What is it?**

The `desiredSize` property on the `WritableStreamDefaultWriter` object represents the amount of data that the writer would like the stream to hold in its queue before becoming paused.

**How does it work?**

If the stream's queue is already at or above the `desiredSize`, the writer will be paused and all write operations will be suspended until the queue size drops below the `desiredSize`.

**Why use it?**

The `desiredSize` property can be used to control the flow of data through the stream. By setting a low `desiredSize`, the writer can prevent the stream from becoming overwhelmed with data and potentially causing a buffer overflow. By setting a high `desiredSize`, the writer can allow the stream to hold more data before becoming paused, which can improve performance in some cases.

**Code Example**

```javascript
const { WritableStream } = require("stream");
const writer = new WritableStream();

// Set the desired size to 1024 bytes
writer.desiredSize = 1024;

// Write some data to the stream
writer.write("Hello world");

// Wait for the stream to become paused
await writer.getWriter().close();

// Check the queue size
console.log(writer.writableEnded); // true
```

**Real World Applications**

The `desiredSize` property can be used in a variety of applications, including:

* Controlling the flow of data in a streaming pipeline
* Preventing buffer overflows
* Improving performance in certain scenarios

***

### `writableStreamDefaultWriter.ready`

This is a promise that is resolved when the writer is ready to be used.

#### Example

```typescript
const stream = new TransformStream();
const writer = stream.writable.getWriter();

writer.ready.then(() => {
  // The writer is now ready to be used.
  writer.write("Hello world!");
});
```

***

#### `writableStreamDefaultWriter.releaseLock()`

Imagine you're playing with two toys: a ball and a box.

* The ball represents data that you want to write to a place called a "stream."
* The box represents a "lock" on the stream that prevents other people from writing to it at the same time.

When you want to write data to the stream, you first need to lock it using the `lock()` method. This makes sure that you're the only one writing to the stream and prevents any conflicts with other writers.

Once you're done writing, you need to unlock the stream using the `releaseLock()` method. This allows other writers to lock and write to the stream again.

Think of it like a traffic light: when you lock the stream, it turns red, stopping other writers. When you release the lock, it turns green, allowing others to proceed.

**Real-world example:**

Imagine you're sending a series of messages over a network connection. Each message is written to a stream. To ensure that the messages are sent in order and without errors, you need to lock the stream before writing each message and release the lock afterwards. This prevents other messages from being sent while you're working on the current one.

***

**Topic: `writableStreamDefaultWriter.write([chunk])`**

**Summary:**

This method is used to add a chunk of data to a writable stream.

**Explanation:**

Think of a writable stream as a pipe that you can send data through. The `write()` method is like dropping a chunk of data into the pipe. The data will then flow through the pipe and be written to its destination.

**Code Example:**

```javascript
const writableStream = new WritableStream();
const writer = writableStream.getWriter();

writer.write(new Uint8Array([1, 2, 3]));
```

In this example, we create a writable stream and get its writer. Then, we use the writer to write an array of bytes (in this case, the numbers 1, 2, and 3) to the stream.

**Real-World Applications:**

* **Logging:** You can use a writable stream to log data to a file or console.
* **Data storage:** You can use a writable stream to store data in a database or file system.
* **Communication:** You can use a writable stream to send data over a network or to a remote server.

***

**What is `WritableStreamDefaultController`?**

It's like the manager of how a stream of data gets written out.

**What does it do?**

* Keeps track of how much data is ready to be written.
* Decides when to stop writing more data (called "backpressure").
* Signals when the stream has no more data to write (called "end of stream").

**Real-World Example:**

Imagine you have a pipe that you're trying to fill with water. The `WritableStreamDefaultController` would manage the rate at which water is flowing into the pipe, making sure it doesn't get too full or too empty.

**Code Example:**

```javascript
// Create a WritableStream
const stream = new WritableStream();

// Get the default controller
const controller = stream.getWriter();

// Write some data to the stream
controller.write("Hello");

// Close the stream (end of stream)
controller.close();
```

**Potential Applications:**

* Sending data over a network.
* Writing data to a file.
* Splitting a large dataset into smaller chunks.
* Throttling the rate at which data is processed.

***

**WritableStreamDefaultController.error()**

Just like your internet connection, a `WritableStream` is used to send data, and can encounter errors. When `error()` is called on a `WritableStreamDefaultController`, it means that an error has occurred and the stream will be stopped. Any data that was waiting to be sent will be canceled.

**Example:**

```javascript
const writableStream = new WritableStream();

writableStream.write("Hello");

// If an error occurs, call error() to stop the stream.
writableStream.error(new Error("Something went wrong!"));

// Any further writes will be canceled.
writableStream.write("Goodbye");
```

**Simplified Explanation:**

Imagine you're writing a letter to a friend. But then, you accidentally spill coffee on the letter. You call `error()` to say that there's a problem and stop writing the letter. Any other writing you try to do will be canceled, because the letter is already messed up.

**Real-World Applications:**

Writable streams are useful in many applications, such as:

* Logging error messages to a file.
* Sending data to a server over a network.
* Compressing data and sending it to a storage device.

**Potential Applications:**

* Tracking errors in a logging system.
* Ensuring that data is sent to a server reliably, even if there are temporary network issues.
* Compressing large files to save storage space.

***

#### `writableStreamDefaultController.signal`

The `writableStreamDefaultController.signal` property of the WritableStreamDefaultController interface is an `AbortSignal` that can be used to cancel pending write or close operations when a WritableStream is aborted.

If the `abort()` method is called on the WritableStream, the `signal` property will be set to an `AbortSignal` object that is in the "aborted" state. Any pending write or close operations will be canceled and will reject with an `AbortError`.

You can use the `signal` property to cancel pending write or close operations if you know that the WritableStream will no longer be used. For example, if you are writing to a file and the user closes the file, you can call `abort()` on the WritableStream to cancel any pending write operations.

Here is an example of how to use the `signal` property:

```
const writableStream = new WritableStream();
const controller = writableStream.getWriter();

controller.write('Hello');
controller.write('World');

writableStream.abort();

controller.write('This will not be written').catch((error) => {
  console.log(error); // AbortError
});
```

In this example, the `writableStream` is aborted after the first two write operations. The third write operation is canceled and will reject with an `AbortError`.

#### Real-world applications

The `writableStreamDefaultController.signal` property can be used in any situation where you need to cancel pending write or close operations. For example, you can use it to:

* Cancel pending write operations if the user closes a file.
* Cancel pending close operations if the user navigates away from a page.
* Cancel pending write or close operations if the network connection is lost.

#### Potential applications

The `writableStreamDefaultController.signal` property has a variety of potential applications, including:

* **Web applications:** You can use the `signal` property to cancel pending write or close operations when the user closes a tab or navigates away from a page.
* **Desktop applications:** You can use the `signal` property to cancel pending write or close operations when the user closes a file or window.
* **Command-line tools:** You can use the `signal` property to cancel pending write or close operations when the user presses Ctrl+C.

***

**TransformStream**

Imagine you have a magic box with two pipes, one for putting things in and one for taking things out. Inside the box, there's a magician who can change the things that go in before they come out. That's what a `TransformStream` is!

**WritableStream** (Input Pipe): You write things to the input pipe, like letters.

**ReadableStream** (Output Pipe): You read transformed things from the output pipe.

**Transform** (Magician): In between the pipes, there's a magical function called "transform." This function can look at the letters you write and change them however it likes. For example, it could make them all uppercase or even replace them with emojis!

**Real-World Example:**

Imagine you have a website that lets users enter their names. You want to display their names on the screen, but only the first letter should be capitalized. You can use a `TransformStream`!

**Code:**

```javascript
const transform = new TransformStream({
  transform(chunk, controller) {
    controller.enqueue(chunk.toUpperCase());
  },
});

// Write the user's name to the input pipe
transform.writable.getWriter().write("john");

// Read the transformed name from the output pipe
transform.readable
  .getReader()
  .read()
  .then((result) => {
    console.log(result.value); // Output: "JOHN"
  });
```

**Applications:**

* Capitalizing text
* Filtering out unwanted characters
* Encrypting or decrypting data
* Compressing or decompressing files

***

**Simplified Explanation of TransformStream**

A TransformStream is a type of Duplex stream that allows you to transform data as it flows from the input to the output. It's like a conveyor belt where you can modify each item before it reaches the end.

