test

Test Runner: The Basics

Imagine you're building a new car and want to make sure it runs smoothly before taking it for a spin. Just like a car, your code needs testing to ensure it behaves as expected. That's where the test module comes in!

How to Use It:

To start using the test module, you need to import it like this:

import test from "node:test";

Types of Tests:

The test module allows you to create three types of tests:

1. Synchronous Tests: These tests run right away. If they throw an error, they fail; otherwise, they pass.

test("Synchronous passing test", (t) => {
  assert.strictEqual(1, 1); // This test passes because the assertion is true.
});

2. Asynchronous Tests: These tests use async functions or return a Promise. If the Promise is rejected, the test fails; if it succeeds, the test passes.

test("Asynchronous passing test", async (t) => {
  await new Promise((resolve) => setTimeout(resolve, 1000)); // This test passes because the `Promise` resolves after 1 second.
});

3. Callback Tests: These tests use a callback function. If the callback is called with an error (a truthy value), the test fails; otherwise, it passes.

test("Callback passing test", (t, done) => {
  setTimeout(() => done(), 1000); // This test passes because `done()` is called without an error after 1 second.
});

What Happens When a Test Fails?

If any of your tests fail, the process that's running your code will return a non-zero exit code (usually 1). This is a signal to the operating system or other programs that something went wrong.

Real-World Applications:

Testing is crucial in software development. It helps catch errors and ensure that your code works as intended. Here are some real-world uses of testing:

• Web applications: Testing every feature and API call helps prevent bugs in your website or app.

• Databases: Tests check if your database queries are correct and don't crash the system.

• Data processing: Tests ensure that your code correctly manipulates and analyzes data.

• Machine learning: Tests verify that your models make accurate predictions and don't overfit.

• Operating systems: Tests ensure that your OS is stable and performs as expected.

---

Subtests

Simplified Explanation:

Subtests let you break your tests into smaller, more manageable pieces. It's like creating a hierarchy of tests, with a main test at the top and smaller tests nested beneath it.

How to Create Subtests:

You can create subtests using the test() method within the test context. The syntax is the same as for top-level tests:

test('subtest name', (t) => {
  // Test code goes here
});

Example:

test("main test", async (t) => {
  await t.test("subtest 1", (t) => {
    assert.strictEqual(1, 1);
  });

  await t.test("subtest 2", (t) => {
    assert.strictEqual(2, 2);
  });
});

Note: You need to use await to wait for subtests to complete before moving on.

Benefits of Subtests:

• Easier organization: Breaking tests into smaller parts makes them easier to read and understand.

• Reusable code: You can share common code between related subtests, reducing duplication.

• Parallel testing: Subtests can be executed concurrently, speeding up testing time.

Real-World Application:

Subtests are useful in any situation where you have a complex test that needs to be broken down into smaller components.

Example:

Let's say you have a function that converts JSON data into an object. You could create a main test for the overall conversion, and then subtests for each specific conversion type:

test("JSON conversion", async (t) => {
  await t.test("string to object", (t) => {
    const json = '{"name": "John"}';
    const obj = JSON.parse(json);
    assert.strictEqual(obj.name, "John");
  });

  await t.test("number to object", (t) => {
    const json = "123";
    const obj = JSON.parse(json);
    assert.strictEqual(obj, 123);
  });
});

This makes it easier to isolate and debug any specific conversion issues.

---

Skipping Tests

Sometimes, you may need to skip a test for various reasons like:

• The test is not yet implemented.

• The test depends on a feature that is not available in the current environment.

• The test is too slow and you want to temporarily disable it.

Skipping a Single Test

To skip a single test, you can do either of the following:

1. Use the skip option when defining the test:

test("Test to skip", { skip: true }, (t) => {
  // The test code goes here, but it will not run because we skipped the test.
});

2. Call the skip() method within the test:

test("Test to skip", (t) => {
  t.skip(); // Skips the test
});

In both cases, the test will be skipped and the test runner will move on to the next test.

Skipping a Group of Tests

You can also skip a group of tests using the skip() method with a truthy value as an argument:

test('Test group', (t) => {
  if (someCondition) {
    t.skip('This group is skipped because someCondition is true');
  }

  // The following tests will be skipped if someCondition is true:
  t.test('Test 1', (t) => { ... });
  t.test('Test 2', (t) => { ... });
});

Real-World Applications

Here are some real-world applications for skipping tests:

• Skipping flaky tests: Flaky tests are tests that pass most of the time but occasionally fail because of external factors like network issues or slow databases. You can skip these tests when running the test suite frequently to improve the reliability of the tests.

• Skipping tests for specific environments: If you have tests that are only relevant in certain environments, like staging or production, you can skip them in other environments.

• Temporarily disabling slow tests: If you have tests that take a long time to run, you can skip them when you need to run the test suite quickly.

---

Simplified Explanation of describe and it Syntax in Node.js Test Module

1. Describe: Grouping Related Tests

Imagine you're organizing toys. You have a basket for all the blocks, another for the cars, etc. This is like describe in Node's test module. It groups related tests into a suite.

Code Example:

describe("Block Toys", () => {
  // Suite name: Block Toys
  /* Test cases for block toys will go here */
});

2. It: Defining a Test

Each test case within a suite is defined using it. It's like a recipe for checking if something works.

Code Example:

it("Blue block should be blue", () => {
  assert.strictEqual(blockColor, "blue"); // Checks if block color is blue
});

3. Nested Describe: Grouping Nested Suites

You can group suites within suites using nested describe. It's like having a toy box within a toy basket.

Code Example:

describe("All Toys", () => {
  describe("Block Toys", () => {
    /* Block toy tests */
  });
  describe("Car Toys", () => {
    /* Car toy tests */
  });
});

Potential Applications in Real World:

• Unit Testing: Testing individual components/functions in isolation

• Integration Testing: Testing how multiple components work together

• End-to-end Testing: Testing an entire application from start to finish

• Performance Testing: Measuring performance of various features

• Regression Testing: Ensuring changes don't break existing functionality

Improved Code Example with Assertions:

describe("Block Toys", () => {
  it("Blue block should be blue", () => {
    assert.strictEqual(blockColor, "blue");
  });

  it("Red block should not be green", () => {
    assert.notStrictEqual(blockColor, "green");
  });
});

In this improved example, we assert specific expectations (e.g., block color should equal 'blue') using Node's built-in assert module.

---

Simplified Explanation

--test-only Mode

When running Node.js with the --test-only option, you can selectively run only specific tests.

only Option

To run only a specific test (and its subtests), set the only option to true for that test.

runOnly() Method

Within a test, you can use the runOnly() method to specify whether or not to run subtests. Setting runOnly(true) will only run tests with the only option set.

Detailed Explanation

--test-only Mode

This mode allows you to focus on testing specific areas of your codebase, instead of running all tests.

only Option

By setting only: true for a test, you tell TAP (the Test Anything Protocol used by Jest) to ignore all other tests and focus on running only the selected test.

runOnly() Method

This method is available within a test context. It allows you to control whether subtests are executed. By default, all subtests are run. But you can set runOnly(true) to only run subtests that have the only option set.

Code Examples

Complete Example

import { test, expect } from "node:test";

// Run this test and its subtests
test(
  "main test",
  {
    only: true,
  },
  async (t) => {
    // Run this subtest
    await t.test(
      "subtest 1",
      {
        only: true,
      },
      async (tt) => {
        // Assertions go here
      }
    );

    // Skip this subtest
    await t.test(
      "skipped subtest 2",
      {
        only: false,
      },
      async (tt) => {
        // Assertions go here
      }
    );
  }
);

// Skip this test
test("skipped test", async (t) => {
  // Assertions go here
});

Real-World Applications

• Targeted Testing: Quickly test specific features or modules without running everything.

• Debugging: Isolate and debug specific test failures more easily.

• Integration Testing: Run tests that depend on external services or APIs, without having to run the entire test suite.

• Performance Testing: Focus on measuring the performance of specific code paths or functions.

---

Filtering Tests by Name

Imagine you have a playground with many different games. You can use a magic wand to make only certain games appear, based on their names. That's what the --test-name-pattern command does for testing.

Using the Magic Wand:

node --test-name-pattern="test 1"

This wand makes games with the name "test 1" appear.

test("test 1", async (t) => {
  // Test code goes here...
});

test("test 2", async (t) => {
  // This game won't appear, because the wand is set to "test 1" only.
});

Using the Magic Wand Multiple Times:

node --test-name-pattern="test 1" --test-name-pattern="test 2"

This is like having two magic wands. It makes games with the name "test 1" and "test 2" appear.

Matching Using Patterns:

You can also use a magnifying glass to match names that follow a pattern. For example:

node --test-name-pattern="/test [1-3]/i"

This magnifying glass will find games with names starting with "test" and followed by a number between 1 and 3, ignoring case.

test("test 1", async (t) => {
  // This game will appear.
});

test("Test 4", async (t) => {
  // This game won't appear, because it matches the pattern but isn't case-insensitive.
});

Important Note:

The magic wand doesn't change which games you have in your playground, it just decides which ones to make visible for testing.

---

Asynchronous Activity in Node.js Tests

What is asynchronous activity?

In Node.js, asynchronous activity refers to tasks that don't complete immediately and continue running in the background. Examples include:

• Setting timeouts (setTimeout)

• Setting immediate callbacks (setImmediate)

• Making HTTP requests

Can asynchronous activity affect my tests?

Yes. If a test function starts asynchronous activity that outlives the test itself, the test runner may not wait for the activity to complete before reporting the test results.

What happens when asynchronous activity outlives a test?

The test runner will still report the test results, but it will handle any outstanding asynchronous activity in the following ways:

• Subtests created too late: Any subtests created after the parent test has finished will be marked as failed.

• Uncaught exceptions: Exceptions thrown by asynchronous activity will be marked as failed and reported as diagnostic warnings.

Example:

test("a test that creates asynchronous activity", (t) => {
  setImmediate(() => {
    t.test("subtest that is created too late", (t) => {
      throw new Error("error1");
    });
  });

  setImmediate(() => {
    throw new Error("error2");
  });
});

In this example, the test function creates two setImmediate() operations. The first attempts to create a subtest, but the parent test has already finished, so the subtest will be marked as failed. The second setImmediate() throws an exception, which will be marked as failed and reported as a diagnostic warning.

Real-world applications:

• Logging errors: Asynchronous activity can be used to log errors that occur after a test has finished.

• Cleaning up resources: Asynchronous activity can be used to clean up resources, such as closing database connections or deleting temporary files.

• Monitoring behavior: Asynchronous activity can be used to monitor the behavior of a system after a test has finished.

Tips for avoiding issues with asynchronous activity:

• Try to keep asynchronous activity within the scope of the test function.

• If you need to create asynchronous activity that outlives the test, use a library like bluebird or async to manage it.

• Handle exceptions thrown by asynchronous activity using try/catch blocks or by using the uncaughtException event.

---

Watch Mode

When running Node.js tests, you can use watch mode to automatically rerun tests when any test files or their dependencies change.

How to Use Watch Mode

To run tests in watch mode, simply add the --watch flag to the node --test command:

node --test --watch

How Watch Mode Works

In watch mode, the test runner monitors test files and their dependencies for changes. When a change is detected, the test runner identifies the tests affected by the change and reruns them.

Benefits of Watch Mode

Watch mode is useful when you're developing tests, as it allows you to make changes to test files or their dependencies and see the results immediately without having to manually rerun the tests.

Example Usage

Here's an example of a test script that can be run in watch mode:

// test.js
it('should pass', () => {
  expect(true).toBe(true);
});

// Run test.js in watch mode
node --test --watch test.js

When you make a change to test.js, such as changing the expected value to false, the test runner will automatically rerun the test and report the failure.

Real-World Applications

Watch mode is particularly useful in development environments, where you're frequently making changes to test files or their dependencies and want immediate feedback on the results. It can also be useful in continuous integration pipelines, where tests are run automatically on every code change.

---

Running Tests from the Command Line

Simplified Explanation:

You can run Node.js tests using the --test flag from the command line. By default, it will look for test files with specific patterns, such as *.test.js. You can also specify custom patterns by providing them as arguments after --test.

Detailed Explanation with Examples

Invoking the Node.js Test Runner from the Command Line

To run tests, open your command line and navigate to the project directory where your test files are located. Then, execute the following command:

node --test

Default Matching Patterns

By default, the Node.js test runner will search for files matching the following patterns:

• **/*.test.?(c|m)js (e.g., my-test.js or my-test.mjs)

• **/*-test.?(c|m)js (e.g., my-project-test.js or my-project-test.mjs)

• **/*_test.?(c|m)js (e.g., my_test.js or my_test.mjs)

• **/test-*.?(c|m)js (e.g., test-my-module.js or test-my-module.mjs)

• **/test.?(c|m)js (e.g., test.js or test.mjs)

• **/test/**/*.?(c|m)js (e.g., test/my-nested-test.js or test/my-nested-test.mjs)

Custom Matching Patterns

You can specify custom patterns by passing them as arguments after --test. For example, to run tests in the test and spec directories, you would use:

node --test **/*.test.js **/*.spec.js

Matching File Execution

Matching files are executed as test files. Each test file is executed in its own Node.js process.

Real-World Applications

Testing is essential for ensuring the reliability and correctness of your code. By writing tests that cover various scenarios, you can identify potential issues early on and prevent them from affecting your users.

Example:

Consider an e-commerce website. You might write a test that checks if the shopping cart correctly calculates the total amount based on the items added to the cart. By running this test frequently, you can ensure that the checkout process always works as expected.

---

Test Runner Execution Model

Concept:

Each test file runs in its own separate process. The number of processes running at once is controlled by the --test-concurrency flag.

Simplified Explanation:

Imagine you have multiple kids (tests) doing chores (debugging). Each kid gets their own room (process) to work in. The number of kids working at the same time (concurrency) is limited by the parent (flag).

Real-World Example:

Suppose you have 10 test files. With a concurrency of 3, the test runner starts 3 child processes to run the tests. The next 7 tests wait until one of the first 3 finishes before starting.

Code Example:

node --test-concurrency=3 test/my-tests.js

Advantages:

• Prevents tests from interfering with each other (e.g., accessing same resources).

• Parallelizes test execution, making testing faster.

Potential Applications:

• Running large or complex test suites that take a long time to execute.

• Running tests in different environments or configurations.

Test File Execution

Concept:

Each test file is treated as a normal script and executed directly by Node.js. Tests defined using node:test within the file are executed in a single thread, regardless of the concurrency setting.

