# Test runner

<!--introduced_in=v18.0.0-->

<!-- YAML
added:
  - v18.0.0
  - v16.17.0
changes:
  - version: v20.0.0
    pr-url: https://github.com/nodejs/node/pull/46983
    description: The test runner is now stable.
-->

> Stability: 2 - Stable

<!-- source_link=lib/test.js -->

The `node:test` module facilitates the creation of JavaScript tests.
To access it:

```mjs
import test from 'node:test';
```

```cjs
const test = require('node:test');
```

This module is only available under the `node:` scheme. The following will not
work:

```mjs
import test from 'test';
```

```cjs
const test = require('test');
```

Tests created via the `test` module consist of a single function that is
processed in one of three ways:

1. A synchronous function that is considered failing if it throws an exception,
   and is considered passing otherwise.
2. A function that returns a `Promise` that is considered failing if the
   `Promise` rejects, and is considered passing if the `Promise` fulfills.
3. A function that receives a callback function. If the callback receives any
   truthy value as its first argument, the test is considered failing. If a
   falsy value is passed as the first argument to the callback, the test is
   considered passing. If the test function receives a callback function and
   also returns a `Promise`, the test will fail.

The following example illustrates how tests are written using the
`test` module.

```js
test('synchronous passing test', (t) => {
  // This test passes because it does not throw an exception.
  assert.strictEqual(1, 1);
});

test('synchronous failing test', (t) => {
  // This test fails because it throws an exception.
  assert.strictEqual(1, 2);
});

test('asynchronous passing test', async (t) => {
  // This test passes because the Promise returned by the async
  // function is settled and not rejected.
  assert.strictEqual(1, 1);
});

test('asynchronous failing test', async (t) => {
  // This test fails because the Promise returned by the async
  // function is rejected.
  assert.strictEqual(1, 2);
});

test('failing test using Promises', (t) => {
  // Promises can be used directly as well.
  return new Promise((resolve, reject) => {
    setImmediate(() => {
      reject(new Error('this will cause the test to fail'));
    });
  });
});

test('callback passing test', (t, done) => {
  // done() is the callback function. When the setImmediate() runs, it invokes
  // done() with no arguments.
  setImmediate(done);
});

test('callback failing test', (t, done) => {
  // When the setImmediate() runs, done() is invoked with an Error object and
  // the test fails.
  setImmediate(() => {
    done(new Error('callback failure'));
  });
});
```

If any tests fail, the process exit code is set to `1`.

## Subtests

The test context's `test()` method allows subtests to be created.
It allows you to structure your tests in a hierarchical manner,
where you can create nested tests within a larger test.
This method behaves identically to the top level `test()` function.
The following example demonstrates the creation of a
top level test with two subtests.

```js
test('top level test', async (t) => {
  await t.test('subtest 1', (t) => {
    assert.strictEqual(1, 1);
  });

  await t.test('subtest 2', (t) => {
    assert.strictEqual(2, 2);
  });
});
```

> **Note:** `beforeEach` and `afterEach` hooks are triggered
> between each subtest execution.

In this example, `await` is used to ensure that both subtests have completed.
This is necessary because tests do not wait for their subtests to
complete, unlike tests created within suites.
Any subtests that are still outstanding when their parent finishes
are cancelled and treated as failures. Any subtest failures cause the parent
test to fail.

## Skipping tests

Individual tests can be skipped by passing the `skip` option to the test, or by
calling the test context's `skip()` method as shown in the
following example.

```js
// The skip option is used, but no message is provided.
test('skip option', { skip: true }, (t) => {
  // This code is never executed.
});

// The skip option is used, and a message is provided.
test('skip option with message', { skip: 'this is skipped' }, (t) => {
  // This code is never executed.
});

test('skip() method', (t) => {
  // Make sure to return here as well if the test contains additional logic.
  t.skip();
});

test('skip() method with message', (t) => {
  // Make sure to return here as well if the test contains additional logic.
  t.skip('this is skipped');
});
```