**Constructor**

To create a TransformStream, you use the `new TransformStream()` constructor:

```
const transformStream = new TransformStream({
  // ... (see below)
});
```

**Transformer**

The transformer object is where you define the functions that will be called to modify the data. It has three functions:

* `start`: Called when the TransformStream is created. Use it to initialize variables or perform any one-time setup.
* `transform`: Called for each chunk of data that enters the stream. You can modify the chunk or create a new one to be forwarded to the output.
* `flush`: Called when the input stream ends. Use it to ensure that all pending data is flushed to the output.

**Writable and Readable Strategies**

These objects control the behavior of the writable (input) and readable (output) sides of the stream. They have two properties:

* `highWaterMark`: The maximum amount of data that can be buffered before backpressure is applied, which slows down the input.
* `size`: A function that calculates the size of each chunk of data. It's used to determine when the high water mark has been reached.

**Example**

Here's a simple example of a TransformStream that converts all lowercase letters in the input to uppercase:

```
const converter = new TransformStream({
  start(controller) {
    // ...
  },

  transform(chunk, controller) {
    const newChunk = chunk.toString().toUpperCase();
    controller.enqueue(newChunk);
  },

  flush(controller) {
    // ...
  }
});

const readable = converter.readable;
const writable = converter.writable;

// Write some lowercase text to the input
writable.write('hello world');
writable.end('goodbye');

// Read the transformed uppercase text from the output
readable.on('data', (chunk) => {
  console.log(chunk.toString()); // HELLO WORLD, GOODBYE
});
```

**Applications**

TransformStreams can be used in a variety of applications, including:

* Data compression and decompression
* Encryption and decryption
* Parsing and reformatting data
* Creating custom filters or transformations

***

#### **`transformStream.readable`**

* **Type:** `ReadableStream`

**Explanation:**

* `transformStream.readable` is a property of a `TransformStream` object.
* It represents the readable side of the stream, which allows data to be read from the stream.
* The `transformStream.readable` property is a `ReadableStream` object.
* You can use this object to read data from the stream.
* The `transformStream.readable` property is useful for reading data from a stream that is being transformed by a `TransformStream`.

**Real-World Example:** Here is an example of how to use the `transformStream.readable` property to read data from a stream:

```
const { Transform } = require('stream');

const transformStream = new Transform();

transformStream.readable.on('data', (chunk) => {
  // Do something with the data.
});

transformStream.write('Hello');
transformStream.end();
```

In this example, the `transformStream` object is created and the `readable` property is accessed.

The `data` event is listened for on the `readable` property.

When data is available to be read from the stream, the `data` event is emitted and the `chunk` parameter is passed to the event handler.

In the event handler, you can do something with the data, such as log it to the console or send it to another stream.

Once you have finished writing data to the `transformStream`, you can call the `end()` method to indicate that there is no more data to be written.

**Potential Applications:** The `transformStream.readable` property can be used in a variety of applications, including:

* Filtering data: You can use a `TransformStream` to filter data from a stream. For example, you could use a `TransformStream` to remove duplicate data from a stream.
* Transforming data: You can use a `TransformStream` to transform data from a stream. For example, you could use a `TransformStream` to convert data from one format to another.
* Aggregating data: You can use a `TransformStream` to aggregate data from a stream. For example, you could use a `TransformStream` to calculate the sum of the numbers in a stream.

***

**`transformStream.writable`**

* Type: `WritableStream`

**Explanation:**

`transformStream.writable` is a writable stream that allows you to transform data as it is written to the stream. A writable stream is a destination for data, meaning you can write data to it. Transforming the data means modifying or changing the data in some way.

**Real-World Example:**

Let's say you have a stream of data that contains comma-separated values (CSV). You want to transform the CSV data into JSON objects. You can use a transform stream to do this by providing a transform function that reads from the input stream and writes JSON objects to the output stream.

**Code Example:**

```
const { Transform } = require('stream');

// Create a transform stream
const csvToJsonStream = new Transform({
  // Transform function is called for each chunk of data written to the stream
  transform(chunk, encoding, callback) {
    // Convert the CSV chunk to a JSON object
    const jsonObject = parseCsvToJson(chunk.toString());

    // Write the JSON object to the output stream
    this.push(jsonObject);

    // Call the callback to indicate completion of transformation
    callback();
  }
});

// Create a readable stream of CSV data
const csvData = 'name,age\nJohn,30\nJane,25';
const csvStream = new Readable({
  read() {
    this.push(csvData);
    this.push(null); // Signal end of stream
  }
});

// Pipe the CSV data through the transform stream
csvStream.pipe(csvToJsonStream).pipe(process.stdout);
```

**Output:**

```
{ name: 'John', age: '30' }
{ name: 'Jane', age: '25' }
```

**Potential Applications:**

* Data filtering: Filtering data based on specific criteria.
* Data encryption/decryption: Transforming data to protect it from unauthorized access.
* Data compression/decompression: Reducing the size of data for more efficient storage or transmission.
* Data conversion: Converting data from one format to another, such as CSV to JSON or XML to HTML.

***

### Transferring with postMessage()

**Simplified Explanation:**

Imagine you want to send a special package (called a `TransformStream`) to a friend who's in another house. But you can't just walk over and hand it to them because there's a huge wall between you!

To get around this, you use a secret passageway called a `MessagePort`. It's like a magical tunnel that allows you to send packages between the two houses without even leaving yours.

You set up two entrances to the tunnel, one in your house and one in your friend's house. Then, you open up the entrance in your house and carefully prepare your package.

Once the package is ready, you "post" it (send it) through the tunnel to your friend. But wait! You also include the secret entrance code so your friend knows how to open the package once it arrives.

Your friend receives the package and uses the secret entrance code to unlock it. They now have the `TransformStream` that you sent them!

**Code Example:**

```
const stream = new TransformStream(); // Create the TransformStream

// Create a secret tunnel with two entrances
const { port1, port2 } = new MessageChannel();

// Create a listener that will handle the incoming package
port1.onmessage = ({ data }) => {
  // Extract the `TransformStream` from the package
  const { writable, readable } = data;

  // Do something with the TransformStream...
};

// Send the TransformStream through the tunnel to your friend
port2.postMessage(stream, [stream]);
```

**Real-World Applications:**

* Sending data between web workers or iframes that run in different processes.
* Transferring large objects or streams without copying them in memory.
* Implementing communication protocols that involve streaming data.

***

### Class: `TransformStreamDefaultController`

The `TransformStreamDefaultController` class is the default controller for the `TransformStream`. It provides methods to control the flow of data through the transform stream, and to get and set the transform stream's state.

#### Methods

**`controller.enqueue(chunk)`**

Enqueues a chunk of data to be transformed by the transform stream. The chunk can be any type of data, and it will be passed to the transform stream's `transform()` method.

```javascript
const transformStream = new TransformStream();
const controller = transformStream.writable.getWriter();

controller.enqueue("Hello");
controller.enqueue("world");
controller.enqueue("!");
```

**`controller.close()`**

Closes the writable side of the transform stream. This will cause the transform stream to finish its current task and then close the readable side of the stream.

```javascript
const transformStream = new TransformStream();
const controller = transformStream.writable.getWriter();

controller.close();
```

**`controller.error(error)`**

Errors the transform stream with the given error. This will cause the transform stream to close the readable side of the stream and reject any pending promises with the error.

```javascript
const transformStream = new TransformStream();
const controller = transformStream.writable.getWriter();

controller.error(new Error("An error occurred"));
```

**`controller.desiredSize`**

Gets the desired size of the writable side of the transform stream. This is the amount of data that the transform stream is willing to accept at any given time.

```javascript
const transformStream = new TransformStream();
const controller = transformStream.writable.getWriter();

console.log(controller.desiredSize); // 0
```

**`controller.terminate()`**

Terminates the transform stream. This will cause the transform stream to close both the readable and writable sides of the stream.

```javascript
const transformStream = new TransformStream();
const controller = transformStream.writable.getWriter();

controller.terminate();
```

#### Properties

**`controller.state`**

Gets the state of the transform stream. The state can be one of the following values:

* `"readable"`: The transform stream is readable.
* `"writable"`: The transform stream is writable.
* `"closed"`: The transform stream is closed.
* `"errored"`: The transform stream has errored.

```javascript
const transformStream = new TransformStream();
const controller = transformStream.writable.getWriter();

console.log(controller.state); // "writable"
```

#### Real-World Applications

Transform streams can be used for a variety of real-world applications, including:

* **Data transformation**: Transform streams can be used to convert data from one format to another. For example, a transform stream could be used to convert a CSV file to a JSON file.
* **Data validation**: Transform streams can be used to validate data before it is passed on to the next stage in a pipeline. For example, a transform stream could be used to check that all of the data in a CSV file has the correct format.
* **Data filtering**: Transform streams can be used to filter out unwanted data from a stream. For example, a transform stream could be used to filter out all of the even numbers from a stream of integers.