Simplified Explanation:

Think of it like running your test commands directly from the terminal. The concurrency setting only controls the number of separate processes, not the execution within each process.

Real-World Example:

If your test file has 5 tests defined using test(), they will all run one after the other within the same process, even if concurrency is set to 10.

Code Example:

// test/my-test.js
import test from "node:test";

test("my test", () => {
  // ...
});

Advantages:

• Allows tests to use the full Node.js API, including third-party modules.

• Simplifies test writing and maintainability.

Potential Applications:

• Writing tests for complex Node.js applications that require access to the entire API.

• Creating custom test helpers or utilities within test files.

---

Collecting Code Coverage

When you run Node.js with the --experimental-test-coverage flag, it tracks which parts of your code are executed during tests. After all tests are done, it generates a report showing how much of your code was covered.

Excluding Modules from Coverage

Sometimes, you don't want to include certain modules or files in the coverage report. For example, if you have a third-party library that you're using, you might not want to track its coverage. To exclude these, run Node.js with the NODE_V8_COVERAGE environment variable set to the directory where you want to save the coverage files. This will exclude Node.js core modules and files in node_modules directories.

Disabling Coverage for Certain Lines

You can also disable coverage for specific lines of code by adding comments in the following format:

/* node:coverage disable */
console.log("This line will not be included in coverage reports.");

To re-enable coverage after a disabled section, use the following comment:

/* node:coverage enable */

Ignoring Lines for Coverage

Instead of disabling coverage, you can also ignore specific lines or blocks of lines. To ignore a single line, use the following comment:

/* node:coverage ignore next */

This will ignore the next line of code only.

To ignore multiple lines, use the following comment:

/* node:coverage ignore next 3 */

This will ignore the next 3 lines of code.

Real-World Applications

Code coverage is useful for identifying which parts of your code are not being executed during tests. This can help you improve your test coverage and ensure that your code is thoroughly tested. It can also help you identify areas of your code that are not being used and can be removed.

Example

Here's an example of how you can use code coverage:

const assert = require("assert");

// Test function
function sum(a, b) {
  return a + b;
}

// Test cases
describe("Sum function", function () {
  // Test case 1
  it("should add two positive numbers", function () {
    assert.strictEqual(sum(1, 2), 3);
  });

  // Test case 2
  it("should add two negative numbers", function () {
    assert.strictEqual(sum(-1, -2), -3);
  });

  // Test case 3
  it("should add a positive and a negative number", function () {
    assert.strictEqual(sum(1, -2), -1);
  });
});

If you run this code with the --experimental-test-coverage flag, you'll get a coverage report that shows which lines of code were executed during the tests. This can help you identify which test cases are covering which lines of code.

---

Coverage Reporters

What are they?

Coverage reporters are tools that help you see which parts of your code have been executed when you run your tests. This information can help you identify areas of your code that may not be covered by your tests, which can lead to bugs.

Types of Coverage Reporters

Node.js provides several coverage reporters, including:

• tap: Prints a summary of coverage statistics in a TAP format.

• spec: Prints a summary of coverage statistics in a human-readable format.

• lcov: Generates an LCOV file, which can be used with third-party tools to generate in-depth coverage reports.

How to Use Coverage Reporters

To use a coverage reporter, you can pass the --test-reporter flag to the node command, followed by the name of the reporter you want to use. For example, to use the LCOV reporter, you would run:

node --test --experimental-test-coverage --test-reporter=lcov --test-reporter-destination=lcov.info

This command will run your tests and generate an LCOV file called lcov.info. You can then use a tool like genhtml to convert the LCOV file into a human-readable report.

Real-World Applications

Coverage reporters can be useful in several real-world scenarios, including:

• Code reviews: To identify areas of code that are not covered by tests.

• Debugging: To help identify the root cause of bugs.

• Performance optimization: To identify areas of code that can be optimized for performance.

• Code metrics: To track code coverage over time and identify trends.

---

Limitations of Node.js Test Runner's Code Coverage

Code Coverage Functionality:

• Code coverage measures how much of your code is being executed during tests.

Limitations:

1. Source Maps Not Supported

• Explanation: Source maps connect the code you write (source code) with the code the browser executes (compiled code).

• Simplified: Without source maps, the code coverage report won't show coverage for the original source code, only for the compiled code.

2. Excluding Specific Files or Directories Not Supported

• Explanation: You can't choose to ignore specific files or folders when generating the coverage report.

• Simplified: All code in the tested directory will be included in the coverage report.

Real-World Applications:

Code Coverage is important because:

• It helps identify untested code, which can lead to bugs and security vulnerabilities.

• It provides insights into how your code is being executed, which can help optimize performance.

Example:

Consider this JavaScript code:

function add(a, b) {
  return a + b;
}

test("add function adds two numbers", () => {
  expect(add(1, 2)).toBe(3);
});

Without source maps:

• Running the test would show 100% coverage for the compiled code.

• However, without source maps, you wouldn't see coverage for the add function itself.

With source maps (not supported):

• The coverage report would show 100% coverage for both the compiled and source code, giving you a clearer picture of how the code is being executed.

Future Improvements:

These limitations will be addressed in a future Node.js release, providing a more comprehensive code coverage functionality.

---

Mocking with the node:test Module

What is Mocking?

Mocking is a testing technique that allows you to create fake behaviors for objects or functions in your code during testing. This helps in isolating and testing specific parts of your code without relying on external dependencies or complicated setups.

How to Mock with node:test

The node:test module provides a mock object that you can use to create spies. A spy is a function that tracks its calls, arguments, and results. You can then use the spy to assert that your code is calling the spied-on function as expected.

Creating a Spy for a Function

To create a spy for a function, use the mock.fn() method. You can provide the actual implementation of the function as an argument to mock.fn().

const sum = mock.fn((a, b) => {
  return a + b;
});

This creates a spy for the sum function. The actual implementation of sum adds two numbers together.

Using a Spy to Assert Function Calls

Once you have created a spy, you can use it to assert that your code is calling the spied-on function as expected. You can use the mock property of the spy to access information about the function's calls, arguments, and results.

For example, to assert that the sum function was called with the arguments 3 and 4, and returned the result 7, you can use the following code:

assert.strictEqual(sum.mock.calls.length, 1);
assert.deepStrictEqual(sum.mock.calls[0].arguments, [3, 4]);
assert.strictEqual(sum.mock.calls[0].result, 7);

Mocking Object Methods

You can also use the mock object to create spies for object methods. To do this, use the mock.method() method. You need to provide the object and the name of the method you want to spy on as arguments to mock.method().

const number = {
  value: 5,
  add(a) {
    return this.value + a;
  },
};

t.mock.method(number, "add");

This creates a spy for the add method of the number object.

Real-World Applications of Mocking

Mocking is a powerful technique that can be used in various real-world scenarios:

• Isolating code dependencies: By mocking external dependencies, you can test your code in isolation without worrying about flaky or unreliable behaviors from the dependency.

• Testing edge cases: You can use mocks to simulate specific scenarios that may be difficult or impossible to test in a real environment.

• Speeding up tests: Mocks can help speed up tests by isolating the code being tested and eliminating the need for external interactions.

---

Mocking Timers

Timers in Node.js, like setInterval and setTimeout, are used to delay the execution of code. Mocking timers allows you to manipulate their behavior during testing.

Why Mock Timers?

• Remove the need to wait for the actual delay, speeding up tests.

• Control the timing precisely, making tests more predictable.

Using MockTimers

To mock timers, use the MockTimers class provided by the test module. You can specify which timer APIs to mock, such as setTimeout or setInterval.

// Mock only setTimeout
mock.timers.enable({ apis: ["setTimeout"] });

Mocking setTimeout

// Create a mock function for the callback
const fn = mock.fn();

// Mock setTimeout with a 9999ms delay
setTimeout(fn, 9999);

// Advance the mocked time by 9999ms
mock.timers.tick(9999);

// Assert that the callback was called once
assert.strictEqual(fn.mock.callCount(), 1);

Mocking in Test Context

You can also mock timers directly in the test context object:

const fn = context.mock.fn();

context.mock.timers.enable({ apis: ["setTimeout"] });
setTimeout(fn, 9999);
context.mock.timers.tick(9999);

assert.strictEqual(fn.mock.callCount(), 1);

Real-World Applications

• Test components that rely on timers without waiting for actual delays.

• Verify the order and timing of events in time-sensitive applications.

• Control timeouts to prevent unexpected behavior during testing.

---

Mocking Dates with Node.js

What is Date Mocking?

Imagine you have a function that sends you an email reminder on your birthday. To test this function, you need to wait a whole year! That's where date mocking comes in. It allows you to pretend it's your birthday without waiting.

How to Mock Dates with MockTimers

Node.js provides a MockTimers class to mock dates. You can use it in your tests like this:

// Import the MockTimers class.
import { test, MockTimers } from 'node:test';

// Create a MockTimers object.
const mockTimers = new MockTimers();

// Use the MockTimers object to mock the Date object.
mockTimers.enable({ apis: ['Date'] });

// Mock the current date to be January 1st, 2023.
mockTimers.setTime(1640995200000);

Features of Date Mocking

• Set initial date: You can choose what date to start with.

• Advance in time: Use mockTimers.tick(milliseconds) to move forward in time.

• Set specific time: Use mockTimers.setTime(milliseconds) to jump to a specific point in time.

• Run all timers: Use mockTimers.runAll() to execute all pending timers at once.

Real-World Applications of Date Mocking

• Testing functions that depend on specific dates, like reminders or scheduling tasks.

• Simulating time-sensitive events for testing purposes, like expiring tokens or timed processes.

• Isolating timing-related issues in your code.

Example: Testing a Reminder Function

// Mock the current date to be January 1st, 2023.
mockTimers.setTime(1640995200000);

// Create a reminder function to send an email on a specific date.
const remind = (date) => {
  if (Date.now() >= date) {
    // Send email reminder.
  }
};

// Set the reminder date to be one day in the future.
const reminderDate = Date.now() + (24 * 60 * 60 * 1000);

// Call the reminder function.
remind(reminderDate);

// Move forward in time by one day.
mockTimers.tick(24 * 60 * 60 * 1000);

// Assert that the reminder was sent.
// ... (your assertion code here)

Tips

• Consider dependencies: If your code uses both timers and dates, mocking both can be complex.

• Use sparingly: Avoid mocking dates excessively, as it can make your tests less reliable.

---

Test Reporters in Node.js

When running tests in Node.js, you can use reporters to display the results in different formats. These reporters are helpful for debugging, tracking progress, and generating reports.

Built-in Reporters

Node.js comes with several built-in reporters:

• TAP (Test Anything Protocol): Outputs results in a machine-readable format, suitable for parsing by other tools.

• Spec: Outputs results in a human-readable format, showing test names and pass/fail status.

• Dot: Outputs a simple dot (.) for each passing test and an X for each failing test.

• JUnit: Outputs results in an XML format compatible with JUnit testing frameworks.

• LCOV: Generates coverage reports when used with the --experimental-test-coverage flag.

Default Reporter

By default, Node.js uses the Spec reporter if you're running tests in a terminal window (TTY). Otherwise, it uses the TAP reporter.

Custom Reporters

You can also create your own custom reporters. To do this, implement the IReporter interface:

interface IReporter {
  report(testRunResult: TestRunResult): void;
  write(text: string): void;
  error(error: Error): void;
}

The report method is called with the test run results, and you can use the write and error methods to output text or error messages.

Example Usage

Here's an example of how to use the TAP reporter:

import { run } from "node:test";
import { tap } from "node:test/reporters";

run({
  reporter: tap,
  filter: {
    pattern: "my-test-file.js",
  },
});

This will run all tests in the my-test-file.js file and output the results in TAP format.

Real-World Applications

Test reporters have several real-world applications, including:

• Debugging: Reporters can help you identify failed tests and track progress.

• Reporting: Reporters can generate reports that can be used for tracking test coverage, generating statistics, or sharing with stakeholders.

• Integration with CI/CD pipelines: Reporters can be used to automatically generate reports and fail builds based on test results.

---

Custom Reporters

In Node.js, you can create custom reporters to receive events from the test runner and display them in your own way. This allows you to customize the output and handle events in a way that suits your needs.

How to create a custom reporter:

You can use --test-reporter flag to specify the path to your custom reporter module. The module should export a function that accepts an input stream of events and returns an output stream of transformed events.

Example of a custom reporter (as a Transform stream):

import { Transform } from "node:stream";

const customReporter = new Transform({
  writableObjectMode: true,
  transform(event, encoding, callback) {
    switch (event.type) {
      case "test:dequeue":
        callback(null, `test ${event.data.name} dequeued`);
        break;
      case "test:enqueue":
        callback(null, `test ${event.data.name} enqueued`);
        break;
      // Other event handling cases...
    }
  },
});

module.exports = customReporter;

Example of a custom reporter (as a generator):

async function* customReporter(source) {
  for await (const event of source) {
    switch (event.type) {
      case "test:dequeue":
        yield `test ${event.data.name} dequeued`;
        break;
      case "test:enqueue":
        yield `test ${event.data.name} enqueued`;
        break;
      // Other event handling cases...
    }
  }
}

module.exports = customReporter;

Usage:

To use your custom reporter, run the test runner with the following command:

node --test-reporter ./path/to/custom-reporter.js

Potential Applications:

• Display test results in a custom HTML format

• Generate a test coverage report using a custom tool

• Send test results to a remote server

• Integrate with a continuous integration system

---

Multiple Reporters

When you run tests in Node.js, you can get results in different formats, like a list of tests (spec) or dots (dot).

Using Multiple Reporters

You can use multiple reporters to get results in all the formats you want. To do this, you use the --test-reporter flag multiple times, like this:

node --test-reporter=spec --test-reporter=dot

Specifying a Destination

For each reporter, you need to specify a place to put the results. This is called a "destination" and can be:

• stdout: Shows results in the terminal window

• stderr: Also shows results in the terminal window, but in a different color

• A file path: Saves results to a file

Matching Reporters and Destinations

The reporters and destinations are matched up in the order you specify them. For example:

node --test-reporter=spec --test-reporter=dot --test-reporter-destination=file.txt --test-reporter-destination=stdout

In this case, the spec reporter will output to file.txt, and the dot reporter will output to the terminal window.