## TODO tests

Individual tests can be marked as flaky or incomplete by passing the `todo`
option to the test, or by calling the test context's `todo()` method, as shown
in the following example. These tests represent a pending implementation or bug
that needs to be fixed. TODO tests are executed, but are not treated as test
failures, and therefore do not affect the process exit code. If a test is marked
as both TODO and skipped, the TODO option is ignored.

```js
// The todo option is used, but no message is provided.
test('todo option', { todo: true }, (t) => {
  // This code is executed, but not treated as a failure.
  throw new Error('this does not fail the test');
});

// The todo option is used, and a message is provided.
test('todo option with message', { todo: 'this is a todo test' }, (t) => {
  // This code is executed.
});

test('todo() method', (t) => {
  t.todo();
});

test('todo() method with message', (t) => {
  t.todo('this is a todo test and is not treated as a failure');
  throw new Error('this does not fail the test');
});
```

## `describe()` and `it()` aliases

Suites and tests can also be written using the `describe()` and `it()`
functions. [`describe()`][] is an alias for [`suite()`][], and [`it()`][] is an
alias for [`test()`][].

```js
describe('A thing', () => {
  it('should work', () => {
    assert.strictEqual(1, 1);
  });

  it('should be ok', () => {
    assert.strictEqual(2, 2);
  });

  describe('a nested thing', () => {
    it('should work', () => {
      assert.strictEqual(3, 3);
    });
  });
});
```

`describe()` and `it()` are imported from the `node:test` module.

```mjs
import { describe, it } from 'node:test';
```

```cjs
const { describe, it } = require('node:test');
```

## `only` tests

If Node.js is started with the [`--test-only`][] command-line option, it is
possible to skip all top level tests except for a selected subset by passing
the `only` option to the tests that should be run. When a test with the `only`
option set is run, all subtests are also run. The test context's `runOnly()`
method can be used to implement the same behavior at the subtest level.

```js
// Assume Node.js is run with the --test-only command-line option.
// The 'only' option is set, so this test is run.
test('this test is run', { only: true }, async (t) => {
  // Within this test, all subtests are run by default.
  await t.test('running subtest');

  // The test context can be updated to run subtests with the 'only' option.
  t.runOnly(true);
  await t.test('this subtest is now skipped');
  await t.test('this subtest is run', { only: true });

  // Switch the context back to execute all tests.
  t.runOnly(false);
  await t.test('this subtest is now run');

  // Explicitly do not run these tests.
  await t.test('skipped subtest 3', { only: false });
  await t.test('skipped subtest 4', { skip: true });
});

// The 'only' option is not set, so this test is skipped.
test('this test is not run', () => {
  // This code is not run.
  throw new Error('fail');
});
```

## Filtering tests by name

The [`--test-name-pattern`][] command-line option can be used to only run tests
whose name matches the provided pattern. Test name patterns are interpreted as
JavaScript regular expressions. The `--test-name-pattern` option can be
specified multiple times in order to run nested tests. For each test that is
executed, any corresponding test hooks, such as `beforeEach()`, are also
run.

Given the following test file, starting Node.js with the
`--test-name-pattern="test [1-3]"` option would cause the test runner to execute
`test 1`, `test 2`, and `test 3`. If `test 1` did not match the test name
pattern, then its subtests would not execute, despite matching the pattern. The
same set of tests could also be executed by passing `--test-name-pattern`
multiple times (e.g. `--test-name-pattern="test 1"`,
`--test-name-pattern="test 2"`, etc.).

```js
test('test 1', async (t) => {
  await t.test('test 2');
  await t.test('test 3');
});

test('Test 4', async (t) => {
  await t.test('Test 5');
  await t.test('test 6');
});
```

Test name patterns can also be specified using regular expression literals. This
allows regular expression flags to be used. In the previous example, starting
Node.js with `--test-name-pattern="/test [4-5]/i"` would match `Test 4` and
`Test 5` because the pattern is case-insensitive.

Test name patterns do not change the set of files that the test runner executes.