***

### `transformStreamDefaultController.desiredSize`

This property represents the number of bytes that the readable side of the stream is willing to accept. If the value is less than 0, the readable side is currently not interested in reading any data. If the value is 0, the readable side is currently full and can't accept any more data. If the value is greater than 0, the readable side is ready to accept that number of bytes.

This property is useful for controlling the flow of data through a stream. For example, a stream that is reading data from a disk file might use this property to limit the number of bytes that are read from the file at any one time. This can help to prevent the stream from overloading the computer's memory.

Here is an example of how to use the `transformStreamDefaultController.desiredSize` property:

```
const { Transform } = require('stream');

const transformStream = new Transform({
  transform(chunk, encoding, callback) {
    // The readable side of the stream is willing to accept 1000 bytes.
    this.desiredSize = 1000;

    // Transform the chunk of data.
    const transformedChunk = chunk.toString().toUpperCase();

    // Write the transformed chunk to the writable side of the stream.
    this.push(transformedChunk);

    // Tell the stream that we're done transforming the chunk.
    callback();
  }
});

// Create a readable stream.
const readableStream = require('fs').createReadStream('file.txt');

// Pipe the readable stream through the transform stream.
readableStream.pipe(transformStream);

// Listen for data events on the writable side of the transform stream.
transformStream.on('data', (chunk) => {
  // Do something with the transformed data.
});
```

In this example, the `transformStream` transforms the data from the readable stream by converting it to uppercase. The `transformStreamDefaultController.desiredSize` property is set to 1000, which means that the readable side of the stream is willing to accept 1000 bytes at a time. This helps to prevent the stream from overloading the computer's memory.

### Applications in the real world

* **Data compression:** A data compression stream can use the `transformStreamDefaultController.desiredSize` property to control the amount of data that is compressed at one time. This can help to improve the performance of the compression algorithm.
* **Data encryption:** A data encryption stream can use the `transformStreamDefaultController.desiredSize` property to control the amount of data that is encrypted at one time. This can help to improve the security of the encryption algorithm.
* **Data filtering:** A data filtering stream can use the `transformStreamDefaultController.desiredSize` property to control the amount of data that is filtered at one time. This can help to improve the performance of the filtering algorithm.

***

### `transformStreamDefaultController.enqueue([chunk])`

* `chunk` {any}

Appends a chunk of data to the readable side's queue.

In order to push data to a TransformStream, we use the `enqueue()` method. This method takes a chunk of data as an argument and appends it to the readable side's queue.

```javascript
// Create a TransformStream.
const transformStream = new TransformStream();

// Get the controller for the TransformStream.
const controller = transformStream.writable.getWriter();

// Enqueue a chunk of data.
controller.enqueue("Hello, world!");

// Close the readable side of the TransformStream.
controller.close();
```

In this example, we create a TransformStream and then get the controller for the TransformStream. We then use the `enqueue()` method to append a chunk of data to the readable side's queue. Finally, we close the readable side of the TransformStream.

The `enqueue()` method is a very important method for working with TransformStreams. It allows us to push data to the readable side of the TransformStream, which can then be processed by the transform function.

***

**Error Handling in Transform Streams**

**Simplified Explanation:**

When processing data in a transform stream, if an error occurs, the stream needs to be closed abruptly. To do this, you can use `transformStreamDefaultController.error()`.

**Code Example:**

```
const { Transform, Writable } = require('stream');

const transformStream = new Transform();
const writableStream = new Writable();

transformStream.on('error', (error) => {
  console.error(error);
  writableStream.end();
});

transformStream.on('data', (data) => {
  try {
    // Do something with the data
  } catch (error) {
    transformStream.error(error);
  }
});

writableStream.on('finish', () => {
  console.log('All data processed.');
});

transformStream.pipe(writableStream);
```

**Real-World Example:**

Imagine you have a stream that is processing financial data. If an error occurs while parsing the data, such as an invalid number format, you would want to abruptly close the stream to prevent further processing and potential data corruption.

**Potential Applications:**

* Error handling in data processing pipelines
* Validating data before further transformation
* Closing streams to prevent data loss or corruption

***

**Abruptly Terminating a Transform Stream**

Imagine a pipe where water flows through. A `transformStreamDefaultController.terminate()` method acts like a valve that suddenly closes at one end of the pipe, abruptly stopping the flow of water.

**How it Works**

When you call this method:

1. Writing to the stream becomes impossible. The writable side is closed.
2. Reading from the stream will eventually return an error, indicating that the stream has been unexpectedly shut down. The readable side is closed.

**Potential Applications**

This method can be useful in situations where you need to:

* **Handle errors:** If an unexpected error occurs in the middle of a data transformation, you can terminate the stream to prevent further data processing and protect your application from potential data inconsistencies or security vulnerabilities.
* **Control stream lifecycle:** In certain scenarios, you may want to manually end the data flow early, such as in an event-driven system where specific conditions trigger a stream termination.

**Real-World Example**

Here's a simplified example of using `transformStreamDefaultController.terminate()`:

```javascript
const { Transform, PassThrough } = require('stream');

const transformStream = new Transform();
transformStream.on('data', chunk => /* Process chunk */);

// Create a stream that produces data
const dataStream = new PassThrough();

// Pipe the data stream through the transform stream
dataStream.pipe(transformStream);

// Suddenly decide to terminate the stream
setTimeout(() => {
  transformStream.terminate();
}, 1000);
```

In this example, `dataStream` sends data to `transformStream`. After one second, `transformStream` is unexpectedly terminated, which will cause errors in any subsequent attempts to write or read data from the stream.

**Additional Tips**

* If you have a stream chain, terminating one stream will close all downstream streams as well.
* You can also specify an error message when terminating the stream, which will be passed on to any consumers of the readable side.
* Be careful using `terminate()` as it can lead to data loss or unexpected behavior if not handled properly.

***

### ByteLengthQueuingStrategy

The `ByteLengthQueuingStrategy` is a queuing strategy that limits the number of bytes in the queue.

#### Properties

* `highWaterMark` - The maximum number of bytes that can be in the queue.
* `size` - The number of bytes currently in the queue.

#### Methods

* `enqueue()` - Adds a new byte to the queue.
* `dequeue()` - Removes a byte from the queue.
* `clear()` - Removes all bytes from the queue.

#### Example

The following example creates a new `ByteLengthQueuingStrategy` with a high water mark of 10 bytes:

```javascript
const strategy = new ByteLengthQueuingStrategy({ highWaterMark: 10 });
```

The following example adds a new byte to the queue:

```javascript
strategy.enqueue(0x42);
```

The following example removes a byte from the queue:

```javascript
strategy.dequeue();
```

The following example clears the queue:

```javascript
strategy.clear();
```

#### Real-World Applications

The `ByteLengthQueuingStrategy` can be used to limit the size of a queue in bytes. This can be useful in applications where memory is limited, or where there is a need to control the flow of data.

For example, the `ByteLengthQueuingStrategy` could be used to limit the size of a queue of network packets. This would help to prevent the network from being overwhelmed with traffic.

Another potential application for the `ByteLengthQueuingStrategy` is to limit the size of a queue of audio data. This would help to prevent the audio from becoming choppy or distorted.