Default Destination

If you only specify one reporter, it will default to stdout. However, you can still specify a destination if you want to:

node --test-reporter=spec --test-reporter-destination=file.txt

Real-World Applications

Using multiple reporters is useful when you want to have different formats of results for different purposes, such as:

• Spec reporter for detailed test results

• Dot reporter for live updates during testing

• HTML reporter for publishing test results online

---

run([options])

• options {Object} Configuration options for running tests. The following properties are supported:

  • concurrency {number|boolean} The number of test processes to run in parallel. If true, it would run os.availableParallelism() - 1 test files in parallel. If false, it would only run one test file at a time. Default: false.

  • files: {Array} An array containing the list of files to run. Default matching files from [test runner execution model][].

  • inspectPort {number|Function} Sets inspector port of test child process. This can be a number, or a function that takes no arguments and returns a number. If a nullish value is provided, each process gets its own port, incremented from the primary's process.debugPort. Default: undefined.

  • only: {boolean} If truthy, the test context will only run tests that have the only option set

  • setup {Function} A function that accepts the TestsStream instance and can be used to setup listeners before any tests are run. Default: undefined.

  • signal {AbortSignal} Allows aborting an in-progress test execution.

  • testNamePatterns {string|RegExp|Array} A String, RegExp or a RegExp Array, that can be used to only run tests whose name matches the provided pattern. Test name patterns are interpreted as JavaScript regular expressions. For each test that is executed, any corresponding test hooks, such as beforeEach(), are also run. Default: undefined.

  • timeout {number} A number of milliseconds the test execution will fail after. If unspecified, subtests inherit this value from their parent. Default: Infinity.

  • watch {boolean} Whether to run in watch mode or not. Default: false.

  • shard {Object} Running tests in a specific shard. Default: undefined.

    • index {number} is a positive integer between 1 and <total> that specifies the index of the shard to run. This option is required.

    • total {number} is a positive integer that specifies the total number of shards to split the test files to. This option is required.

Returns: {TestsStream}

Real World Example:

import { tap } from "node:test/reporters";
import { run } from "node:test";
import process from "node:process";
import path from "node:path";

// Run tests in watch mode, tap reporter provides structured test output
run({ files: [path.resolve("./tests/test.js")], watch: true })
  .compose(tap)
  .pipe(process.stdout);

Real World Complete Code Implementation:

import { tap } from "node:test/reporters";
import { run } from "node:test";
import path from "node:path";

// Run tests in watch mode, tap reporter provides structured test output
run({ files: [path.resolve("./tests/test.js")], watch: true })
  .compose(tap)
  .pipe(process.stdout);

---

Simplified Explanation of test() Function from Node.js's Test Module

What is test()?

test() is a function provided by Node.js to help you write and run tests for your code. It allows you to create individual tests within your codebase and check if they pass or fail.

Parameters of test():

• name: A short description of your test.

• options: Optional settings for your test, such as whether to skip it or set a timeout.

• fn: The actual test code you want to run.

What does test() do?

When you call test(), it creates a new test and runs the code you provide in the fn parameter. The test will either pass or fail based on the outcome of your code.

TestContext Object:

The fn parameter receives a TestContext object that provides you with various methods to interact with the test. These methods include:

• Skipping the test: t.skip()

• Setting a timeout: t.timeout(milliseconds)

• Creating subtests: t.test()

How to use test():

Write a function that runs your test code, then call test() with the following syntax:

test("Test Name", async (t) => {
  // Your test code goes here
});

Example:

test("My Test", async (t) => {
  const expected = 10;
  const actual = 5 + 5;

  t.equal(actual, expected);
});

In this example, we're testing if the sum of 5 and 5 equals 10. The t.equal() method ensures that the actual value (10) is equal to the expected value (10). If they match, the test passes; otherwise, it fails.

Applications in the Real World:

test() is essential for ensuring the reliability and correctness of your codebase. It allows you to write comprehensive tests that cover different scenarios and edge cases, giving you confidence that your code works as intended.

---

Sure, here's a simplified explanation of test.skip function:

test.skip is a function provided by the test module in Node.js for skipping tests. It's useful when you want to temporarily disable a test or group of tests without having to delete or comment them out.

Syntax:

test.skip([name][, options][, fn])

Parameters:

• name: (optional) Name of the test.

• options: (optional) Options object.

• fn: (optional) Test function.

Example:

// Skip a single test
test.skip("my test", () => {
  // Test code goes here
});

// Skip a group of tests
describe("My tests", () => {
  test.skip("test 1", () => {
    /* ... */
  });
  test.skip("test 2", () => {
    /* ... */
  });
});

Real-World Applications:

test.skip can be useful in several scenarios:

• Skipping incomplete tests: If you have a test that's not yet complete or does not have proper assertions, you can skip it to avoid failing the entire test suite.

• Skipping tests based on environment: You can use test.skip to skip tests that are specific to a particular environment, such as a development or production environment.

• Skipping tests temporarily: Sometimes you may need to temporarily disable a test or group of tests while debugging or refactoring code. test.skip allows you to do this without having to manually comment them out.

Tips:

• You can use test.skip.only to skip only a single test or group of tests, ignoring all others.

• You can also use the --skip command-line flag when running tests to skip all tests marked with test.skip.

---

test.todo([name][, options][, fn])

The test.todo() function is used to mark a test as a "TODO" item. This means that the test is not yet implemented, but you intend to implement it in the future.

The syntax for test.todo() is as follows:

test.todo([name][, options][, fn])

The following parameters are supported:

• name: The name of the test.

• options: An object containing options for the test. The following options are supported:

  • todo: A boolean value indicating whether or not the test is a TODO item.

• fn: The function to be executed when the test is run.

The following example shows how to use test.todo() to mark a test as a TODO item:

test.todo('my test', function() {
  // Implement the test here.
});

When a test is marked as a TODO item, it will be skipped when the test suite is run. However, the test will still appear in the test results, with a status of "TODO".

Real-world applications

TODO items can be useful for tracking the progress of your test suite. They can also be used to remind yourself of tests that need to be implemented in the future.

For example, you might use a TODO item to mark a test that you know will be difficult to implement. This will allow you to keep track of the test and make sure that it gets implemented eventually.

Potential applications

• Tracking the progress of your test suite.

• Reminding yourself of tests that need to be implemented in the future.

• Marking tests that are known to be difficult to implement.

---

test.only([name][, options][, fn])

What it is:

test.only() is a function that lets you run only a specific test or group of tests in your test suite. This can be useful when you are debugging a specific part of your code or when you want to focus on a particular set of tests.

How it works:

test.only() takes the same arguments as the test() function, but it adds a only option. When you set the only option to true, only the tests that you specify will run.

test.only('my test', () => {
  // Test code
});

In this example, only the 'my test' will run. All other tests in the test suite will be skipped.

Why you would use it:

You would use test.only() when you want to:

• Debug a specific part of your code

• Focus on a particular set of tests

• Run a specific test or group of tests before deploying your code

Real-world example:

Let's say you are working on a new feature for your application. You have written a number of tests to verify that the new feature works as expected. You can use test.only() to run only the tests that are related to the new feature. This will help you to focus on the specific code that you have changed.

test.only('new feature tests', () => {
  // Tests for the new feature
});

Potential applications:

test.only() can be used in a variety of ways, including:

• Debugging code

• Testing specific features

• Verifying that code changes do not break existing functionality

• Running tests before deploying code

---

describe([name][, options][, fn])

Overview

In Node.js, we have the test module that comes with Jest-like assertions and test runner.

The describe() function allows you to group related tests into suites, which helps organize your test code and improve its readability.

Syntax

describe([name][, options][, fn]): Promise<undefined>;

Parameters

• name (optional): The name of the suite.

• options (optional): Configuration options for the suite.

• fn (optional): A function that declares all subtests and subsuites within the suite.

Return Value

A Promise that is immediately fulfilled with undefined.

Function Definition

The describe() function can be used in two ways:

• As a decorator:

@describe("My Suite")
class MySuite {
  @test("subtest 1")
  subtest1() {}

  @test("subtest 2")
  subtest2() {}
}

• As a function:

describe("My Suite", () => {
  test("subtest 1", () => {});
  test("subtest 2", () => {});
});

Real-World Example

Let's say we have a simple calculator class that we want to test. We can use describe() to group our tests by operation:

describe("Calculator", () => {
  describe("Addition", () => {
    test("adds two numbers", () => {
      const calculator = new Calculator();
      expect(calculator.add(1, 2)).toBe(3);
    });

    test("adds multiple numbers", () => {
      const calculator = new Calculator();
      expect(calculator.add(1, 2, 3)).toBe(6);
    });
  });

  describe("Subtraction", () => {
    test("subtracts two numbers", () => {
      const calculator = new Calculator();
      expect(calculator.subtract(3, 1)).toBe(2);
    });

    test("subtracts multiple numbers", () => {
      const calculator = new Calculator();
      expect(calculator.subtract(6, 2, 1)).toBe(3);
    });
  });
});

Potential Applications

Grouping tests into suites can be useful in various scenarios:

• Organizing large test suites: Helps break down large suites into smaller, more manageable chunks.

• Enhancing code readability: Improves the structure and flow of your test code, making it easier to understand and maintain.

• Facilitating test maintenance: Allows for easier identification and updating of specific tests within a suite.

---

Simplified explanation:

describe.skip() is a function that "skips" a suite of tests, meaning that they will not be run. It's useful for temporarily disabling tests that are not currently working or for skipping tests that are not relevant to the current situation.

Detailed explanation:

A "suite" in testing is a group of related tests that are executed together. describe.skip() allows you to skip a suite of tests by passing in the name of the suite and an optional callback function.

The following code shows how to use describe.skip():

describe.skip("My Suite", function () {
  it("should do something", function () {
    // This test will be skipped
  });
});

In this example, the suite named "My Suite" will be skipped, along with all the tests inside it.

You can also pass in an options object to describe.skip(), like this:

describe.skip("My Suite", { skip: true }, function () {
  it("should do something", function () {
    // This test will be skipped
  });
});

The skip option is set to true to explicitly indicate that the suite should be skipped.

Real-world example:

One potential application of describe.skip() is to skip tests that are not relevant to the current platform or environment. For example, if you have a suite of tests that use a particular API that is not available on all platforms, you could use describe.skip() to skip those tests on the platforms where the API is not available.

Another potential application is to skip tests that are known to be broken or unstable. By skipping these tests, you can avoid having your test suite fail due to these issues.

Improved code snippet:

Here is an improved version of the code snippet above that includes an options object:

describe.skip("My Suite", { skip: true }, function () {
  it("should do something", function () {
    // This test will be skipped
  });
});

---

Simplified Explanation:

The describe.todo() function is used to create a test case that is marked as "To Do." This means that the test has not yet been written or implemented.

Usage:

describe.todo("My test", function () {
  // This test has not yet been implemented.
});

Parameters:

• name: The name of the test suite.

• options: An optional object that can include the following properties:

  • todo: A boolean value that indicates whether the test is marked as "To Do."

• fn: The function that contains the test code.

Real-World Example:

Suppose you are working on a new feature for an application and you have not yet implemented the tests for it. You can use describe.todo() to mark the placeholder tests as "To Do":

describe.todo("New feature tests", function () {
  // Tests for the new feature have not yet been implemented.
});

This lets you keep track of the missing tests and remind you to come back and implement them later.

Potential Applications:

• Creating placeholder tests for features that are still under development.

• Marking tests that have been temporarily disabled or are known to fail.

• Documenting the scope of planned testing.

---

describe.only([name][, options][, fn])

The describe.only function is used to mark a test suite as the only suite to be run. This is useful when you want to focus on debugging a specific test or suite of tests.

The syntax for describe.only is as follows:

describe.only([name][, options][, fn])

where:

• name is the name of the test suite.

• options is an optional object that can be used to configure the test suite.

• fn is the function that contains the test suite's code.

Here is an example of how to use describe.only:

describe.only('My Suite', () => {
  it('should pass', () => {
    expect(true).to.be.true;
  });
});

In this example, the My Suite test suite will be the only suite that is run. All other test suites will be skipped.

Real-world applications

describe.only can be useful in a number of real-world scenarios, such as:

• Debugging a specific test or suite of tests. By marking a test or suite as only, you can focus on debugging that specific test or suite without having to run all of your other tests.

• Running a specific test or suite of tests in a CI/CD pipeline. By marking a test or suite as only, you can ensure that that test or suite is always run as part of your CI/CD pipeline, even if other tests are failing.

Potential applications

Here are a few potential applications for describe.only in real-world projects:

• Unit testing: You can use describe.only to focus on testing a specific unit of code, such as a function or class.

• Integration testing: You can use describe.only to focus on testing a specific integration between two or more components of your system.

• End-to-end testing: You can use describe.only to focus on testing a specific end-to-end flow through your system.

---

1. Description:

The it() function in Node.js is a shorthand for the test() function, which is used for writing tests in Node.js. Tests are used to check if a particular piece of code works as expected.

2. Usage:

To use the it() function, you call it like this:

it("should do something", () => {
  // Your test code here
});

• name: A string that describes the test.

• options: An optional object that can be used to configure the test.

• fn: A function that contains the test code.

3. Example:

Here's an example of a test using the it() function:

const assert = require("assert");

it("should return 2", () => {
  assert.strictEqual(1 + 1, 2);
});

In this example, the it() function is used to test that the sum of 1 and 1 is equal to 2. The assert module is used to check if the result of the test is correct.

4. Real-World Application:

Tests are essential for ensuring the quality of your code. By writing tests, you can check if your code works as expected, which can prevent errors and improve the reliability of your software.

Here are some potential applications of tests in real-world scenarios:

• Testing APIs to ensure they return the correct data.

• Testing web pages to ensure they load correctly and display the correct content.

• Testing database queries to ensure they return the correct rows.

---

it.skip()

The it.skip() function is a way to skip a test in Jest. It takes a name, options, and a function as arguments.

Name

The name is a string that identifies the test. It is used to group tests together and to identify them in the test runner.

Options

The options are an object that can contain the following properties:

• skip - A boolean value that indicates whether or not to skip the test. If this property is set to true, the test will be skipped.

• timeout - A number that specifies the maximum amount of time that the test can run before it is considered a failure.

• retries - A number that specifies the number of times that the test will be retried if it fails.

Function

The function is the body of the test. It is the code that will be executed when the test is run.

Example

The following example shows how to use the it.skip() function:

it.skip('should skip this test', () => {
  // Code that will not be executed
});

This test will be skipped when the test runner is executed.

Real-World Applications

The it.skip() function can be used to skip tests that are not yet implemented or that are not relevant to the current context. For example, you might skip a test that depends on a feature that has not yet been implemented.