## Extraneous asynchronous activity

Once a test function finishes executing, the results are reported as quickly
as possible while maintaining the order of the tests. However, it is possible
for the test function to generate asynchronous activity that outlives the test
itself. The test runner handles this type of activity, but does not delay the
reporting of test results in order to accommodate it.

In the following example, a test completes with two `setImmediate()`
operations still outstanding. The first `setImmediate()` attempts to create a
new subtest. Because the parent test has already finished and output its
results, the new subtest is immediately marked as failed, and reported later
to the {TestsStream}.

The second `setImmediate()` creates an `uncaughtException` event.
`uncaughtException` and `unhandledRejection` events originating from a completed
test are marked as failed by the `test` module and reported as diagnostic
warnings at the top level by the {TestsStream}.

```js
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');
  });

  // The test finishes after this line.
});
```

## Watch mode

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> Stability: 1 - Experimental

The Node.js test runner supports running in watch mode by passing the `--watch` flag:

```bash
node --test --watch
```

In watch mode, the test runner will watch for changes to test files and
their dependencies. When a change is detected, the test runner will
rerun the tests affected by the change.
The test runner will continue to run until the process is terminated.

## Running tests from the command line

The Node.js test runner can be invoked from the command line by passing the
[`--test`][] flag:

```bash
node --test
```

By default, Node.js will recursively search the current directory for
JavaScript source files matching a specific naming convention. Matching files
are executed as test files. More information on the expected test file naming
convention and behavior can be found in the [test runner execution model][]
section.

Alternatively, one or more paths can be provided as the final argument(s) to
the Node.js command, as shown below.

```bash
node --test test1.js test2.mjs custom_test_dir/
```

In this example, the test runner will execute the files `test1.js` and
`test2.mjs`. The test runner will also recursively search the
`custom_test_dir/` directory for test files to execute.

### Test runner execution model

When searching for test files to execute, the test runner behaves as follows:

* Any files explicitly provided by the user are executed.
* If the user did not explicitly specify any paths, the current working
  directory is recursively searched for files as specified in the following
  steps.
* `node_modules` directories are skipped unless explicitly provided by the
  user.
* If a directory named `test` is encountered, the test runner will search it
  recursively for all all `.js`, `.cjs`, and `.mjs` files. All of these files
  are treated as test files, and do not need to match the specific naming
  convention detailed below. This is to accommodate projects that place all of
  their tests in a single `test` directory.
* In all other directories, `.js`, `.cjs`, and `.mjs` files matching the
  following patterns are treated as test files:
  * `^test$` - Files whose basename is the string `'test'`. Examples:
    `test.js`, `test.cjs`, `test.mjs`.
  * `^test-.+` - Files whose basename starts with the string `'test-'`
    followed by one or more characters. Examples: `test-example.js`,
    `test-another-example.mjs`.
  * `.+[\.\-\_]test$` - Files whose basename ends with `.test`, `-test`, or
    `_test`, preceded by one or more characters. Examples: `example.test.js`,
    `example-test.cjs`, `example_test.mjs`.
  * Other file types understood by Node.js such as `.node` and `.json` are not
    automatically executed by the test runner, but are supported if explicitly
    provided on the command line.

Each matching test file is executed in a separate child process. The maximum
number of child processes running at any time is controlled by the
[`--test-concurrency`][] flag. If the child process finishes with an exit code
of 0, the test is considered passing. Otherwise, the test is considered to be a
failure. Test files must be executable by Node.js, but are not required to use
the `node:test` module internally.

Each test file is executed as if it was a regular script. That is, if the test
file itself uses `node:test` to define tests, all of those tests will be
executed within a single application thread, regardless of the value of the
`concurrency` option of [`test()`][].

## Collecting code coverage

> Stability: 1 - Experimental

When Node.js is started with the [`--experimental-test-coverage`][]
command-line flag, code coverage is collected and statistics are reported once
all tests have completed. If the [`NODE_V8_COVERAGE`][] environment variable is
used to specify a code coverage directory, the generated V8 coverage files are
written to that directory. Node.js core modules and files within
`node_modules/` directories are not included in the coverage report. If
coverage is enabled, the coverage report is sent to any [test reporters][] via
the `'test:coverage'` event.