#### Improved Code Snippets

The following code snippet provides a complete implementation of a `ByteLengthQueuingStrategy`:

```javascript
class ByteLengthQueuingStrategy {
  constructor({ highWaterMark }) {
    this.highWaterMark = highWaterMark;
    this.size = 0;
    this.queue = [];
  }

  enqueue(byte) {
    if (this.size + byte.length > this.highWaterMark) {
      throw new Error("Queue is full.");
    }

    this.queue.push(byte);
    this.size += byte.length;
  }

  dequeue() {
    if (this.size === 0) {
      throw new Error("Queue is empty.");
    }

    const byte = this.queue.shift();
    this.size -= byte.length;

    return byte;
  }

  clear() {
    this.queue = [];
    this.size = 0;
  }
}
```

The following code snippet shows how to use the `ByteLengthQueuingStrategy` to limit the size of a queue of network packets:

```javascript
const strategy = new ByteLengthQueuingStrategy({ highWaterMark: 1000 });

const socket = new WebSocket("ws://example.com");

socket.addEventListener("message", (event) => {
  strategy.enqueue(event.data);
});

socket.addEventListener("open", () => {
  // Start sending packets.
});
```

The following code snippet shows how to use the `ByteLengthQueuingStrategy` to limit the size of a queue of audio data:

```javascript
const strategy = new ByteLengthQueuingStrategy({ highWaterMark: 10000 });

const audioContext = new AudioContext();

const audioBuffer = audioContext.createBuffer(1, 44100, 44100);

const audioSource = audioContext.createBufferSource();

audioSource.buffer = audioBuffer;

audioSource.connect(audioContext.destination);

audioSource.addEventListener("ended", () => {
  // Stop playing audio.
});

audioSource.addEventListener("error", (event) => {
  // Handle error.
});

audioSource.start();

const buffer = new ArrayBuffer(1000);

const dataView = new DataView(buffer);

for (let i = 0; i < 1000; i++) {
  dataView.setInt16(i, Math.random() * 32767);
}

strategy.enqueue(buffer);
```

***

#### `new ByteLengthQueuingStrategy(init)`

The `ByteLengthQueuingStrategy` class in `webcrypto` represents a byte length queuing strategy for controlling the flow of data to and from the `ReadableStream` and `WritableStream` objects.

**Constructor**

The `ByteLengthQueuingStrategy` constructor takes one argument:

* `init` {Object}: An object with the following properties:
  * `highWaterMark` {number}: The high water mark for the strategy. Once the underlying byte length data reaches this value, the strategy will pause the `ReadableStream` or `WritableStream` object.

**Methods**

The `ByteLengthQueuingStrategy` class has the following methods:

* `size()` {number}: Returns the current byte length of the strategy.

**Example**

The following code creates a `ByteLengthQueuingStrategy` object with a high water mark of 1024 bytes:

```js
const strategy = new ByteLengthQueuingStrategy({
  highWaterMark: 1024,
});
```

This strategy can then be used to create a `ReadableStream` or `WritableStream` object:

```js
const readableStream = new ReadableStream({
  strategy,
});

const writableStream = new WritableStream({
  strategy,
});
```

When the byte length of the `ReadableStream` reaches 1024 bytes, the stream will be paused. Similarly, when the byte length of the `WritableStream` reaches 1024 bytes, the stream will be paused.

**Real-World Applications**

The `ByteLengthQueuingStrategy` class can be used to control the flow of data in a variety of applications, such as:

* Streaming media: The strategy can be used to ensure that the media player does not buffer too much data at once, which can lead to stuttering.
* File transfer: The strategy can be used to ensure that the file transfer does not overload the network, which can lead to dropped packets.
* Real-time communication: The strategy can be used to ensure that the communication channel does not become overwhelmed with data, which can lead to dropped packets or delayed messages.

***

#### `byteLengthQueuingStrategy.highWaterMark`

The `byteLengthQueuingStrategy.highWaterMark` property specifies the maximum amount of bytes that can be queued before the readable stream is paused.

Simplified explanation:

Imagine a pipe carrying water. The `highWaterMark` property tells the pipe how much water it can hold before it becomes full and stops flowing. In this case, the pipe carries bytes instead of water, and it will stop receiving new bytes when it reaches the `highWaterMark` limit.

Real-world use case:

Let's say you have a process that reads data from a stream and processes it. If the process is slow and cannot keep up with the incoming data, the stream buffer may fill up and cause the process to crash. To prevent this, you can set a `highWaterMark` value so that the stream will pause when the buffer reaches that limit, giving the process time to catch up.

Here's an example of setting the `highWaterMark` property using the Web Cryptography API:

```javascript
const stream = new ReadableStream({
  highWaterMark: 1024,
  // ...
});
```

In this example, the stream will pause when the buffer reaches 1024 bytes, giving the process time to catch up.

***

#### `byteLengthQueuingStrategy.size`

The `byteLengthQueuingStrategy.size` method returns the size, in bytes, of the provided chunk.

**Syntax**

```typescript
byteLengthQueuingStrategy.size(chunk: any): number;
```

**Parameters**

* `chunk` The chunk to measure.

**Returns**

The size of the chunk in bytes.

**Example**

```typescript
const chunk = new Uint8Array([1, 2, 3, 4, 5]);
const size = byteLengthQueuingStrategy.size(chunk);
console.log(size); // 5
```

**Real World Applications**

The `byteLengthQueuingStrategy.size` method can be used to calculate the size of a chunk of data in bytes. This information can be used to determine how much space a chunk will take up in memory, or to calculate the total size of a stream of chunks.

For example, the following code uses the `byteLengthQueuingStrategy.size` method to calculate the total size of a stream of chunks:

```typescript
const chunks = [
  new Uint8Array([1, 2, 3, 4, 5]),
  new Uint8Array([6, 7, 8, 9, 10]),
  new Uint8Array([11, 12, 13, 14, 15]),
];

const totalSize = chunks.reduce(
  (total, chunk) => total + byteLengthQueuingStrategy.size(chunk),
  0
);
console.log(totalSize); // 45
```

***

**CountQueuingStrategy**

The `CountQueuingStrategy` class is a queuing strategy that controls the number of tasks that can be queued in the task queue.

**Topics:**

1. **Creating a `CountQueuingStrategy`**

```
const {CountQueuingStrategy} = require('queue-microtask');

// Create a queuing strategy that allows a maximum of 10 tasks to be queued.
const strategy = new CountQueuingStrategy({highWaterMark: 10});
```

2. **Queuing Tasks**

The `enqueue()` method is used to queue tasks. The `size` property can be used to check the number of tasks currently queued.

```
// Queue a task.
strategy.enqueue(() => {
  // Task code goes here.
});

// Check the number of tasks queued.
const currentSize = strategy.size;
```

3. **Dequeuing Tasks**

The `dequeue()` method is used to dequeue tasks from the queue. The `close()` method can be used to close the queue, preventing further tasks from being queued.

```
// Dequeue a task.
const task = strategy.dequeue();

// Close the queue.
strategy.close();
```

**Real-World Applications:**

* **Limiting concurrency:** By setting the `highWaterMark` to a specific value, you can control the maximum number of concurrent tasks that can be executed.
* **Preventing queue overflow:** If the task queue fills up, the `enqueue()` method will throw an error, preventing the queue from overflowing.
* **Managing resource usage:** By limiting the number of tasks that can be queued, you can prevent excessive resource consumption.

**Example:**

```
// Create a function that takes a list of numbers and returns the sum of the numbers.
const sumNumbers = (numbers) => {
  let sum = 0;
  for (const number of numbers) {
    sum += number;
  }
  return sum;
};

// Create a queuing strategy that allows a maximum of 5 tasks to be queued.
const strategy = new CountQueuingStrategy({highWaterMark: 5});

// Create a task queue.
const queue = new QueueMicrotask(strategy);

// Queue multiple tasks to calculate the sum of different lists of numbers.
queue.enqueue(() => {
  console.log(sumNumbers([1, 2, 3])); // 6
});

queue.enqueue(() => {
  console.log(sumNumbers([4, 5, 6])); // 15
});

queue.enqueue(() => {
  console.log(sumNumbers([7, 8, 9])); // 24
});
```

This example demonstrates how to use the `CountQueuingStrategy` to control the number of concurrent tasks that can be executed.

***

**CountQueuingStrategy Constructor**

The `CountQueuingStrategy` constructor in `webcrypto` creates a queuing strategy that limits the number of queued items to a specified limit.

**Parameters:**

* `init`: An object with the following properties:
  * `highWaterMark`: The maximum number of queued items allowed.

**Example:**

```js
const strategy = new CountQueuingStrategy({
  highWaterMark: 10,
});
```

This creates a queuing strategy that allows up to 10 queued items.

**Real-World Applications:**

Queuing strategies are used to manage the flow of data between different components of a system. In the case of the `CountQueuingStrategy`, it can be used to limit the number of queued requests to a server or other resource. This can help to prevent overloading the resource and ensures that requests are processed in a timely manner.

**Simplified Explanation:**

A queuing strategy is like a line of people waiting to be served. The `CountQueuingStrategy` is a rule that says the line can only have a certain number of people waiting in it. This helps to keep the line from getting too long and overwhelming the person serving.