Potential Applications

The it.skip() function can be used in a variety of applications, including:

• Skipping tests that are not yet implemented

• Skipping tests that are not relevant to the current context

• Skipping tests that are known to fail

• Skipping tests that are computationally expensive

---

it.todo([name][, options][, fn])

Brief Explanation

it.todo() is a function used for marking a test as incomplete or not yet implemented. It's similar to it(), but it specifies that the test is not ready to be run.

Detailed Explanation

Parameters:

• name: (optional) A string specifying the name of the test.

• options: (optional) An object specifying options for the test.

• fn: (optional) A function containing the test code.

Purpose:

it.todo() is used when you want to create a placeholder for a test that you intend to write in the future. It allows you to mark the test as pending without actually implementing it, helping you keep track of what needs to be done.

Real-World Use Case

Imagine you're developing a new feature for a website. You know there should be a test to verify the functionality, but you haven't written it yet. To avoid missing this test, you can use it.todo():

it.todo("verify new feature functionality");

This will create a placeholder test that will be skipped when the test suite is run. Once you're ready to implement the test, you can replace it.todo() with it().

Benefits of Using it.todo()

• Organized Test Suite: Keeps track of incomplete tests, helping you prioritize and avoid forgetting about them.

• Visibility: Lets other developers know that a test is still under development, preventing them from assuming it's complete.

• Time-Saving: Allows you to focus on other aspects of the feature development without worrying about writing a test that you're not ready for.

---

Simplified Explanation of it.only() Function:

Imagine you have a bunch of tests in your code. The it.only() function allows you to choose one test as the "only" test to run. This means that when you run your tests, only the it.only() test will be executed, and all other tests will be skipped.

Uses of it.only() Function:

You might use the it.only() function if:

• You want to focus on testing a specific part of your code.

• You want to isolate a test to find out why it's failing.

• You want to temporarily disable all other tests while working on something.

Example Usage:

it.only("should calculate the area of a circle", () => {
  // Your test code here
});

// Other tests will be skipped
it("should calculate the perimeter of a square", () => {
  // Your test code here
});

In this example, the it.only() function is used to focus on testing the calculation of the area of a circle. The other test for calculating the perimeter of a square will be skipped.

Real-World Applications:

• Debugging: You can use it.only() to isolate a failing test and focus on fixing it without having to run all the other tests.

• Maintenance: When working on a specific feature, you can use it.only() to temporarily disable all other tests that interact with that feature. This helps prevent unexpected failures during your changes.

• Focus: If you have a complex test suite and only want to run a few specific tests, you can use it.only() to select those tests.

---

What is before?

before is a function that allows you to run some code before a test suite starts running. It's useful for setting up any necessary conditions or resources before the tests run.

How to use before?

You can use before by passing a function as the first argument:

describe("tests", async () => {
  before(() => console.log("about to run some test"));
  it("is a subtest", () => {
    assert.ok("some relevant assertion here");
  });
});

The function you pass to before will be called before any of the tests in the suite run. You can use it to do anything you need to set up before the tests run.

What are the options for before?

before takes an optional second argument, which is an object that can contain the following options:

• signal: An AbortSignal object that allows you to abort an in-progress hook.

• timeout: A number of milliseconds the hook will fail after. If unspecified, subtests inherit this value from their parent.

When to use before?

before is useful for any setup that needs to be done before the tests in a suite run. This could include:

• Creating a database connection

• Loading data into a database

• Starting a server

• Creating a temporary directory

Real-world example

Here's a real-world example of using before to create a database connection before running a test suite:

describe("database tests", () => {
  before(async () => {
    // Connect to the database
    await connectToDatabase();
  });

  after(async () => {
    // Close the database connection
    await closeDatabaseConnection();
  });

  it("should insert a new record", async () => {
    // Insert a new record into the database
    await insertRecord();
  });
});

Potential applications

before can be used in any situation where you need to set up some conditions or resources before running a test suite. Some potential applications include:

• Setting up a database connection

• Loading data into a database

• Starting a server

• Creating a temporary directory

• Creating temporary files

• Loading configuration files

• Creating test doubles (e.g., mocks and spies)

Conclusion

before is a useful function for setting up any necessary conditions or resources before a test suite starts running. It can be used in a variety of situations to make your tests more reliable and easier to write.

---

after Hook

What is a hook? A hook is a special function that runs at a specific point during the testing process.

The after Hook:

• Runs after all the tests in a suite have completed, regardless of whether they passed or failed.

• Useful for cleaning up resources, closing database connections, or performing any other actions that need to be done after the tests.

How to use the after Hook:

describe("My Tests", () => {
  // Define the test suite

  // `after` hook: Will run after all tests in the suite have completed
  after(() => {
    // Cleanup code goes here
    console.log("Finished running tests!");
  });

  it("Test 1", () => {
    // Test implementation
  });

  it("Test 2", () => {
    // Test implementation
  });
});

Real-World Example:

In a database testing suite, you could use the after hook to close the database connection after all tests have run:

after(() => {
  database.close();
});

Applications in Real World:

• Closing database connections

• Clearing temporary files or resources

• Resetting application state

• Logging test results or cleanup activities

---

beforeEach() is a function in Node.js's test module that allows you to run code before each subtest in a suite of tests.

Parameters:

• fn: The function to run before each subtest. This function can be either synchronous or asynchronous.

• options (optional): An object containing configuration options for the hook. The following properties are supported:

  • signal: An AbortSignal object that allows you to abort an in-progress hook.

  • timeout: A number of milliseconds after which the hook will fail. If unspecified, subtests inherit this value from their parent.

Example:

describe("tests", async () => {
  beforeEach(() => {
    console.log("about to run a test");
  });

  it("is a subtest", () => {
    assert.ok("some relevant assertion here");
  });
});

In this example, the beforeEach() function will print a message to the console before each subtest in the 'tests' suite.

Real-world applications:

beforeEach() can be used to:

• Set up test data

• Create mocks

• Start a server or database

• Perform any other actions that need to be performed before each test

Potential applications:

• Unit testing: beforeEach() can be used to set up test data and create mocks for unit tests.

• Integration testing: beforeEach() can be used to start a server or database for integration tests.

• End-to-end testing: beforeEach() can be used to perform any actions that need to be performed before each end-to-end test.

---

afterEach

The afterEach function is used to create a hook that runs after each subtest of the current test.

Syntax:

afterEach([fn][, options])

Parameters:

• fn: The hook function. It can be a regular function or an async function. If the hook uses callbacks, the callback function is passed as the second argument.

• options: Configuration options for the hook. The following properties are supported:

  • signal: AbortSignal - Allows aborting an in-progress hook.

  • timeout: number - A number of milliseconds the hook will fail after. If unspecified, subtests inherit this value from their parent.

Return value:

The afterEach function does not return a value.

Usage:

The afterEach hook is used to perform cleanup or other actions after each subtest of the current test.

For example, the following code logs a message to the console after each subtest:

describe("tests", async () => {
  afterEach(() => console.log("finished running a test"));
  it("is a subtest", () => {
    assert.ok("some relevant assertion here");
  });
});

Real-world example:

The afterEach hook can be used to perform cleanup actions after each test. For example, the following code deletes a temporary file after each test:

const fs = require("fs");

describe("tests", async () => {
  afterEach(() => {
    fs.unlinkSync("/tmp/test.txt");
  });
  it("is a subtest", () => {
    fs.writeFileSync("/tmp/test.txt", "Hello world!");
    assert.ok("some relevant assertion here");
  });
});

Potential applications:

The afterEach hook can be used for a variety of purposes, including:

• Performing cleanup actions after each test

• Logging information about each test

• Checking the test environment for errors

I hope this simplified explanation is helpful. Please let me know if you have any other questions.

---

Class: MockFunctionContext

The MockFunctionContext class is used to inspect or manipulate the behavior of mocks created via the [MockTracker][] APIs.

Simplified Explanation:

Imagine you have a function that you want to test, but the function relies on other functions or services that are difficult to test directly. The MockFunctionContext allows you to create mock versions of these other functions or services so that you can test your main function independently.

Real-World Example:

Let's say you have a function that interacts with a database. To test this function, you could use the MockFunctionContext to create a mock database that returns specific data for your test cases. This allows you to verify that your function behaves as expected without having to worry about the actual database implementation.

Code Implementation:

const {
  MockFunctionContext,
} = require("@google-cloud/functions-framework/testing");
const myFunction = require("./my-function");

// Create a mock instance of the database service
const mockDatabase = new MockFunctionContext({
  database: {
    get: (cb) => {
      // Return a mock response from the database
      cb(null, { data: "test data" });
    },
  },
});

// Test the function using the mock database
myFunction(mockDatabase.callback());

Potential Applications:

• Testing functions that rely on external services or databases

• Isolating the behavior of specific functions for unit testing

• Mocking out error conditions to test error handling

---

ctx.calls

Simplified Explanation:

ctx.calls is like a record-keeper for your mocked function. It stores information about all the times someone called the function, including what arguments were passed, what was returned, and if there were any errors.

Detailed Explanation:

ctx.calls is an array of objects. Each object represents a single call to the mocked function. The object has the following properties:

• arguments: An array of the arguments passed to the function.

• error: If the function threw an error, this property contains the error message.

• result: The value returned by the function.

• stack: A stack trace that shows where the function was called from.

• target: If the function is a constructor, this property contains the class that was constructed.

• this: The this value of the function when it was called.

Real-World Code Example:

const assert = require("assert");

const mock = sinon.mock();

// When the mock function is called with any arguments, return the value 42.
mock.onCall(0).returns(42);

// When the mock function is called with the arguments (1, 2), return the value 43.
mock.onCall(1).withArgs(1, 2).returns(43);

// Now let's call the mock function a few times and check the `ctx.calls` property.
const result1 = mock(5, 10);
const result2 = mock(1, 2);
const result3 = mock(3, 4);

assert.deepEqual(ctx.calls[0].arguments, [5, 10]);
assert.equal(ctx.calls[0].result, 42);
assert.deepEqual(ctx.calls[1].arguments, [1, 2]);
assert.equal(ctx.calls[1].result, 43);
assert.deepEqual(ctx.calls[2].arguments, [3, 4]);
assert.equal(ctx.calls[2].result, undefined);

Potential Applications:

• Testing: Verifying that a function was called with the expected arguments and returned the expected value.

• Debugging: Finding out what arguments were passed to a function and what value it returned.

• Profiling: Measuring the performance of a function by tracking how many times it was called and how long it took to execute.

---

Explanation:

The callCount() function in the test module returns the number of times a mock function has been called.

Simplified Explanation:

Imagine you have a fake function that pretends to be a real function. Whenever you call the fake function, it counts how many times it has been called.

Code Snippet:

const { test, mock } = require("ava");

test("foo", (t) => {
  const foo = mock((a, b) => a + b);

  // Call foo three times
  foo(1, 2);
  foo(3, 4);
  foo(5, 6);

  // Check how many times foo was called
  t.is(foo.callCount, 3);
});

Real-World Applications:

• Unit Testing: To ensure that code interacts with other functions the expected number of times.

• Dependency Injection: To verify that dependencies are being injected into code as expected.

• Performance Testing: To measure the number of times a function is called in a given scenario.

---

ctx.mockImplementation(implementation)

• implementation {Function|AsyncFunction} The function to be used as the mock's new implementation.

This function is used to change the behavior of an existing mock.

Imagine you have a function that you want to test, but it depends on another function that you don't want to test. You can use a mock function to replace the other function, and then you can control the behavior of the mock function to test the first function.

Here is an example of how to use ctx.mockImplementation() to change the behavior of a mock function:

const t = require("tap");

const fn = t.mock.fn(function (a, b) {
  return a + b;
});

fn(1, 2); // 3

fn.mockImplementation(function (a, b) {
  return a - b;
});

fn(1, 2); // -1

In this example, we first create a mock function and call it with the arguments 1 and 2. The mock function returns 3. We then change the implementation of the mock function to a new function that returns the difference of its arguments. We call the mock function again with the arguments 1 and 2, and it now returns -1.

Real-world applications

Mocking is a powerful technique that can be used in a variety of real-world applications. Here are a few examples:

• Testing: Mocking can be used to test functions that depend on other functions that are difficult or impossible to test. For example, you could use a mock function to replace a function that makes a network request, or a function that reads from a database.

• Debugging: Mocking can be used to debug functions by isolating the behavior of individual functions. For example, you could use a mock function to replace a function that is causing an error, and then you could step through the code of the mock function to see what is going wrong.

• Performance testing: Mocking can be used to test the performance of functions by simulating the behavior of other functions. For example, you could use a mock function to replace a function that makes a network request, and then you could use a performance testing tool to measure the time it takes for the function to complete.

Conclusion

ctx.mockImplementation() is a powerful function that can be used to change the behavior of existing mocks. This function can be used for testing, debugging, and performance testing.

---

ctx.mockImplementationOnce(implementation[, onCall])

This function is used to change the behavior of an existing mock for a single invocation.

Parameters:

• implementation: The function to be used as the mock's implementation for the invocation number specified by onCall.

• onCall: The invocation number that will use implementation. If the specified invocation has already occurred then an exception is thrown. Default: The number of the next invocation.

Example:

// Create a mock function
const mockFn = jest.fn();

// Call the mock function
mockFn();

// Change the mock implementation for the next invocation
mockFn.mockImplementationOnce(() => 10);

// Call the mock function again
console.log(mockFn()); // 10

// The mock function reverts to its original implementation
console.log(mockFn()); // undefined

Real-world Applications:

• Mocking a function that returns different values for different invocations.

• Controlling the behavior of a function during a specific test case.

• Stubbing out a function that performs asynchronous operations, such as making HTTP requests.

---

Simplified Explanation of ctx.resetCalls()

What is ctx.resetCalls()? In testing, we use mock functions to simulate real functions. When a mock function is called, it records the details of the call, such as the arguments passed and the return value. ctx.resetCalls() is a method that clears this call history.

Why use ctx.resetCalls()? It's useful to reset the call history between tests to ensure that each test starts with a clean slate. This prevents previous call information from interfering with the current test's results.

How to use ctx.resetCalls()?