Coverage can be disabled on a series of lines using the following
comment syntax:

```js
/* node:coverage disable */
if (anAlwaysFalseCondition) {
  // Code in this branch will never be executed, but the lines are ignored for
  // coverage purposes. All lines following the 'disable' comment are ignored
  // until a corresponding 'enable' comment is encountered.
  console.log('this is never executed');
}
/* node:coverage enable */
```

Coverage can also be disabled for a specified number of lines. After the
specified number of lines, coverage will be automatically reenabled. If the
number of lines is not explicitly provided, a single line is ignored.

```js
/* node:coverage ignore next */
if (anAlwaysFalseCondition) { console.log('this is never executed'); }

/* node:coverage ignore next 3 */
if (anAlwaysFalseCondition) {
  console.log('this is never executed');
}
```

### Coverage reporters

The tap and spec reporters will print a summary of the coverage statistics.
There is also an lcov reporter that will generate an lcov file which can be
used as an in depth coverage report.

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

### Limitations

The test runner's code coverage functionality does not support excluding
specific files or directories from the coverage report.

## Mocking

The `node:test` module supports mocking during testing via a top-level `mock`
object. The following example creates a spy on a function that adds two numbers
together. The spy is then used to assert that the function was called as
expected.

```mjs
import assert from 'node:assert';
import { mock, test } from 'node:test';

test('spies on a function', () => {
  const sum = mock.fn((a, b) => {
    return a + b;
  });

  assert.strictEqual(sum.mock.calls.length, 0);
  assert.strictEqual(sum(3, 4), 7);
  assert.strictEqual(sum.mock.calls.length, 1);

  const call = sum.mock.calls[0];
  assert.deepStrictEqual(call.arguments, [3, 4]);
  assert.strictEqual(call.result, 7);
  assert.strictEqual(call.error, undefined);

  // Reset the globally tracked mocks.
  mock.reset();
});
```

```cjs
'use strict';
const assert = require('node:assert');
const { mock, test } = require('node:test');

test('spies on a function', () => {
  const sum = mock.fn((a, b) => {
    return a + b;
  });

  assert.strictEqual(sum.mock.calls.length, 0);
  assert.strictEqual(sum(3, 4), 7);
  assert.strictEqual(sum.mock.calls.length, 1);

  const call = sum.mock.calls[0];
  assert.deepStrictEqual(call.arguments, [3, 4]);
  assert.strictEqual(call.result, 7);
  assert.strictEqual(call.error, undefined);

  // Reset the globally tracked mocks.
  mock.reset();
});
```

The same mocking functionality is also exposed on the [`TestContext`][] object
of each test. The following example creates a spy on an object method using the
API exposed on the `TestContext`. The benefit of mocking via the test context is
that the test runner will automatically restore all mocked functionality once
the test finishes.

```js
test('spies on an object method', (t) => {
  const number = {
    value: 5,
    add(a) {
      return this.value + a;
    },
  };

  t.mock.method(number, 'add');
  assert.strictEqual(number.add.mock.calls.length, 0);
  assert.strictEqual(number.add(3), 8);
  assert.strictEqual(number.add.mock.calls.length, 1);

  const call = number.add.mock.calls[0];

  assert.deepStrictEqual(call.arguments, [3]);
  assert.strictEqual(call.result, 8);
  assert.strictEqual(call.target, undefined);
  assert.strictEqual(call.this, number);
});
```

### Timers

Mocking timers is a technique commonly used in software testing to simulate and
control the behavior of timers, such as `setInterval` and `setTimeout`,
without actually waiting for the specified time intervals.

Refer to the [`MockTimers`][] class for a full list of methods and features.

This allows developers to write more reliable and
predictable tests for time-dependent functionality.

The example below shows how to mock `setTimeout`.
Using `.enable({ apis: ['setTimeout'] });`
it will mock the `setTimeout` functions in the [node:timers](./timers.md) and
[node:timers/promises](./timers.md#timers-promises-api) modules,
as well as from the Node.js global context.