***

### Breaking Down `countQueuingStrategy.highWaterMark`

**Concept:**

`countQueuingStrategy.highWaterMark` represents the maximum number of items that can be queued in a readable stream before the stream becomes paused.

**Analogy for a Child:**

Imagine you have a water tank with a hose attached to it. The water tank represents the readable stream, and the hose represents the queue. The `highWaterMark` is like a level indicator in the tank. When the water level reaches the indicator, it means that the tank is full and needs to be paused to prevent overflowing.

**Detailed Explanation:**

When you create a readable stream, you can specify a `countQueuingStrategy` to control how the stream buffers data. The `highWaterMark` property of this strategy determines how many items can be queued in the stream before it becomes paused.

**Simplified Example:**

```javascript
const { Readable } = require("stream");

const stream = new Readable({
  highWaterMark: 2,
  read() {
    this.push("item1");
    this.push("item2");
    this.push("item3");
  },
});

stream.on("readable", () => {
  // Handle data events
});
```

In this example, the stream is configured with a `highWaterMark` of 2, meaning it can buffer up to 2 items at a time. When the stream emits `'readable'` events, it means that at least one item is available in the queue to be read. The event handler reads the first two items (item1 and item2) and then waits for the next `'readable'` event before continuing.

**Real-World Applications:**

* **Backpressure Control:** `highWaterMark` helps prevent backpressure in stream pipelines. When a downstream consumer is slow to process data, the stream will pause when the queue reaches the `highWaterMark`, preventing the upstream producer from overwhelming the consumer.
* **Resource Management:** By limiting the number of queued items, `highWaterMark` helps manage memory usage and prevent performance bottlenecks.

**Potential Improvement:**

Instead of using a static value for `highWaterMark`, you can use a dynamic value based on the performance characteristics of your system. This allows you to optimize the stream's behavior based on real-time conditions.

***

#### `countQueuingStrategy.size`

The `countQueuingStrategy.size` method returns the number of chunks currently in the queue.

**Example:**

```javascript
const queue = new CountQueuingStrategy({ highWaterMark: 3 });

// Add three chunks to the queue
queue.enqueue("a");
queue.enqueue("b");
queue.enqueue("c");

// The size of the queue is now 3
console.log(queue.size); // 3
```

**Applications:**

The `countQueuingStrategy.size` method can be used to track the number of chunks in the queue and prevent the queue from becoming too large. This can be useful in situations where the queue is being used to buffer data that is being processed asynchronously.

***

**TextEncoderStream**

This class can be used in browsers to encode a stream of data using the TextEncoder interface.

**Decoder**

1. **Creation:** You create a `TextEncoderStream` object like this:

```
const encoderStream = new TextEncoderStream();
```

2. **Usage:** You can then write data to the stream using the `write()` method. The data will be encoded using the TextEncoder interface.

```
encoderStream.write('Hello, world!');
```

3. **Reading:** To read the encoded data, you can use the `readableStream` property of the `TextEncoderStream` object.

```
const readableStream = encoderStream.readableStream;

readableStream.on('data', (chunk) => {
  // Do something with the encoded data.
});
```

**Potential Applications**

This class can be used in any situation where you need to encode data using the TextEncoder interface. For example, you could use it to:

* Encode data for transmission over a network.
* Encode data for storage in a database.
* Encode data for use in a web application.

**Code Implementation**

Here is a complete code implementation of a simple web application that uses the `TextEncoderStream` class to encode data:

```
const encoderStream = new TextEncoderStream();

const form = document.getElementById('form');

form.addEventListener('submit', (event) => {
  event.preventDefault();

  const data = new FormData(form);

  encoderStream.write(data.get('text'));

  const readableStream = encoderStream.readableStream;

  readableStream.on('data', (chunk) => {
    console.log(chunk);
  });
});
```

***

#### `new TextEncoderStream()`

Creates a new `TextEncoderStream` instance.

A `TextEncoderStream` is a `TransformStream` that encodes strings into a stream of bytes using a specified encoding. The default encoding is "utf-8".

Here is an example of using `TextEncoderStream` to encode a string into a stream of bytes:

```js
const { TextEncoderStream } = require("stream/web");

const text = "Hello, world!";

const encoder = new TextEncoderStream();

encoder.on("data", (chunk) => {
  console.log(chunk);
});

encoder.write(text);
encoder.end();
```

This will output the following to the console:

```
<Buffer 48 65 6c 6c 6f 2c 20 77 6f 72 6c 64 21>
```

which is the UTF-8 encoding of the string 'Hello, world!'.

#### Potential applications in the real world

`TextEncoderStream` can be used in a variety of applications, including:

* Encoding data for transmission over a network
* Encoding data for storage in a database
* Encoding data for use in a web application

***

**`textEncoderStream.encoding`**

Imagine you have a text file written in English. To send this file to a friend overseas who speaks Spanish, you need to encode it into a format they can understand. This is where `textEncoderStream.encoding` comes in.

`textEncoderStream.encoding` allows you to specify the encoding method used to convert your text file into a format that can be easily read and understood by the recipient. Just like choosing a language to speak in, you need to choose the correct encoding to ensure your message is conveyed correctly.

For example, if your friend's computer uses UTF-8 encoding, you would set `textEncoderStream.encoding` to "utf-8" to convert your English text into a format that their computer can understand. This way, they can open the file and read it without any issues.

**Here's a real-world example:**

```
const textEncoderStream = new TextEncoderStream();
const writer = textEncoderStream.getWriter();
writer.write("Hello, World!");
writer.close();

const encodedText = textEncoderStream.readable.getReader().read().value;

console.log(encodedText);
```

In this example, we create a `TextEncoderStream` and write the text "Hello, World!" to it. The `TextEncoderStream` automatically encodes the text using UTF-8 encoding since that's the default. The encoded text is stored in `encodedText` and can now be sent to our friend's computer.

**Potential applications in the real world:**

* Sending text messages across borders
* Storing internationalized text data
* Displaying text in different languages on websites and applications
* Translating documents between languages

***

### `textEncoderStream.readable`

The `textEncoderStream.readable` property is a readable stream that contains the encoded data.

#### Usage

The following code sample shows you how to use the `textEncoderStream.readable` property:

```
const {TextEncoderStream} = require('util');
const encoder = new TextEncoderStream('utf-8');

encoder.write('Hello world');
encoder.end();

encoder.readable.on('data', (chunk) => {
  console.log(chunk);
});
```

#### Output

```
<Buffer 48 65 6c 6c 6f 20 77 6f 72 6c 64>
```

***

**`textEncoderStream.writable`**

The `textEncoderStream.writable` property is a WritableStream object that you can use to encode text data into a stream of bytes.

**Example:**

```typescript
const textEncoderStream = new TextEncoderStream();
const writableStream = textEncoderStream.writable;

writableStream.write("Hello, world!");
writableStream.close();

const encodedBytes = await textEncoderStream.readable.getReader().read();
console.log(encodedBytes); // Uint8Array containing the encoded bytes
```

**Applications:**

* Sending text data over a network connection
* Storing text data in a binary format
* Encrypting text data

***

**Class: `TextDecoderStream`**

The `TextDecoderStream` class is a Node.js stream that allows you to decode a stream of binary data into a stream of text data using a specified character encoding.

**How to use it:**

To use the `TextDecoderStream`, you first need to create a new instance of the class. You can do this by passing the desired character encoding to the constructor, for example:

```
const decoder = new TextDecoderStream('utf-8');
```

Once you have created a `TextDecoderStream`, you can pipe a stream of binary data into it. The `TextDecoderStream` will automatically decode the binary data into text data and emit the resulting text data as a stream of strings.

For example, the following code decodes a stream of binary data from a file and prints the decoded text to the console:

```
const fs = require('fs');
const decoder = new TextDecoderStream('utf-8');

fs.createReadStream('file.bin')
  .pipe(decoder)
  .pipe(process.stdout);
```

**Real-world applications:**

The `TextDecoderStream` can be used in a variety of real-world applications, including:

* Decoding text data from a web server
* Decoding text data from a database
* Decoding text data from a file
* Decoding text data from a stream of binary data

**Potential applications:**

* Receiving text data from a web server and displaying it in a web browser
* Storing text data in a database and retrieving it as text
* Loading text data from a file and processing it
* Decoding text data from a stream of binary data and analyzing it

***

### `TextDecoderStream([encoding[, options]])`

#### Summary

This method creates a new `TextDecoderStream` instance. A `TextDecoderStream` is a transform stream that can be used to decode a stream of bytes into a stream of strings.