Simply call ctx.resetCalls() on the mock function:

const mockFunction = sinon.mock();
mockFunction.resetCalls();

Real-World Example

Let's say we have a function greet(name) that returns a greeting. We write a test using Sinon's mock function to verify that the function is called with the correct name.

const greet = (name) => `Hello, ${name}!`;

describe("greet", () => {
  it("should greet the person by name", () => {
    const mockGreet = sinon.mock(greet);
    // Verify that the function is called with the argument "Alice"
    mockGreet.expects("greet").withArgs("Alice");

    greet("Alice"); // Call the actual function

    mockGreet.verify(); // Assert that the expected call was made
  });
});

To ensure that this test doesn't interfere with other tests, it's important to reset the call history using ctx.resetCalls() before each test:

beforeEach(() => {
  mockGreet.resetCalls();
});

Potential Applications

ctx.resetCalls() is useful in any testing scenario where you need to:

• Isolate tests from each other

• Ensure that mock functions are not affected by previous calls

• Reset the state of mock functions between tests

---

ctx.restore()

Explanation:

Imagine your mock function as a puppet. You can control the puppet's behavior by setting up expectations and responses. But sometimes, you want to reset the puppet to its original behavior, like if you want to use it for a different test. That's what ctx.restore() does.

Code Snippet to Use ctx.restore():

// Create a mock function
const myMock = jest.fn();

// Set up an expectation for the mock function
myMock.mockImplementation(() => "Hello");

// Call the mock function
myMock(); // Returns 'Hello'

// Restore the mock function to its original behavior
myMock.mockRestore();

// Call the mock function again
myMock(); // Returns `undefined` (original behavior)

Real-World Example:

Suppose you have a service that performs a calculation. You want to test that the service is using the correct input to perform the calculation. You can use the following steps:

1. Create a mock function to replace the original function used by the service.

2. Set up expectations on the mock function to ensure that it's called with the correct input.

3. Perform the test and assert that the mock function was called with the expected input.

4. Reset the mock function to its original behavior using ctx.restore().

This ensures that the service is using the correct input and that the mock function is not interfering with the normal operation of the service.

Potential Applications of ctx.restore():

• Resetting mock functions to their original behavior after a test.

• Allowing mock functions to be reused for multiple tests.

• Isolating the effects of mock functions to specific tests.

---

MockTracker

Simplified Explanation:

Imagine you're building a game where you pretend to drive a race car. The MockTracker is like a special tool that lets you give imaginary commands to the car and make it do whatever you want. It allows you to test the game without actually having a real race car.

Technical Details:

The MockTracker class helps you create fake or "mock" versions of objects and functions in your code. This is useful for testing because it allows you to control how the mock objects behave, making it easier to check that your code is working correctly.

Usage

Real-World Example:

Let's say you have a function that fetches data from a server. Instead of actually connecting to the server, you can use a MockTracker to create a mock version of the function that returns the data you specify. This way, you can test your code's logic without having to worry about external factors like network issues.

Complete Code Implementation:

// Create a mock function using MockTracker
const mockFetchData = test.mock.fn();
// Specify the data to return when the function is called
mockFetchData.mockImplementation(() => ({ id: 1, name: "John Doe" }));

// Use the mock function in your test
test("get user data", () => {
  const userData = getUserData(mockFetchData);
  expect(userData).toEqual({ id: 1, name: "John Doe" });
});

Potential Applications

• Testing Code Isolation: Create mock objects to isolate the behavior of specific components, allowing you to test their functionality independently.

• Simulating External Dependencies: Mock external APIs or services to test your code's behavior without relying on actual integrations.

• Creating Predictable Data: Use mocks to return specific data sets or behaviors, ensuring that your tests have consistent input and results.

---

Mocking Functions with test.mock.fn()

What is a Mock Function?

A mock function is a fake version of a real function that you can create to control and verify its behavior in your tests.

Creating a Mock Function with test.mock.fn()

To create a mock function, use the test.mock.fn() function. You can provide two optional arguments:

1. Original Function: An existing function that you want to mock. This is optional, and if not provided, a no-op function will be used.

2. Implementation: A function that you want to use as the mock behavior. This is also optional, and if not provided, the original function will be used as the implementation.

Configuration Options

You can also provide an optional options object to configure the mock function:

• times: The number of times the mock function will use the implementation before restoring to its original behavior.

Using the Mock Function

Once you have created a mock function, you can use it just like a regular function. The mock function will automatically record the number of times it has been called, the arguments it has been called with, and the values it has returned.

Example:

// Create a mock function to increment a counter
const counterMock = test.mock.fn();

// Call the mock function several times
counterMock();
counterMock();
counterMock();

// Check how many times the mock function has been called
const callCount = counterMock.mock.calls.length; // 3

// Get the arguments of the first call to the mock function
const firstCallArgs = counterMock.mock.calls[0]; // []

Real-World Applications

Mock functions are useful in testing for several reasons:

• They allow you to control the behavior of external dependencies, such as HTTP requests or database queries.

• They help you verify that your code interacts with dependencies as expected.

• They can help you isolate and test specific parts of your code.

---

Simplified Explanation:

mock.getter() is a helper function that sets up a mock for a getter method (a method that gets a property value) within an object.

Detailed Explanation:

• Mock: A mock is a fake object or function that simulates the behavior of a real object, allowing you to test code that depends on it without relying on the actual implementation.

• Getter Method: A getter method is a special type of method that returns the value of a property. For example, the getAge method is a getter for the age property.

• Implementation: The implementation parameter specifies the behavior the mock should have when the getter method is called. It can be a real function or a value that should be returned by the mock.

• Options: The options parameter allows you to customize the mock behavior, including whether or not the mock should be called as a getter.

Code Example:

// Create a mock object
const mockObject = mock();

// Mock the "getAge" getter method
mock.getter(mockObject, 'getAge').returns(20);

// Call the "getAge" getter method on the mock object (dot notation)
const age = mockObject.getAge;

// Call the "getAge" getter method on the mock object (bracket notation)
const age = mockObject['getAge'];

// Assert that the mock was called with the correct arguments
expect(mockObject.getAge).toHaveBeenCalled();

Real-World Applications:

• Testing Class Implementations: You can use mock getters to verify that getter methods in your classes return the expected values.

• Simulating Object Behavior: You can use mock getters to simulate the behavior of external objects or services that are not available or difficult to access during testing.

• Isolating Code Dependencies: You can use mock getters to isolate code that depends on specific getters, making it easier to test specific functionality.

---

Mocks

What are mocks?

Mocks are ways to fake or modify the behavior of methods in an object. They are commonly used in testing to test specific behaviors of objects without having to create the actual implementation of the methods. For example, you could create a mock to simulate the behavior of a database, so that you could test your code without having to set up a real database.

How to create a mock?

You can create a mock by using the mock.method function. This function takes three arguments:

1. object: The object whose method you want to mock.

2. methodName: The name of the method you want to mock.

3. implementation: The function that you want to use as the mock implementation.

For example, the following code shows how to create a mock for the subtract method of the number object:

const number = {
  value: 5,
  subtract(a) {
    return this.value - a;
  },
};

test("spies on an object method", (t) => {
  t.mock.method(number, "subtract", (a) => a + 1);
  assert.strictEqual(number.subtract(3), 4);
});

What can you do with mocks?

Once you have created a mock, you can use it to do the following:

• Inspect the calls that have been made to the mocked method.

• Control the behavior of the mocked method.

• Restore the original behavior of the mocked method.

For example, the following code shows how to inspect the calls that have been made to the subtract method of the number object:

const number = {
  value: 5,
  subtract(a) {
    return this.value - a;
  },
};

test("spies on an object method", (t) => {
  t.mock.method(number, "subtract");
  number.subtract(3);
  const calls = number.subtract.mock.calls;
  assert.deepStrictEqual(calls, [[3]]);
});

Real-world applications

Mocks are used in a variety of real-world applications, including:

• Testing: Mocks can be used to test the behavior of objects without having to create the actual implementation of the methods.

• Debugging: Mocks can be used to help debug code by isolating the behavior of specific methods.

• Performance testing: Mocks can be used to test the performance of code by simulating the behavior of slow or unreliable systems.

---

Simplified Explanation

What is a MockTracker?

A MockTracker is a tool that helps you create and manage "mock objects" in your tests. Mock objects are fake versions of real objects that you can control and test separately.

What does mock.reset() do?

mock.reset() is a function that:

• Resets all mock objects created by the MockTracker back to their default behavior.

• Removes the mock objects from the MockTracker so you can no longer interact with them through the tracker.

• However, the mock objects still exist and can be used independently.

Why is mock.reset() useful?

mock.reset() is useful because it allows you to:

• Clean up your tests: After each test, you can reset all mock objects to make sure they don't interfere with subsequent tests.

• Isolate test cases: By resetting mock objects, you ensure that each test case runs independently and doesn't rely on the state of mock objects from previous tests.

Example:

const { mock, MockTracker } = require('test');

const tracker = new MockTracker();
const myMock = tracker.mock('myModule', 'myFunction');

myMock.on('call', () => {
  console.log('myFunction was called');
});

// Run the test
myMock.call(); // Logs "myFunction was called"

// Reset the mock after the test
tracker.reset();

myMock.call(); // Doesn't log anything (because the mock was reset)

Applications in the Real World:

mock.reset() is commonly used in:

• Unit testing to isolate test cases and ensure that mock objects don't interfere with each other.

• Integration testing to clean up mock objects after each test scenario.

---

mock.restoreAll() function in Node.js

The mock.restoreAll() function is used to restore the original behavior of all mocks that were previously created by the MockTracker. This means that any changes made to the mocks will be undone, and they will behave as if they were never created.

Unlike the mock.reset() function, mock.restoreAll() does not disassociate the mocks from the MockTracker instance. This means that the mocks can still be used to track calls and verify expectations, but they will no longer affect the behavior of the code under test.

Syntax

mock.restoreAll(): void;

Example

The following example shows how to use the mock.restoreAll() function:

const mock = new MockTracker();
const myMock = mock.mock("myMock");

myMock.expects("foo").once();
myMock.foo();

// Restore the original behavior of the mock
mock.restoreAll();

// The mock will now behave as if it was never created
myMock.foo();

Real-world applications

The mock.restoreAll() function can be used in a variety of real-world applications, such as:

• Testing: To restore the original behavior of mocks after a test has been run. This can help to prevent unexpected side effects from mocks in subsequent tests.

• Debugging: To identify the cause of a problem by restoring the original behavior of mocks and then re-running the code. This can help to isolate the problem and find the root cause.

• Performance testing: To measure the performance of code without the overhead of mocks. This can help to identify bottlenecks and optimize the code.

---

mock.setter(object, methodName[, implementation][, options])

This function is a convenient way to set up a mock for a setter method. It takes the following parameters:

• object: The object to mock.

• methodName: The name of the setter method to mock.

• implementation (optional): The function to be called when the setter method is invoked.

• options (optional): An object containing configuration options for the mock.

The options object can contain the following properties:

• setter: A boolean value indicating whether the mock is for a setter method.

• value: The value to return when the setter method is invoked.

Here is an example of how to use mock.setter() to mock a setter method:

// Create a mock object.
const mockObject = sinon.mock(object);

// Set up a mock for the `setName` setter method.
mockObject.setter("setName", function (value) {
  this.name = value;
});

// Call the `setName` setter method.
object.setName("John Doe");

// Assert that the `name` property was set to 'John Doe'.
assert.strictEqual(object.name, "John Doe");

In this example, the mock.setter() function is used to create a mock for the setName setter method on the object object. When the setName method is called, the mock function will be invoked and the name property of the object object will be set to the value passed to the setName method.

Real-world applications

Mocks can be used in a variety of real-world applications, including:

• Unit testing: Mocks can be used to isolate individual units of code and test them independently of their dependencies.

• Integration testing: Mocks can be used to test how different components of a system interact with each other.

• Performance testing: Mocks can be used to create a realistic environment for performance testing, without the need for expensive hardware or software.

• Debugging: Mocks can be used to help debug problems by isolating and testing specific parts of a system.

---

Mocking Timers in Node.js with MockTimers

MockTimers is a feature in Node.js that allows you to control and simulate timers like setInterval and setTimeout in your tests.

Why use MockTimers?

In software testing, we often want to control the timing of our tests to speed them up or make them more reliable. For example, if you have a function that triggers a timer that takes 10 seconds to execute, using MockTimers, you can simulate the 10-second delay and cause the function to execute immediately.

How to use MockTimers:

1. Require the MockTimers module:

const { MockTimers } = require("jest");

2. Create an instance of MockTimers:

const timers = new MockTimers();

3. Replace the global timers object with your mocked instance:

jest.useFakeTimers(timers);

4. Now you can control and simulate timers using the timers instance.

Methods:

• runAllTimers(): Runs all pending timers.

• runTimersToTime(time): Runs all timers that have elapsed before the specified time.

• runOnlyPendingTimers(): Runs only timers that have not yet elapsed.

• clearAllTimers(): Clears all pending timers.

• advanceTimersByTime(time): Advances the current time by the specified amount.

Real-world example:

Suppose you have the following code:

function doSomethingExpensive() {
  setTimeout(() => {
    console.log("Something expensive is done!");
  }, 10000);
}

This code triggers a timer that will log a message to the console after 10 seconds. Using MockTimers, we can test this function without waiting the full 10 seconds:

const timers = new MockTimers();
jest.useFakeTimers(timers);

test("doSomethingExpensive", () => {
  doSomethingExpensive();
  timers.runAllTimers();

  expect(console.log).toHaveBeenCalledWith("Something expensive is done!");
});

In this test, we replace the global timers object with our mocked instance, trigger the timer, and then run all pending timers. This simulates the 10-second delay and allows us to assert that the log message was printed to the console.

Potential applications:

• Speeding up tests: Shortening the time it takes for timers to execute can make tests faster.

• Testing time-based events: MockTimers allows you to reliably test functions that rely on timing, even if the timing is unreliable in the real world.

• Error handling: You can simulate timer-related errors and test how your code handles them.

• Concurrency testing: MockTimers can be used to control the execution of multiple timers in parallel and test potential race conditions or concurrency issues.

---

timers.enable([enableOptions])

What it is: This function helps you pretend to move time forward in your tests. It's like having a remote control for time! You can pause, fast-forward, or rewind the clock to test how your code behaves at different times.

How to use it: You can tell the function which timers you want to control (like setInterval or setTimeout) and even set the starting time.

Example: Let's say you have a function that calls setInterval to do something every second. To test how it behaves after 5 seconds have passed:

// Imagine this is the code you're testing
function doSomethingEverySecond() {
  console.log("Doing something");
}

// This is the test code
import { mock } from "node:test";
mock.timers.enable();
jest.fn(doSomethingEverySecond); // This is a function that will be called by setInterval
setInterval(doSomethingEverySecond, 1000); // Set up the interval to run every second
mock.timers.advance(5000); // Advance the clock by 5 seconds
expect(doSomethingEverySecond).toHaveBeenCalledTimes(5); // Check if the function was called 5 times

Real-world applications:

• Testing how your code handles different times, such as daylight savings time changes.

• Mocking time-dependent APIs, like fetch or XMLHttpRequest, to simulate real-world scenarios.

• Creating repeatable tests that don't rely on real-time.

---

timers.reset()

This function is used to restore the default behavior of all the timers that were previously created using the MockTimers instance. It also disassociates the mocks from the MockTracker instance.

In simple terms: It's like erasing all the changes you made to the timers and setting them back to their original state.

Code snippet:

import { mock } from "node:test";

// Create a mock timer
const timer = mock.timers.createTimeout(1000);

// Reset the mock timer
mock.timers.reset();

// Check if the mock timer is still active
console.log(timer.running); // false

Real-world application:

You can use this function to test code that relies on timers. For example, you can test that a function is called after a certain amount of time or that a timer is stopped when it should be.

---

timers[Symbol.dispose]

Description

timers is an array of timers that have been set using the setTimeout() or setInterval() functions.

timers[Symbol.dispose] calls timers.reset(), which clears all the timers in the timers array and removes their references from the global clearTimeout() and clearInterval() functions.

Example

// Set a timer to expire after 1 second
setTimeout(() => {
  console.log("Hello world!");
}, 1000);

// Clear the timer before it expires
timers[Symbol.dispose]();

This will prevent the message "Hello world!" from being logged to the console.

Real World Applications

timers[Symbol.dispose] can be used to prevent timers from firing when they are no longer needed. This can be useful for performance optimization and to prevent unwanted side effects.

For example, if you have a timer that updates the UI of a web page, you can clear the timer when the user navigates away from the page. This will prevent the timer from continuing to update the UI, which could lead to performance problems.

---

Timers.tick() method in Node.js test module

The timers.tick() method in the test module allows you to advance the time for all mocked timers. This is useful for testing code that relies on timers, such as setTimeout or setInterval.

Syntax:

timers.tick(milliseconds)

Parameters:

• milliseconds: The amount of time, in milliseconds, to advance the timers.

Example:

const assert = require("node:assert");
const { test } = require("node:test");

test("mocks setTimeout to be executed synchronously without having to actually wait for it", (context) => {
  const fn = context.mock.fn();

  context.mock.timers.enable({ apis: ["setTimeout"] });

  setTimeout(fn, 9999);

  assert.strictEqual(fn.mock.callCount(), 0);

  // Advance in time
  context.mock.timers.tick(9999);

  assert.strictEqual(fn.mock.callCount(), 1);
});

In this example, we are mocking the setTimeout function and using .tick to advance in time, triggering all pending timers.

Potential applications:

• Testing code that relies on timers, such as setTimeout or setInterval

• Simulating the passage of time in a test environment

• Controlling the execution of asynchronous tasks in a test environment

Real world example:

Imagine you have a function that sends a request to a server every 10 seconds. You can use the timers.tick() method to simulate the passage of time and test the function without having to wait 10 seconds for each request to complete.

const { test } = require("node:test");

test("tests a function that sends a request to a server every 10 seconds", (context) => {
  const fn = context.mock.fn();

  context.mock.timers.enable({ apis: ["setTimeout"] });

  // Mock the function that sends the request
  context.mock.fn(() => {
    // Do something...
  });

  // Advance in time by 10 seconds
  context.mock.timers.tick(10000);

  // Assert that the function was called once
  assert.strictEqual(fn.mock.callCount(), 1);
});

---

Mocking Timers with clearTimeout

Timers in JavaScript

Timers in JavaScript allow you to schedule tasks to run at a specific time in the future. The most common timers are setTimeout and setInterval.

Mocking Timers with clearTimeout

When writing tests, it's often useful to mock timers to avoid waiting for the actual timer to fire. This can speed up your tests and make them more reliable.

The clearTimeout function is used to cancel a previously scheduled timer. When you mock timers, the clearTimeout function is also mocked, allowing you to cancel the timer without actually waiting for it to fire.

Example

Here's an example of how to mock setTimeout and clearTimeout:

// Import the test module
import { test } from "node:test";

// Create a mock function for our timer
const fn = jest.fn();

// Mock the setTimeout function
jest.useFakeTimers();

// Schedule a timer to call the mock function after 10 seconds
setTimeout(fn, 10000);

// Immediately cancel the timer
clearTimeout(id);

// Advance the fake timer by 10 seconds
jest.advanceTimersByTime(10000);

// Assert that the mock function was not called
expect(fn).not.toHaveBeenCalled();

Real-World Applications

Mocking timers can be useful in a variety of real-world applications, such as:

• Testing code that relies on timers, such as UI animations or background tasks.

• Debugging code that interacts with timers.

• Speeding up integration tests that involve waiting for timers to fire.

---

Timers module in Node.js

The Node.js timers module provides an API for working with timers in your code. This allows you to schedule tasks to be executed at a specific time or after a certain delay.

Mocking timers

When writing tests, it can be useful to mock the timers module so that you can control the passage of time in your tests. This makes it easier to test code that relies on timers, without having to wait for the actual timers to fire.

Enabling mocking

To enable mocking of the timers module, you first need to enable it in your test context. You can do this with the following code:

context.mock.timers.enable();

Once you have enabled mocking, you can use the tick() method to advance the current time in your test. This is useful for testing code that runs on a timer, as it allows you to simulate the passage of time and see how your code responds.

Example

The following example shows how to mock the timers module in a Node.js test:

import assert from "node:assert";
import { test } from "node:test";
import nodeTimersPromises from "node:timers/promises";

test("mocks setTimeout to be executed synchronously without having to actually wait for it", async (context) => {
  context.mock.timers.enable();

  const timerSpy = context.mock.fn();

  setTimeout(timerSpy, 9999);
  nodeTimersPromises.setTimeout(9999);

  // Advance in time
  context.mock.timers.tick(9999);

  assert.strictEqual(timerSpy.mock.callCount(), 1);
});

In this example, we mock the setTimeout() function and the setTimeout() function from the timers/promises module. We then advance the current time by 9999 milliseconds using the tick() method. This causes the setTimeout() functions to be executed, and we can then assert that the timerSpy function was called once.

Real-world applications

Mocking the timers module can be useful in a variety of real-world applications, such as:

• Testing code that relies on timers, without having to wait for the actual timers to fire

• Simulating the passage of time in a controlled manner

• Testing code that uses the event loop

---

timers.runAll()

Function Definition

runAll(): void;

Description

The timers.runAll() function is used to trigger all pending mocked timers immediately. If the Date object is also mocked, it will also advance the Date object to the furthest timer's time.

How it Works

Timers are used in JavaScript to schedule functions to be executed at a specific time or after a specific delay. When you mock timers, you can control when these functions are executed.

The runAll() function triggers all pending mocked timers immediately. This means that any functions that were scheduled to be executed at a specific time or after a specific delay will be executed immediately.

If the Date object is also mocked, the runAll() function will also advance the Date object to the furthest timer's time. This means that the Date object will reflect the time at which the furthest timer was scheduled to be executed.

Example

The following example shows how to use the runAll() function:

import { test, expect } from "node:test";
import { mock } from "node:timers";

test("runAll functions following the given order", () => {
  const timer = mock.timers.createMock();
  timer.enable();

  const results = [];
  timer.setTimeout(() => results.push(1), 9999);

  // Notice that if both timers have the same timeout,
  // the order of execution is guaranteed
  timer.setTimeout(() => results.push(3), 8888);
  timer.setTimeout(() => results.push(2), 8888);

  expect(results).toEqual([]);

  timer.runAll();
  expect(results).toEqual([3, 2, 1]);
  expect(timer.now()).toEqual(9999);
});

Potential Applications

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

• Testing code that uses timers

• Controlling the timing of events in a simulation

• Advancing the time in a system to trigger a specific event

Conclusion

The timers.runAll() function is a powerful tool that can be used to control the timing of events in a mocked environment. It is an essential tool for testing code that uses timers.

---

timers.setTime(milliseconds)

Simplified Explanation:

You can use setTime() to change the current time for your tests. This is useful if you want to test how your code behaves at a specific time.

Detailed Explanation:

timers.setTime() sets the current Unix timestamp. The Unix timestamp is a number that represents the number of milliseconds since January 1, 1970 at midnight.

By setting the Unix timestamp, you can control the current time for your tests. This is useful if you want to test how your code behaves at a specific time.

For example, you might want to test how your code behaves when a deadline is reached. You can set the current time to be just before the deadline and then test how your code handles the deadline.

Real-World Example:

const assert = require("assert");
const { test } = require("node:test");

test("test deadline handling", (context) => {
  // Set the current time to be just before the deadline
  context.mock.timers.enable({ apis: ["Date"] });
  context.mock.timers.setTime(deadline - 1000);

  // Test how your code handles the deadline
  assert.strictEqual(handleDeadline(), "deadline reached");
});

Potential Applications:

timers.setTime() can be used in a variety of situations, including:

• Testing how your code behaves at a specific time

• Simulating delays or timeouts

• Testing how your code handles deadlines

---

Dates and Timers in Node.js Testing

Dates and Times in Node.js

• Node.js uses the Date object to represent dates and times.

• You can create a Date object by calling new Date().

• The Date object has methods to get the current time, set the time, and add or subtract time.

Timers in Node.js

• Node.js uses timers to schedule functions to run at a later time.

• You can create a timer using the setTimeout() or setInterval() functions.

• The setTimeout() function schedules a function to run once after a specified delay.

• The setInterval() function schedules a function to run repeatedly at a specified interval.

Interaction between Dates and Timers

• Dates and timers are dependent on each other.

• If you set the time of the Date object, the timers will not be affected.

• However, if you use the tick() method to advance the time of the Date object, the timers will be triggered.

Example

The following example shows how to use dates and timers in Node.js testing:

const assert = require("assert");
const { test } = require("node:test");

test("runAll functions following the given order", (context) => {
  context.mock.timers.enable({ apis: ["setTimeout", "Date"] });
  const results = [];
  setTimeout(() => results.push(1), 9999);

  assert.deepStrictEqual(results, []);
  context.mock.timers.setTime(12000);
  assert.deepStrictEqual(results, []);
  // The date is advanced but the timers don't tick
  assert.strictEqual(Date.now(), 12000);
});

In this example, we use the mock.timers.enable() function to enable timers in our test. We then create a timer using the setTimeout() function. We then use the setTime() function to advance the time of the Date object. We expect that the timer will not be triggered because the timers are not affected by the setTime() function.

Real-World Applications

Dates and timers can be used in a variety of real-world applications, such as:

• Scheduling tasks to run at a later time

• Tracking the time spent on a task

• Creating animations

• Generating random numbers

---

Class: TestsStream

Imagine you have a series of tests you want to run. You can use a TestsStream to get a real-time stream of the results of those tests.

• A TestsStream is a special kind of readable stream that emits events representing the execution of the tests.

• You can get a TestsStream by calling the run() method on an instance of the Test class.

• The events emitted by a TestsStream will be in the order of the tests definition.

• You can use a TestsStream to listen for the results of the tests and take action accordingly. For example, you could use a TestsStream to display the results of the tests in a UI or to send notifications to a team member.

Example

const { Test } = require("@google-cloud/test-platform");

const test = new Test();
const stream = test.run();

stream.on("data", (event) => {
  console.log(event);
});

stream.on("end", () => {
  console.log("All tests completed.");
});

Potential Applications

TestsStreams can be used in a variety of real-world applications, including:

• Continuous integration: TestsStreams can be used to monitor the progress of tests in a continuous integration pipeline.

• Test reporting: TestsStreams can be used to generate test reports that can be shared with stakeholders.

• Test debugging: TestsStreams can be used to help debug tests by providing a real-time view of the test execution.

---

Coverage Event in Jest

What is Coverage?

Coverage measures how much of your code is being executed during tests. This helps you identify areas that are not being covered and may contain bugs or unexpected behavior.

When is the Coverage Event Emitted?

The 'test:coverage' event is emitted after all tests have finished running and coverage collection is complete.

Event Data

The event data contains the following information:

• summary: An object with coverage statistics:

  • files: An array of coverage reports for individual files, including:

    • path: The file path

    • coveredLineCount: Number of lines covered

    • coveredBranchCount: Number of branches covered

    • coveredFunctionCount: Number of functions covered

    • coveredLinePercent: Percentage of lines covered

    • coveredBranchPercent: Percentage of branches covered

    • coveredFunctionPercent: Percentage of functions covered

  • totals: A summary of coverage for all files, including the same statistics as files.

  • workingDirectory: The directory where the tests were run.

• nesting: The level of nesting for the test (e.g., 1 for top-level tests).

Code Example

To listen for the coverage event, add the following code to your test file:

test.on("test:coverage", (coverage) => {
  console.log(coverage);
});

Real-World Applications

Coverage can be used to:

• Improve code quality by identifying areas that need more testing.

• Identify dead code (code that is never executed) that can be removed.

• Enhance test confidence by ensuring that a majority of your code is covered by tests.

Example Code

Here's an example of a test file that demonstrates how to use coverage:

const assert = require("assert");

describe("My Test", () => {
  it("should pass", () => {
    assert.strictEqual(1, 1);
  });

  it("should fail", () => {
    assert.strictEqual(1, 2);
  });
});

When you run this test, you'll see the following coverage report:

{
  summary: {
    files: [
      {
        path: '/path/to/my-test.js',
        totalLineCount: 10,
        totalBranchCount: 2,
        totalFunctionCount: 2,
        coveredLineCount: 5,
        coveredBranchCount: 1,
        coveredFunctionCount: 1,
        coveredLinePercent: 50,
        coveredBranchPercent: 50,
        coveredFunctionPercent: 50,
        functions: [],
        branches: [],
        lines: []
      }
    ],
    totals: {
      totalLineCount: 10,
      totalBranchCount: 2,
      totalFunctionCount: 2,
      coveredLineCount: 5,
      coveredBranchCount: 1,
      coveredFunctionCount: 1,
      coveredLinePercent: 50,
      coveredBranchPercent: 50,
      coveredFunctionPercent: 50
    },
    workingDirectory: '/path/to/my-project'
  },
  nesting: 1
}

This report shows that:

• 50% of the lines were covered during the tests.

• 50% of the branches were covered.

• 50% of the functions were covered.

Based on this information, you can identify areas in your code that need more testing to improve coverage and confidence in your application.

---

Dequeuing Tests in Node.js's Test Module

Event: 'test:dequeue'

What it is: This event is emitted whenever a test is removed from the queue and is about to be executed.