#### Parameters

\| Parameter | Type | Description | | ---------- | ----------- | --------------------------------------------------------------------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | `encoding` | string | The encoding that this `TextDecoder` instance supports. Default: `'utf-8'`. | | `options` | object | An object that contains the following optional properties: | | | `fatal` | boolean | If `true`, decoding failures are fatal. Default: `false`. | | | `ignoreBOM` | boolean | If `true`, the `TextDecoderStream` will include the byte order mark in the decoded result. If `false`, the byte order mark will be removed from the output. This option is only used when `encoding` is `'utf-8'`, `'utf-16be'`, or `'utf-16le'`. Default: `false`. |

#### Returns

A new `TextDecoderStream` instance.

#### Example

The following code creates a new `TextDecoderStream` instance to decode a stream of bytes into a stream of UTF-8 strings:

```javascript
const { TextDecoderStream } = require("stream/web");

const textDecoderStream = new TextDecoderStream();

textDecoderStream.on("data", (chunk) => {
  console.log(chunk.toString());
});

textDecoderStream.end(Buffer.from("Hello, world!"));
```

#### Potential Applications

`TextDecoderStream` can be used in any application that needs to decode a stream of bytes into a stream of strings. Some potential applications include:

* Decoding data from a network connection
* Decoding data from a file
* Decoding data from a database
* Decoding data from a web socket

#### Real-World Example

The following code uses `TextDecoderStream` to decode a stream of bytes from a web socket into a stream of UTF-8 strings:

```javascript
const { WebSocket, TextDecoderStream } = require("stream/web");

const webSocket = new WebSocket("ws://localhost:8080");

webSocket.on("message", (message) => {
  const textDecoderStream = new TextDecoderStream();

  textDecoderStream.on("data", (chunk) => {
    console.log(chunk.toString());
  });

  textDecoderStream.write(message);
  textDecoderStream.end();
});
```

***

#### `textDecoderStream.encoding`

**Simplified Explanation:**

The `textDecoderStream.encoding` property tells you what encoding format the `TextDecoderStream` is using. This means it specifies how the stream will convert binary data into human-readable text.

**Type:**

String

**Possible Values:**

* `"utf-8"`: Unicode Transformation Format, 8-bit
* `"utf-16"`: Unicode Transformation Format, 16-bit
* `"utf-32"`: Unicode Transformation Format, 32-bit
* `"ascii"`: American Standard Code for Information Interchange
* `"iso-8859-1"`: ISO 8859-1 Latin Alphabet No. 1
* `"latin1"`: Alias for `"iso-8859-1"`
* `"binary"`: Raw binary data (not converted to text)

**Real-World Example:**

Let's say you have a stream of binary data that represents a JSON object. You want to decode this data into a JavaScript object, so you use a `TextDecoderStream` with the `"utf-8"` encoding:

```javascript
const data = new Uint8Array([
  123, 34, 116, 101, 115, 116, 34, 58, 34, 48, 49, 34, 125,
]);

const decoderStream = new TextDecoderStream({
  encoding: "utf-8",
});

const readableStream = decoderStream.readable;
const writableStream = decoderStream.writable;

writableStream.write(data);
writableStream.close();

readableStream
  .getReader()
  .read()
  .then(function ({ value, done }) {
    if (done) {
      console.log("Decoded JSON:", value);
    }
  });
```

In this example, the `decoderStream` will decode the binary data using UTF-8 encoding and output a readable stream containing the decoded JSON string.

**Potential Applications:**

* Decoding text data received over a network connection
* Reading text files from a file system
* Converting binary data into human-readable form for debugging or analysis

***

**`textDecoderStream.fatal`**

The `fatal` property of the `textDecoderStream` object determines how the stream handles decoding errors.

**Simplified explanation:**

Imagine you have a stream of encoded data, like a text file encoded in UTF-8. The `textDecoderStream` converts this encoded data into readable text. If there are any problems with the encoding, like a missing character, the stream can either:

* Throw an error (if `fatal` is `true`)
* Ignore the error and continue decoding (if `fatal` is `false`)

**Type:**

The `fatal` property is a boolean value.

**Default value:**

The default value for `fatal` is `false`.

**Real-world example:**

Suppose you have a text file encoded in UTF-8. Here's how you would use the `textDecoderStream` to decode it:

```
const textDecoderStream = new TextDecoderStream();
const input = "This is a test string";
const encodedData = new Uint8Array(input.length);
for (let i = 0; i < input.length; i++) {
  encodedData[i] = input.charCodeAt(i);
}
textDecoderStream.write(encodedData);
textDecoderStream.close();

textDecoderStream.readable
  .pipe(process.stdout)
  .on("finish", () => {
    console.log("Decoded data: " + textDecoderStream.read());
  });
```

**Potential applications:**

-Decoding text data from network requests -Converting encoded files into readable text -Processing large text files efficiently

***

### `textDecoderStream.ignoreBOM`

* **Type:** `boolean`
* **Default:** `false`
* **Description:**

  The `ignoreBOM` option of the `TextDecoderStream` specifies whether the byte order mark (BOM) should be ignored when decoding the input stream.

  A BOM is a special character that appears at the beginning of a text file to indicate the encoding of the file. For example, the UTF-8 BOM is `0xEF` `0xBB` `0xBF`.

  If `ignoreBOM` is set to `true`, the decoder will skip over any BOM it encounters at the beginning of the stream. If it is set to `false`, the BOM will be included in the decoded output.
* **Example:**

  The following example shows how to create a `TextDecoderStream` that ignores the BOM:

  ```
  const decoder = new TextDecoderStream({
    ignoreBOM: true
  });
  ```
* **Real-World Applications:**

  Ignoring the BOM can be useful when decoding text files that may have been created with different encodings. By ignoring the BOM, you can ensure that the decoder will not try to interpret the BOM as part of the actual text.

***

### `textDecoderStream.readable`

The `readable` property of the `textDecoderStream` returns a `ReadableStream` object that represents the stream of decoded data. This stream can be used to read the decoded data as it becomes available.

Here is an example of how to use the `readable` property:

```typescript
const textDecoderStream = new TextDecoderStream();
const readableStream = textDecoderStream.readable;

readableStream.on("data", (chunk) => {
  console.log(chunk);
});

textDecoderStream.write(new Uint8Array([104, 101, 108, 108, 111]));
textDecoderStream.write(new Uint8Array([119, 111, 114, 108, 100]));
textDecoderStream.end();
```

In this example, the `readableStream` event listener will be called twice, once for each chunk of data that is decoded. The first chunk will be the string "hello", and the second chunk will be the string "world".

### Real-world applications

The `textDecoderStream` can be used in a variety of real-world applications, such as:

* Decoding data from a network stream
* Decoding data from a file
* Decoding data from a database
* Decoding data from a web socket

### Potential applications

Here are some potential applications for the `textDecoderStream`:

* A web application that decodes data from a network stream and displays it in a web browser.
* A desktop application that decodes data from a file and saves it to a database.
* A mobile application that decodes data from a web socket and displays it in a user interface.

***

**What is `textDecoderStream.writable`?**

`textDecoderStream.writable` is a writable stream that you can use to write encoded text data (such as a string) and have it decoded into a format that is suitable for consumption by another process.

**How to use `textDecoderStream.writable`?**

To use `textDecoderStream.writable`, you first need to create a new TextDecoderStream object:

```
const textDecoderStream = new TextDecoderStream();
```

Once you have created a TextDecoderStream object, you can use its `writable` property to write data to the stream:

```
textDecoderStream.writable.write('Hello, world!');
```

The data that you write to the `writable` stream will be decoded into a format that is suitable for consumption by another process. For example, if you write a string to the stream, the stream will decode the string into a buffer.

**Real-world examples**

Here is a real-world example of how you can use `textDecoderStream.writable`:

```
const textDecoderStream = new TextDecoderStream();

const readableStream = getReadableStream();

readableStream.pipeThrough(textDecoderStream.writable).pipeTo(writableStream);
```

In this example, the `readableStream` is a stream that contains encoded text data. The `textDecoderStream` is used to decode the data into a format that is suitable for consumption by the `writableStream`.

**Potential applications**

`textDecoderStream.writable` can be used in a variety of applications, such as:

* Decoding data from a file
* Decoding data from a network socket
* Decoding data from a web server

***

#### Simplified Explanation of CompressionStream Class

The `CompressionStream` class in Node.js's Web Cryptography API allows you to create a stream of compressed data. This is useful when you want to send large amounts of data over a network or store it in a compressed format to save space.