Data Properties:

• column: The column number where the test is defined (or undefined if run through the REPL).

• file: The path of the test file (or undefined if run through the REPL).

• line: The line number where the test is defined (or undefined if run through the REPL).

• name: The name of the test.

• nesting: The nesting level of the test (how deeply it is nested within other tests).

Usage:

This event is useful for tracking the progress of tests and for debugging when tests are not properly dequeued. Here's an example of how to use it:

// Register a listener for the 'test:dequeue' event
test.on("test:dequeue", (data) => {
  console.log(
    `Test "${data.name}" dequeued at column ${data.column}, line ${data.line} from ${data.file}`
  );
});

// Run the tests
test.run();

Output:

Test "myTest" dequeued at column 10, line 5 from my-test.js

Real-World Applications:

• Monitoring the execution order of tests for debugging purposes.

• Logging the progress of tests for reporting or tracking purposes.

• Customizing the behavior when a test is dequeued, such as adding additional setup or cleanup logic.

---

Event: 'test:diagnostic'

• data is an object that contains information about the diagnostic:

  • column is the column number where the test is defined, or undefined if the test was run through the REPL.

  • file is the path of the test file, or undefined if the test was run through the REPL.

  • line is the line number where the test is defined, or undefined if the test was run through the REPL.

  • message is the diagnostic message.

  • nesting is the nesting level of the test.

This event is emitted when [context.diagnostic][] is called.

Real-world example:

// test.js
test("my test", (t) => {
  t.diagnostic("This is a diagnostic message");
});

When this test is run, the 'test:diagnostic' event will be emitted with the following data:

{
  column: 10,
  file: 'test.js',
  line: 3,
  message: 'This is a diagnostic message',
  nesting: 0
}

You can listen for this event to handle diagnostic messages from your tests. For example, you could use this event to print diagnostic messages to the console:

test.on("test:diagnostic", (data) => {
  console.log(data.message);
});

Potential applications:

• Debugging: Diagnostic messages can be used to help debug your tests. For example, you could use diagnostic messages to print the values of variables at different points in your test.

• Reporting: Diagnostic messages can be used to generate reports about your tests. For example, you could use diagnostic messages to generate a list of all the tests that failed and the reasons for their failure.

---

Event: 'test:enqueue'

• data (object):

  • column (number or undefined): The column number where the test is defined, or undefined if the test was run in the REPL (JavaScript prompt).

  • file (string or undefined): The path of the test file, or undefined if the test was run in the REPL.

  • line (number or undefined): The line number where the test is defined, or undefined if the test was run in the REPL.

  • name (string): The test name.

  • nesting (number): The nesting level of the test (how deeply nested it is within other tests).

This event is emitted when a test is added to the queue for execution. It provides information about the test, including its name, location (file and line number), and nesting level.

Real-World Application:

In a testing framework, this event can be used to:

• Track the progress of test execution.

• Identify tests that are slow or failing.

• Generate reports on test coverage.

Example:

const { TestRunner } = require("test");

const runner = new TestRunner();

// Listen for the 'test:enqueue' event.
runner.on("test:enqueue", (data) => {
  console.log(`Test enqueued: ${data.name}`);
});

// Run the tests.
runner.run();

---

Event: 'test:fail'

What it is:

This event is triggered whenever a test fails.

Data:

The event data includes information about the failed test:

• column: The column number where the test is defined (if applicable, otherwise undefined).

• details: Additional metadata about the execution:

  • duration_ms: How long the test took to run in milliseconds.

  • error: The error that caused the test to fail.

  • type: If the test is a suite (a collection of tests).

• file: The path to the file where the test is defined (if applicable, otherwise undefined).

• line: The line number where the test is defined (if applicable, otherwise undefined).

• name: The name of the test.

• nesting: How deeply the test is nested within other tests.

• testNumber: The numerical order of the test.

• todo: If the test was marked as a TODO using context.todo().

• skip: If the test was skipped using context.skip().

Example:

import { test } from "node:test";

test("my test", async () => {
  // ...
  throw new Error("Oops"); // This will cause the test to fail
});

test.on("test:fail", (data) => {
  console.log(`Test "${data.name}" failed due to: ${data.details.error}`);
});

Real-World Applications:

You can use this event to:

• Log failed tests for further analysis.

• Trigger actions when a test fails, such as sending an alert or restarting the test suite.

• Track the performance of your tests by monitoring the duration_ms property.

---

Event: 'test:pass'

Description:

This event is emitted when a test in a test suite passes. Here's a breakdown of what the event data contains:

• column: The column number where the test is defined in the source code. This is undefined if the test was run in the REPL (interactive command window).

• details: Additional information about the test execution:

  • duration_ms: The time it took for the test to run, in milliseconds.

  • type: The type of test, such as "test" or "suite".

• file: The path to the test file. This is undefined if the test was run in the REPL.

• line: The line number where the test is defined in the source code. This is undefined if the test was run in the REPL.

• name: The name of the test.

• nesting: The nesting level of the test within the test suite.

• testNumber: The number of the test within the test suite.

• todo: A string or boolean value indicating a TODO item. This is set if context.todo() is called during the test.

• skip: A string or boolean value indicating a skipped test. This is set if context.skip() is called during the test.

Real-World Application:

This event is useful for debugging and monitoring test execution progress. For example, you can use it to:

• Identify tests that are taking excessively long.

• Track the order in which tests are running.

• Identify any skipped or TODO tests.

Example Code:

// Subscribe to the 'test:pass' event
test.on("test:pass", (data) => {
  console.log(`Test ${data.name} passed`);
});

// Run a test suite
test.run();

---

Event: 'test:plan'

The 'test:plan' event is emitted when all subtests have completed for a given test. This event is useful for determining when all tests have finished running and for gathering information about the test run.

The event data includes the following properties:

• column: The column number where the test is defined, or undefined if the test was run through the REPL.

• file: The path of the test file, undefined if test was run through the REPL.

• line: The line number where the test is defined, or undefined if the test was run through the REPL.

• nesting: The nesting level of the test.

• count: The number of subtests that have ran.

Here is an example of using the 'test:plan' event:

test("parent test", (t) => {
  t.test("child test 1", (t) => {
    t.end();
  });

  t.test("child test 2", (t) => {
    t.end();
  });

  t.end();
});

test.on("test:plan", (data) => {
  console.log(`Test ${data.file} completed at line ${data.line}.`);
});

This example will log the following output:

Test parent_test.js completed at line 1.

The 'test:plan' event can be used for a variety of purposes, such as:

• Gathering information about the test run, such as the number of tests that were run and the time it took to run the tests.

• Determining when all tests have finished running, which can be useful for triggering post-test actions, such as sending a notification or generating a report.

• Identifying which tests failed, which can be useful for debugging purposes.

---

Event: 'test:start'

• data {Object}

  • column {number|undefined} The column number where the test is defined, or undefined if the test was run through the REPL.

  • file {string|undefined} The path of the test file, undefined if test was run through the REPL.

  • line {number|undefined} The line number where the test is defined, or undefined if the test was run through the REPL.

  • name {string} The test name.

  • nesting {number} The nesting level of the test.

Emitted when a test starts reporting its own and its subtests status. This event is guaranteed to be emitted in the same order as the tests are defined.

Explanation:

The 'test:start' event is emitted when a test begins executing. It provides information about the test, such as its name, location (file and line number), and nesting level.

Real World Example:

test.on("test:start", (data) => {
  console.log(
    `Test ${data.name} started at line ${data.line} of ${data.file}.`
  );
});

Potential Applications:

• Logging test execution information

• Tracking the progress of a test suite

---

Event: 'test:stderr'

Explanation:

When you run your tests using the --test flag, Node.js will emit several events to help you debug and track your tests. One of these events is the 'test:stderr' event, which is emitted whenever one of your tests writes something to the stderr stream.

Example:

test("test with stderr", () => {
  console.error("This is an error message written to stderr");
});

When you run this test, you will see the following output in your terminal:

This is an error message written to stderr

And you will also see the 'test:stderr' event being emitted:

{
  column: undefined,
  file: 'test.js',
  line: 4,
  message: 'This is an error message written to stderr'
}

Applications:

The 'test:stderr' event can be useful for debugging your tests. For example, if you are seeing unexpected output in your terminal, you can use this event to track down the source of the output.

You can also use this event to programmatically handle stderr output from your tests. For example, you could write a script that automatically captures all stderr output and saves it to a file.

Additional Notes:

• The 'test:stderr' event is only emitted if you are running your tests using the --test flag.

• The data object passed to the event handler contains several properties, including the column number, file name, line number, and message written to stderr.

---

Event: 'test:stdout'

This event is emitted when a running test writes to the standard output stream (stdout). It is only emitted if the --test flag is passed when running the Node.js process.

Data Object

The event data object has the following properties:

• column: The column number where the test is defined, or undefined if the test was run through the REPL.

• file: The path of the test file.

• line: The line number where the test is defined, or undefined if the test was run through the REPL.

• message: The message written to stdout.

Example

The following code shows how to listen for the 'test:stdout' event:

const { test } = require("ava");

test("stdout example", (t) => {
  t.plan(1);

  console.log("Hello, stdout!");

  t.pass();
});

When you run this test, you will see the following output:

Hello, stdout!

Real-World Applications

The 'test:stdout' event can be used to:

• Assert that a test writes the expected message to stdout.

• Capture the output of a test for further analysis.

• Redirect the output of a test to a file or another stream.

Potential Applications

Here are some potential applications of the 'test:stdout' event:

• Testing the output of a command-line tool: You can write a test that runs a command-line tool and asserts that it writes the expected output to stdout.

• Capturing the output of a long-running test: You can write a test that captures the output of a long-running test and saves it to a file for later analysis.

• Redirecting the output of a test to a different stream: You can write a test that redirects the output of a test to a different stream, such as a file or another process.

---

Event: 'test⌚drained'

Explanation

When running tests in watch mode, the test runner continuously checks for changes in the code and re-runs the tests if necessary.

The 'test:watch:drained' event is emitted when there are no more tests queued for execution. This means that the test runner has finished running all the tests and is waiting for new changes.

Example

test.on("test:watch:drained", () => {
  console.log("All tests have finished running.");
});

Applications

The 'test:watch:drained' event can be used to perform cleanup tasks after all tests have finished running. For example, you could use it to close a database connection or stop a server.

---

Class: TestContext

Imagine you're running a test race with multiple runners (test functions). Each runner needs a "test context" that provides them with information and controls about the race. The TestContext is like a race organizer that gives each runner the following:

Topics and Explanations:

Current Test Title

• The TestContext knows the name of the test function you're currently running.

• Real-world use: It can be helpful for logging or debugging purposes.

Current Test Status

• The context keeps track of the status of the test. It can be "passed," "failed," or "skipped."

• Real-world use: You can use this to determine the overall test result.

Time Measuring

• The context provides a timer that allows you to measure how long a test takes.

• Real-world use: This can help you identify performance issues or optimize tests.

Test Assertions

• The context provides methods for making assertions, which are checks that verify the expected behavior of your code.

• Real-world use: Assertions help you ensure that your code is working as expected.

Test Fixtures

• The context allows you to set up and tear down "fixtures" before and after each test. Fixtures are shared resources that can be useful for testing.

• Real-world use: You can use fixtures to create test data, initialize databases, or connect to external services.

Test Metadata

• The context provides a way to store additional information about the test, such as tags or descriptions.

• Real-world use: This information can help you organize and categorize tests.

Example Code

import { assert, time } from "test";

test("Example Test", async () => {
  // Get the test title
  console.log("Test Title:", context.testName);

  // Measure the execution time
  const start = time.time();
  await doSomething();
  const end = time.time();
  console.log("Execution Time:", end - start);

  // Assert an expectation
  assert.equal(result, expectedResult);
});

Potential Applications

The TestContext is a powerful tool that can enhance your testing experience. Here are some potential applications:

• Debugging Tests: The context provides information that can help you identify and fix problems with your tests.

• Test Customization: You can use the context to configure and customize tests based on specific requirements.

• Logging and Reporting: The context allows you to capture and log test results for analysis and reporting purposes.

---

Simplified Explanation of context.before([fn][, options]):

Purpose:

context.before() lets you set up tasks to run before each subtest (child test) within the current test.

How it Works:

1. You define a function (called the "hook function") that contains the setup tasks you want to perform.

2. Call context.before() passing your hook function as an argument.

Options:

• signal: A signal that can be used to abort the hook if it's taking too long.

• timeout: The number of milliseconds after which the hook will automatically fail.

Example:

// Set up a hook to run before each subtest
test("Parent test", async (context) => {
  context.before(async (t) => {
    // Setup tasks for the subtests go here
  });

  // Subtests go here
  await t("Subtest 1", async () => {});
  await t("Subtest 2", async () => {});
});

Real-World Applications:

• Database setup: Create a database connection and populate it with data before running subtests.

• Environment cleanup: Ensure that any changes made during subtests are cleaned up before the next subtest runs.

• Resource allocation: Acquire resources (e.g., API tokens) before subtests and release them afterward.

Potential Improvements:

• Error handling: Add error handling to the hook function to prevent subtests from failing due to hook errors.

• Async functions: If your setup tasks are asynchronous, you can use an async hook function.

• Nested hooks: You can nest context.before() hooks to create a hierarchy of setup tasks.

---

beforeEach Hook

The beforeEach hook in Jest is a function that runs before each subtest within a test case. It allows you to perform setup tasks before each subtest.

Syntax:

context.beforeEach([fn][, options]);

Parameters:

• fn: The function to run before each subtest. It takes a TestContext object as its first argument.

• options: Optional configuration options:

  • signal: An AbortSignal that can be used to abort the hook.

  • timeout: A number of milliseconds after which the hook will fail. Defaults to Infinity.

Usage:

The beforeEach hook is useful for setting up reusable fixtures or performing assertions that should be true before each subtest.

Here's an example:

test("My test case", async (t) => {
  // Create a variable that will be shared between subtests
  let sharedFixture;

  // Set up the fixture before each subtest
  t.beforeEach((t) => {
    sharedFixture = createFixture();
  });

  // Use the fixture in a subtest
  t.test("Subtest 1", (t) => {
    assert.ok(sharedFixture);
  });

  // Use the fixture in another subtest
  t.test("Subtest 2", (t) => {
    assert.strictEqual(sharedFixture.value, "foo");
  });
});

In this example, the beforeEach hook is used to create a shared fixture before each subtest. The fixture is then used in the subtests to perform assertions.

Real-World Applications:

The beforeEach hook can be used in various real-world scenarios:

• Setting up database connections: You can create a database connection in the beforeEach hook and close it in the afterEach hook.

• Loading data into the database: You can load test data into the database in the beforeEach hook and clear it out in the afterEach hook.

• Creating mock objects: You can create mock objects in the beforeEach hook and use them in the subtests.

• Performing assertions that should be true before each subtest: You can use the beforeEach hook to assert that certain conditions are met before each subtest.

---

What is a hook function?

A hook function is a function that is called before or after a test or assertion. It can be used to set up or tear down the test environment, or to perform other tasks, such as logging or collecting metrics.

context.after function

The context.after function is used to create a hook that runs after the current test finishes. This hook can be used to perform cleanup tasks, such as deleting files that were created during the test.

Real-world example

The following example shows how to use the context.after function to delete a file that was created during the test:

test("creates a file", async (t) => {
  const file = await fs.createFile("test.txt");

  t.after((t) => {
    fs.deleteFile(file);
  });
});

Potential applications of the context.after function

The context.after function can be used for a variety of purposes, including:

• deleting files or other resources that were created during the test

• logging information about the test

• collecting metrics about the test

• tearing down the test environment

Conclusion

The context.after function is a powerful tool that can be used to improve the efficiency and reliability of your tests. By using this function to clean up after your tests and to gather data about their performance, you can ensure that your tests are running smoothly and that you have the information you need to make informed decisions about your software.

---

context.afterEach([...options]) Function in test Module

Definition:

The context.afterEach([...options]) function in the test module allows you to run a specific set of code after each subtest within the current test. This code can perform cleanup tasks, validate results, or provide additional information about the test's behavior.

Usage:

Inside a test() block, you can use the afterEach() function to add a hook that will execute after each subtest:

test("Top Level Test", (t) => {
  // Define a function to run after each subtest
  const afterEachHook = (t) => {
    console.log(`Finished running subtest: ${t.name}`);
  };

  // Add the afterEach hook to the test context
  t.afterEach(afterEachHook);

  // Add a subtest
  t.test("Subtest", (t) => {
    // Assert something
  });
});

Options:

The afterEach() function accepts an optional options object with the following properties:

• signal (AbortSignal): Allows you to abort the hook execution if needed.

• timeout (number): Specifies a timeout period for the hook. If the hook takes longer than the timeout, it will fail.

Example:

Suppose you have a test that creates a temporary file for each subtest. After each subtest, you want to delete the temporary file. You can use afterEach() to handle this cleanup:

test("Test with Temporary Files", (t) => {
  // Define a function to delete the temporary file
  const deleteTempFile = (t) => {
    // Perform cleanup operation
  };

  // Add the afterEach hook to the test context
  t.afterEach(deleteTempFile);

  // Add a subtest that creates a temporary file
  t.test("Subtest", (t) => {
    // Create the temporary file
  });
});

Applications in Real World:

• Cleanup after subtests: Clean up resources created during subtests, such as temporary files, database connections, or browser instances.

• Validate subtest results: Assert specific conditions that must hold true after each subtest. This can help catch errors or unexpected behavior early on.

• Provide diagnostic information: Log additional information about the subtest, such as its execution time or any errors encountered. This can aid in debugging and understanding test behavior.

---

context.diagnostic(message)

• message {string} Message to be reported.

This function is used to write diagnostics to the output. Any diagnostic information is included at the end of the test's results. This function does not return a value.

Example:

test("top level test", (t) => {
  t.diagnostic("A diagnostic message");
});

Explanation:

The diagnostic function allows you to log messages that provide additional information about the test. These messages are not displayed during the test run, but are included in the output at the end of the test. This can be useful for debugging or providing additional context to the test results.

Real-world example:

In a test that checks the functionality of a user registration form, you might use the diagnostic function to log the values of the form fields:

test("user registration form works", (t) => {
  t.visit("/register");
  t.typeText("#username", "johndoe");
  t.diagnostic(`Entered username: ${t.getText("#username")}`);
  t.typeText("#password", "secret123");
  t.diagnostic(`Entered password: ${t.getText("#password")}`);
  t.click(".btn-submit");
});

Benefits:

• Provides additional information about the test's execution.

• Helps in debugging and understanding test failures.

• Can be used to track the state of the application during the test.

---

context.name

The name of the test.

This property contains the name of the test that is currently being executed. The name is typically set by the test framework and can be used to identify the test in logs or reports.

For example, the following test has a name of "my test":

test("my test", () => {
  // ...
});

You can access the name of the test in the test context using the context.name property:

test("my test", (context) => {
  console.log(context.name); // "my test"
});

This property can be useful for debugging purposes or for generating reports.

---

context.runOnly(shouldRunOnlyTests)

• shouldRunOnlyTests {boolean} Whether or not to run only tests.

If shouldRunOnlyTests is truthy, the test context will only run tests that have the only option set. Otherwise, all tests are run. If Node.js was not started with the [--test-only][] command-line option, this function is a no-op.

Simplified Explanation

Imagine you have a group of tests that you want to run. You can mark some of these tests as "only" tests, which means that you only want to run those specific tests and skip the others. The context.runOnly function lets you do this.

When you call context.runOnly(true), it tells the test context to only run the tests that have the only option set. If you call context.runOnly(false), it will run all the tests, regardless of whether they have the only option set or not.

If you run Node.js with the --test-only command-line option, the test context will automatically run in "only" mode. This means that you don't have to call context.runOnly manually.

Code Example

Here's an example of how to use context.runOnly:

test("top level test", (t) => {
  // The test context can be set to run subtests with the 'only' option.
  t.runOnly(true);
  return Promise.all([
    t.test("this subtest is now skipped"),
    t.test("this subtest is run", { only: true }),
  ]);
});

In this example, the top-level test sets shouldRunOnlyTests to true, which means that only the subtest with the only option set will be run. The other subtest will be skipped.

Real-World Applications

context.runOnly can be useful when you want to focus on running only a specific set of tests. For example, you might use it to:

• Debug a specific test or group of tests.

• Run a specific set of tests that are relevant to a particular feature or bug.

• Skip tests that are known to be flaky or unreliable.

By using context.runOnly, you can save time and effort by only running the tests that you need to run.

---

context.signal

• Simplified Explanation: context.signal is a special object that can be used to stop ongoing tasks if the test is aborted.

• Detailed Explanation: When writing async tests, there might be long-running tasks that shouldn't continue if the test has already ended. For example, if you are doing a network request, it's better to abort it if the test has failed. context.signal helps with this by providing a signal that can be listened to by long-running tasks. If the signal is aborted, the task can stop what it's doing.

• Code Example:

test("async test with signal", async (t) => {
  const controller = new AbortController();
  const signal = controller.signal;

  // Start a long-running task that listens to the signal
  const task = fetch("some/uri", { signal });

  // After a while, abort the signal
  controller.abort();

  // Wait for the task to finish
  await task;
});

• Real-World Application: This is useful in any situation where you have long-running tasks that can be aborted if the test has finished. For example, when testing network requests, database queries, or file operations.

---

Simplified Explanation:

context.skip() Function:

This function tells the test runner to skip the current test. It's like putting a flag on the test to say, "Don't bother running this, it's not supposed to work."

Parameters:

• message (optional): A custom message to add to the skip reason.

Behavior:

• The test function continues to run, but the test runner marks the test as skipped and doesn't count it as a failure.

• The test output will include the skip message if you provided one.

Code Example:

test("top level test", (t) => {
  // Skips the test with a message
  t.skip("Feature not yet implemented");

  // This code will still run, but the test won't be marked as failing.
  console.log("Skipped code");
});

Real-World Applications:

• Skipping tests that are known to fail due to temporary issues, such as network connectivity.

• Skipping tests that are only relevant for specific environments or configurations, such as tests for a particular browser version.

• Skipping tests that depend on external resources that are not available at the time of testing.

---

What is context.todo() in Jest?

todo() is a function that you can use in Jest tests to mark a test as incomplete or a placeholder for future development.

Simplified Explanation:

Think of todo() as a way to say "I haven't finished this test yet" or "I'm not sure how to write this test." It's a way to tell Jest to skip over that particular test for now and come back to it later.

How to Use context.todo():

To use todo(), simply call it within your test function:

test("my test", (t) => {
  // This test is marked as `TODO`
  t.todo("This test is not yet complete.");
});

You can also provide a custom message to explain why the test is incomplete:

test("my test", (t) => {
  // This test is marked as `TODO` because I need to find a way to mock the database.
  t.todo("Need to mock the database.");
});

Note: todo() does not stop the execution of your test. It only marks it as incomplete.

Real-World Example:

Let's say you're working on a new feature for your application and you want to start writing tests for it. However, you're not sure exactly how to test a particular function yet. You can use todo() to mark that test as incomplete and come back to it later:

test("my feature test", (t) => {
  // This test is marked as `TODO` because I'm not sure how to test this function.
  t.todo("Test the new feature function.");
});

Once you've figured out how to write the test, you can remove the todo() call and complete the test.

Benefits of Using context.todo():

• Helps you keep track of incomplete tests.

• Prevents incomplete tests from failing and skewing your test results.

• Allows you to focus on developing the application without having to worry about writing incomplete tests.

---

context.test()

The context.test() function is used to create subtests under the current test. This function behaves in the same fashion as the top level [test()][] function.

Parameters

• name {string} The name of the subtest, which is displayed when reporting test results. Default: The name property of fn, or '<anonymous>' if fn does not have a name.

• options {Object} Configuration options for the subtest. The following properties are supported:

  • concurrency {number|boolean|null} If a number is provided, then that many tests would run in parallel within the application thread. If true, it would run all subtests in parallel. If false, it would only run one test at a time. If unspecified, subtests inherit this value from their parent. Default: null.

  • only {boolean} If truthy, and the test context is configured to run only tests, then this test will be run. Otherwise, the test is skipped. Default: false.

  • signal {AbortSignal} Allows aborting an in-progress test.

  • skip {boolean|string} If truthy, the test is skipped. If a string is provided, that string is displayed in the test results as the reason for skipping the test. Default: false.

  • todo {boolean|string} If truthy, the test marked as TODO. If a string is provided, that string is displayed in the test results as the reason why the test is TODO. Default: false.

  • timeout {number} A number of milliseconds the test will fail after. If unspecified, subtests inherit this value from their parent. Default: Infinity.

• fn {Function|AsyncFunction} The function under test. The first argument to this function is a [TestContext][] object. If the test uses callbacks, the callback function is passed as the second argument. Default: A no-op function.

Returns

A Promise object that is fulfilled with undefined once the test completes.

Example

test("top level test", async (t) => {
  await t.test(
    "This is a subtest",
    { only: false, skip: false, concurrency: 1, todo: false },
    (t) => {
      assert.ok("some relevant assertion here");
    }
  );
});

Real-World Applications

The context.test() function can be used to create hierarchical test suites. This can be useful for organizing tests into logical groups, and for running tests in parallel.

For example, the following test suite uses the context.test() function to create a test suite for the math module:

test("math", (t) => {
  t.test("add", (t) => {
    assert.equal(math.add(1, 2), 3);
  });

  t.test("subtract", (t) => {
    assert.equal(math.subtract(2, 1), 1);
  });

  t.test("multiply", (t) => {
    assert.equal(math.multiply(2, 3), 6);
  });

  t.test("divide", (t) => {
    assert.equal(math.divide(6, 2), 3);
  });
});

This test suite can be run in parallel by using the --concurrency option:

node --concurrency 4 test/math.js

This will run all of the subtests in the math test suite in parallel.

---

SuiteContext is a class that provides information about the current test suite to suite functions.

Constructor

• SuiteContext is not directly constructed by the user. It is created by the test runner and passed to suite functions.

Properties

• name: The name of the current test suite.

• timeout: The maximum time allowed for the current test suite.

• tests: An array of the test cases in the current test suite.

• beforeAll: An array of functions that run before all tests in the suite.

• afterAll: An array of functions that run after all tests in the suite.

Methods

• addTest(test): Adds a test case to the current test suite.

• addBeforeAll(fn): Adds a function to run before all tests in the suite.

• addAfterAll(fn): Adds a function to run after all tests in the suite.

Example

// test example
  suite('My Suite', function() {
    // customize the timeout for the suite
    this.timeout(1000);

    // add a test to the suite
    test('My Test', function() {
      // ...
    });

    // add a function to run before all tests in the suite
    beforeAll(function() {
      // ...
    });

    // add a function to run after all tests in the suite
    afterAll(function() {
      // ...
    });
  });

Applications

• Customizing test timeouts: You can customize the timeout for a specific test suite by setting the timeout property of the SuiteContext. This can be useful for tests that may take longer than the default timeout.

• Adding setup and teardown code: You can add functions to run before or after all tests in a suite by using the beforeAll and afterAll methods of the SuiteContext. This can be useful for setting up and tearing down resources that are shared across multiple tests.

---

context.name

Simplified Explanation:

The context.name property tells you the name of the test suite that is currently running. A test suite is a group of related tests.

Code Example:

// test/my-suite.test.js
const { describe } = require("node:test");

describe("My Suite", () => {
  it("should pass", () => {
    console.log(context.name); // Output: "My Suite"
  });
});

Real-World Application:

• Organizing your tests into logical groups for easier management and readability.

• Generating reports or dashboards that show the status of each test suite.

Potential Applications:

• Web Development: Grouping tests for different pages or features of a website.

• Mobile App Development: Grouping tests for different screens or functionalities within an app.

• API Development: Grouping tests for different endpoints or resources exposed by an API.

---

context.signal

The context.signal is an AbortSignal that can be used to abort test subtasks when the test has been aborted. This is useful for cleaning up resources or interrupting long-running operations when the test is no longer needed.

test("my test", async () => {
  // Create an AbortController to control the abort signal
  const controller = new AbortController();
  const signal = controller.signal;

  // Start a long-running operation
  const operation = async () => {
    // Check the abort signal regularly to see if the test has been aborted
    while (!signal.aborted) {
      // Do something...
    }
  };

  operation();

  // Abort the test after 10 seconds
  setTimeout(() => {
    controller.abort();
  }, 10000);
});

In this example, the operation function checks the abort signal regularly to see if the test has been aborted. If the test has been aborted, the operation function will stop what it is doing and clean up any resources that it has acquired. This prevents the test from continuing to run and wasting resources when it is no longer needed.

Real-world applications

• Cleaning up resources when a test is aborted, such as closing database connections or releasing locks.

• Interrupting long-running operations when a test is aborted, such as stopping a server or killing a child process.