#### Creating a CompressionStream

To create a `CompressionStream`, you use the `createDeflate()` or `createGzip()` methods, depending on the compression algorithm you want to use:

```js
const crypto = require("crypto");

// Create a Deflate compression stream
const deflateStream = crypto.createDeflate();

// Create a GZip compression stream
const gzipStream = crypto.createGzip();
```

#### Writing Data to the CompressionStream

To write data to the compression stream, you use the `write()` method. The data you write will be compressed and sent out the stream as a single chunk.

```js
// Write some data to the compression stream
deflateStream.write("Hello, world!");
```

#### Reading Data from the CompressionStream

To read data from the compression stream, you use the `pipe()` method to connect it to a readable stream. The data will be decompressed and passed to the readable stream.

```js
// Pipe the compression stream to a readable stream
deflateStream.pipe(process.stdout);

// Read the decompressed data from the readable stream
process.stdout.on("data", (data) => {
  console.log(data.toString());
});
```

#### Real-World Applications

Compression streams can be used in a variety of real-world applications, such as:

* Sending compressed data over a network to save bandwidth
* Storing compressed data on a disk to save space
* Compressing data before encrypting it for added security

#### Complete Code Implementation

Here is a complete code implementation that uses a `CompressionStream` to compress a file:

```js
const crypto = require("crypto");
const fs = require("fs");

const inputFile = "input.txt";
const outputFile = "output.gz";

// Create a GZip compression stream
const gzipStream = crypto.createGzip();

// Open the input file
const input = fs.createReadStream(inputFile);

// Pipe the input file to the compression stream
input.pipe(gzipStream);

// Open the output file
const output = fs.createWriteStream(outputFile);

// Pipe the compression stream to the output file
gzipStream.pipe(output);

// Wait for the compression to finish
gzipStream.on("finish", () => {
  console.log("File compressed successfully");
});
```

***

#### `new CompressionStream(format)`

The `CompressionStream` constructor is used to create a new stream that compresses chunks of data as they pass through.

The `format` parameter specifies the compression format to use. Valid formats are:

* `'deflate'`: The standard DEFLATE compression algorithm.
* `'deflate-raw'`: A variant of DEFLATE that doesn't use a header or footer.
* `'gzip'`: The GZIP compression format, which is a combination of DEFLATE and a header and footer.

Here is an example of how to use the `CompressionStream` constructor:

```
const compressor = new CompressionStream('deflate');
```

The `compressor` object can now be used to compress data. To do this, you can use the `pipeThrough()` method to pass chunks of data through the stream. For example:

```
const data = 'Hello, world!';
const compressedData = await compressor.pipeThrough(data);
```

The `compressedData` variable will now contain the compressed data.

You can also use the `pipeTo()` method to pass data from one stream to another. For example:

```
const inputStream = new ReadableStream();
const outputStream = new WritableStream();

inputStream.pipeTo(compressor).pipeTo(outputStream);
```

In this example, the data from the `inputStream` will be compressed by the `compressor` and written to the `outputStream`.

#### Real-world applications

Compression streams can be used in a variety of real-world applications, including:

* **Data compression**: Compression streams can be used to compress data, making it more efficient to store and transmit.
* **Network optimization**: Compression streams can be used to optimize network traffic by reducing the size of data that is sent over the network.
* **Performance improvement**: Compression streams can be used to improve the performance of applications by reducing the amount of time it takes to load and process data.

***

#### `compressionStream.readable`

* **Type**: `ReadableStream`

A ReadableStream object that provides access to the compressed data as it is being produced.

#### Real World Complete Code Implementation and Example

```javascript
const { createGzip } = require("zlib");
const fs = require("fs");

const gzip = createGzip();

const input = fs.createReadStream("input.txt");
const output = fs.createWriteStream("output.gz");

input.pipe(gzip).pipe(output);

gzip.on("data", (chunk) => {
  console.log("Compressed chunk:", chunk);
});

gzip.on("end", () => {
  console.log("Compression complete.");
});
```

This example creates a GZIP compression stream and uses it to compress the contents of the `input.txt` file and write the compressed data to the `output.gz` file. The `compressionStream.readable` event is used to listen for and log the compressed chunks of data as they are produced.

#### Potential Applications in Real World

Compression streams can be used in a variety of real-world applications, including:

* **Data compression**: Compressing data can reduce its size, making it easier to store and transmit.
* **Encryption**: Compressed data can be encrypted to protect it from unauthorized access.
* **Streaming**: Compressed data can be streamed over a network, allowing it to be accessed and processed as it is being produced.

***

#### `compressionStream.writable`

The `compressionStream.writable` property is a `WritableStream` object that represents the writable side of the compression stream. Data written to this stream will be compressed and sent to the underlying sink.

**Example:**

```
const {createGzip} = require('zlib');
const {Transform} = require('stream');

const compressor = createGzip();
const transformStream = new Transform();
const outputStream = fs.createWriteStream('output.txt.gz');

compressor.pipe(transformStream).pipe(outputStream);

// Write some data to the compressor
compressor.write('Hello, world!');
compressor.end();
```

In this example, the `createGzip()` function is used to create a compression stream. The `Transform` stream is used to transform the data before it is compressed. The `outputStream` is used to write the compressed data to a file.

The `compressor.pipe(transformStream).pipe(outputStream)` method is used to connect the streams together. Data written to the `compressor` will be compressed, transformed, and then written to the `outputStream`.

***

**Class: `DecompressionStream`**

The `DecompressionStream` class in Node.js is a type of transform stream that decompresses data as it passes through. It's used to decompress data that has been compressed using a specific compression algorithm, such as GZIP or Brotli.

**Usage:**

To use the `DecompressionStream`, you first need to create a new instance of the class, passing in the decompression algorithm you want to use. For example, to decompress GZIP-compressed data, you would do the following:

```
const zlib = require('zlib');
const decompressStream = zlib.createDecompressionStream('gzip');
```

Once you have created a `DecompressionStream` instance, you can pipe data into it and it will automatically decompress the data as it passes through. For example, to decompress a file from disk, you could do the following:

```
const fs = require('fs');
const path = require('path');

const inputFile = fs.createReadStream(path.join(__dirname, 'input.gz'));
const outputFile = fs.createWriteStream(path.join(__dirname, 'output.txt'));

inputFile.pipe(decompressStream).pipe(outputFile);
```

This code will read the compressed file `input.gz`, decompress it using the `decompressStream`, and write the decompressed data to the file `output.txt`.

**Applications:**

The `DecompressionStream` class has many potential applications in real-world scenarios, including:

* Decompressing data that has been transferred over a network
* Decompressing data that has been stored in a compressed format on disk
* Decompressing data that has been received from a web service

By using the `DecompressionStream` class, you can easily and efficiently decompress data using a variety of compression algorithms.

***

### `new DecompressionStream(format)`

This class helps you decompress data that was compressed using a given format.

#### Parameters

* `format` {string} One of `'deflate'`, `'deflate-raw'`, or `'gzip'`.

#### Usage

```
const decompressor = new DecompressionStream('deflate');
decompressor.write(compressedData);
const decompressedData = decompressor.read();
```

#### Real World Applications

* Decompressing data that was stored in a compressed format.
* Decompressing data that was sent over a network in a compressed format.

***

### `decompressionStream.readable`

The `decompressionStream.readable` property of the `DecompressionStream` interface represents a readable stream from which compressed data that has been decompressed can be read.

#### Example

```typescript
let stream = new DecompressionStream("deflate");

// Initialize the stream with uncompressed data.
let a = new Uint8Array([72, 101, 108, 108, 111, 32, 119, 111, 114, 108, 100]);
stream.init(a);

let reader = stream.readable.getReader();

reader.read().then((result) => {
  console.log(result.value);
  // Output: "Hello world"
});
```

***

**1. What is `decompressionStream.writable`?**

* `decompressionStream.writable` is a writable stream that can be used to decompress data.

**2. How to use `decompressionStream.writable`?**

* To use `decompressionStream.writable`, you first need to create a decompression stream. You can do this using the `createDecompressionStream()` method of the `crypto` module.

```
const crypto = require('crypto');

const decompressionStream = crypto.createDecompressionStream('zlib');
```

* Once you have created a decompression stream, you can pipe data to it using the `writable` property.

```
stream.pipe(decompressionStream);
```

* As the data is piped through the decompression stream, it will be decompressed. You can then read the decompressed data from the `readable` property of the decompression stream.

```
decompressionStream.on('data', (chunk) => {
  // Do something with the decompressed data.
});
```

**3. Real-world applications of `decompressionStream.writable`:**

* Decompression streams can be used in a variety of applications, such as:
  * Decompressing data that has been downloaded from the internet.
  * Decompressing data that has been stored in a compressed format.
  * Decompressing data that has been encrypted.

***

**Utility Consumers for Streams**

Imagine you have a stream of data flowing through a pipe. These consumer functions are like special tools that you can attach to the pipe to capture the data in specific formats.

**1. arrayBuffer**

* **What it does:** Captures the data as an ArrayBuffer, which is a raw binary data format.
* **Example:**

```
const stream = fs.createReadStream('my-image.png');
const { arrayBuffer } = require('node:stream/consumers');
stream.pipe(arrayBuffer((data) => {
  // data is an ArrayBuffer containing the image data
}));
```

* **Real-world use:** Loading images or other binary files into memory.

**2. blob**

* **What it does:** Captures the data as a Blob, which is a file-like object.
* **Example:**

```
const stream = fetch('https://example.com/image.jpg');
const { blob } = require('node:stream/consumers');
stream.pipe(blob((data) => {
  // data is a Blob object containing the image
}));
```

* **Real-world use:** Saving files downloaded from the Internet to the disk.

**3. buffer**

* **What it does:** Captures the data as a Buffer, which is a sequence of bytes.
* **Example:**

```
const stream = process.stdout;
const { buffer } = require('node:stream/consumers');
stream.pipe(buffer((data) => {
  // data is a Buffer containing the output from the command
}));
```

* **Real-world use:** Capturing the output of a command or process.

**4. json**

* **What it does:** Captures the data as a JSON object.
* **Example:**

```
const stream = fs.createReadStream('data.json');
const { json } = require('node:stream/consumers');
stream.pipe(json((data) => {
  // data is the JSON object parsed from the stream
}));
```

* **Real-world use:** Reading data from a JSON file or API response.

**5. text**

* **What it does:** Captures the data as a string.
* **Example:**

```
const stream = process.stdin;
const { text } = require('node:stream/consumers');
stream.pipe(text((data) => {
  // data is a string containing the user's input
}));
```

* **Real-world use:** Reading text data from a keyboard or other text source.

***

**streamConsumers.arrayBuffer(stream)**

**Purpose:**

Converts a readable stream into a single `ArrayBuffer`.

**Parameters:**

* `stream`: A readable stream containing binary data.

**Return Value:**

A `Promise` that resolves to an `ArrayBuffer` holding the entire contents of the stream.

**Simplified Explanation:**

Imagine you have a pipe (stream) filled with water. `arrayBuffer()` is like a container that collects all the water from the pipe and stores it in one place. Once the pipe is empty, `arrayBuffer()` will return the container with all the water.

**Code Example:**

```javascript
import { Readable } from "stream";
import { arrayBuffer } from "stream/consumers";

const readableStream = Readable.from(["Hello", "World"]);

const arrayBufferPromise = arrayBuffer(readableStream);

arrayBufferPromise.then((arrayBuffer) => {
  const textDecoder = new TextDecoder();
  const text = textDecoder.decode(arrayBuffer);
  console.log(text); // Output: 'HelloWorld'
});
```

**Real-World Application:**

* **File Download:** Download a file from a remote server as a stream and convert it to an `ArrayBuffer` for processing.
* **Audio Streaming:** Convert an audio stream into an `ArrayBuffer` to play it using the Web Audio API.
* **Image Processing:** Load an image from disk as a stream, convert it to an `ArrayBuffer`, and apply image processing algorithms to it.

***

**Simplified Explanation:**

The `blob()` function takes a readable stream and converts it into a Blob object, which represents a chunk of data that can be stored and manipulated as a file.

**Detailed Explanation:**

A stream is a sequence of data that is processed incrementally, one piece at a time. Readable streams allow you to read data from them, while writable streams allow you to write data to them.

A Blob is an object that represents a file-like data. It has a size property, which indicates the size of the data in bytes, and a type property, which indicates the MIME type of the data (e.g., "text/plain", "image/png"). You can create a Blob from an array of strings, an array buffer, or a stream.

The `blob()` function takes a readable stream as its argument and returns a Promise that resolves to a Blob object. The Promise will be fulfilled when the entire stream has been consumed and the Blob has been created.

**Example:**

The following code snippet shows how to use the `blob()` function to convert a readable stream into a Blob object:

```javascript
const dataBlob = new Blob(["hello world from consumers!"]);

const readable = dataBlob.stream();
const data = await blob(readable);
console.log(`from readable: ${data.size}`);
// Prints: from readable: 27
```

**Real World Applications:**

The `blob()` function can be used in a variety of real-world applications, such as:

* Downloading files from the internet
* Saving data to a file on the local computer
* Sending files to other applications or servers
* Creating custom file formats

**Potential Applications:**

Here are some potential applications of the `blob()` function:

* **File uploading:** You can use the `blob()` function to convert a user-selected file into a Blob object, which can then be uploaded to a server.
* **Image processing:** You can use the `blob()` function to convert an image file into a Blob object, which can then be processed using image processing techniques.
* **Data storage:** You can use the `blob()` function to convert data into a Blob object, which can then be stored in a database or file system.

***

**`streamConsumers.buffer(stream)`**

* **Purpose**: `streamConsumers.buffer()` is a function in Node.js that collects all the data from a readable stream and returns it as a single `Buffer`. This is useful when you want to read the entire contents of a stream into memory at once, rather than processing it incrementally.
* **Parameters**:
  * `stream`: The readable stream to buffer. This can be a `ReadableStream`, a `stream.Readable`, or an asynchronous iterator.
* **Return Value**: A `Promise` that resolves with a `Buffer` containing the full contents of the stream.
* **Example**: The following code snippet shows how to use `streamConsumers.buffer()` to read the entire contents of a readable stream into a `Buffer`:

```
import { buffer } from 'node:stream/consumers';
import { Readable } from 'node:stream';

const readable = Readable.from('hello world');

buffer(readable).then((data) => {
  console.log(data.toString());
});
```

Output:

```
hello world
```

* **Real World Applications**: `streamConsumers.buffer()` can be used in a variety of real-world applications, such as:
  * Reading the entire contents of a file into memory.
  * Collecting all the data from a network request.
  * Accumulating data from a serial port or other hardware device.
  * Creating a checksum or hash of a stream of data.

***

**`streamConsumers.json(stream)`**

**Description:**

This function converts the contents of a stream into a JavaScript object by first decoding the stream as a UTF-8 string and then parsing it as JSON.

**Parameters:**

* `stream`: The stream to convert into a JavaScript object. Can be a ReadableStream, a stream.Readable object, or an AsyncIterator.

**Return Value:**

* A Promise that resolves to the JavaScript object.

**Example:**

```javascript
import { json } from "node:stream/consumers";
import { Readable } from "node:stream";

const items = Array.from(
  {
    length: 100,
  },
  () => ({
    message: "hello world from consumers!",
  })
);

const readable = Readable.from(JSON.stringify(items));
const data = await json(readable);
console.log(`from readable: ${data.length}`);
// Prints: from readable: 100
```

In this example, we create a Readable stream and populate it with 100 JSON objects. We then use the `json` function to convert the stream into a JavaScript object. The `data` variable will contain an array of the 100 objects.

**Real-World Applications:**

* Parsing JSON data from a network request
* Parsing JSON data from a file
* Converting a stream of events into a JSON object for further processing

***

**`streamConsumers.text(stream)`**

* `stream` {ReadableStream|stream.Readable|AsyncIterator}
* Returns: {Promise} Fulfills with the contents of the stream parsed as a UTF-8 encoded string.

**Simplified Explanation:**

`text` is a function that takes a stream of data and returns a promise that resolves with the data parsed as a string.

**Code Snippet:**

```
import { text } from 'node:stream/consumers';
import { Readable } from 'node:stream';

const readable = Readable.from('Hello world from consumers!');
const data = await text(readable);
console.log(`from readable: ${data.length}`); // Prints: from readable: 27
```

**Real-World Implementation:**

**Using a stream of text data**

```js
const textStream = fs.createReadStream("text.txt", "utf8");

text(textStream).then((data) => {
  // `data` is a string containing the contents of 'text.txt'
});
```

**Potential Application:**

* Reading text files in a streaming manner.