**Note:** Destructuring functions such as
`import { setTimeout } from 'node:timers'`
is currently not supported by this API.

```mjs
import assert from 'node:assert';
import { mock, test } from 'node:test';

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

  // Optionally choose what to mock
  mock.timers.enable({ apis: ['setTimeout'] });
  setTimeout(fn, 9999);
  assert.strictEqual(fn.mock.callCount(), 0);

  // Advance in time
  mock.timers.tick(9999);
  assert.strictEqual(fn.mock.callCount(), 1);

  // Reset the globally tracked mocks.
  mock.timers.reset();

  // If you call reset mock instance, it will also reset timers instance
  mock.reset();
});
```

```cjs
const assert = require('node:assert');
const { mock, test } = require('node:test');

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

  // Optionally choose what to mock
  mock.timers.enable({ apis: ['setTimeout'] });
  setTimeout(fn, 9999);
  assert.strictEqual(fn.mock.callCount(), 0);

  // Advance in time
  mock.timers.tick(9999);
  assert.strictEqual(fn.mock.callCount(), 1);

  // Reset the globally tracked mocks.
  mock.timers.reset();

  // If you call reset mock instance, it will also reset timers instance
  mock.reset();
});
```

The same mocking functionality is also exposed in the mock property on the [`TestContext`][] object
of each test. The benefit of mocking via the test context is
that the test runner will automatically restore all mocked timers
functionality once the test finishes.

```mjs
import assert from 'node:assert';
import { test } from 'node:test';

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

  // Optionally choose what to mock
  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);
});
```

```cjs
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();

  // Optionally choose what to mock
  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);
});
```

### Dates

The mock timers API also allows the mocking of the `Date` object. This is a
useful feature for testing time-dependent functionality, or to simulate
internal calendar functions such as `Date.now()`.

The dates implementation is also part of the [`MockTimers`][] class. Refer to it
for a full list of methods and features.

**Note:** Dates and timers are dependent when mocked together. This means that
if you have both the `Date` and `setTimeout` mocked, advancing the time will
also advance the mocked date as they simulate a single internal clock.

The example below show how to mock the `Date` object and obtain the current
`Date.now()` value.

```mjs
import assert from 'node:assert';
import { test } from 'node:test';

test('mocks the Date object', (context) => {
  // Optionally choose what to mock
  context.mock.timers.enable({ apis: ['Date'] });
  // If not specified, the initial date will be based on 0 in the UNIX epoch
  assert.strictEqual(Date.now(), 0);

  // Advance in time will also advance the date
  context.mock.timers.tick(9999);
  assert.strictEqual(Date.now(), 9999);
});
```

```cjs
const assert = require('node:assert');
const { test } = require('node:test');

test('mocks the Date object', (context) => {
  // Optionally choose what to mock
  context.mock.timers.enable({ apis: ['Date'] });
  // If not specified, the initial date will be based on 0 in the UNIX epoch
  assert.strictEqual(Date.now(), 0);

  // Advance in time will also advance the date
  context.mock.timers.tick(9999);
  assert.strictEqual(Date.now(), 9999);
});
```

If there is no initial epoch set, the initial date will be based on 0 in the
Unix epoch. This is January 1st, 1970, 00:00:00 UTC. You can set an initial date
by passing a `now` property to the `.enable()` method. This value will be used
as the initial date for the mocked `Date` object. It can either be a positive
integer, or another Date object.

```mjs
import assert from 'node:assert';
import { test } from 'node:test';

test('mocks the Date object with initial time', (context) => {
  // Optionally choose what to mock
  context.mock.timers.enable({ apis: ['Date'], now: 100 });
  assert.strictEqual(Date.now(), 100);

  // Advance in time will also advance the date
  context.mock.timers.tick(200);
  assert.strictEqual(Date.now(), 300);
});
```

```cjs
const assert = require('node:assert');
const { test } = require('node:test');

test('mocks the Date object with initial time', (context) => {
  // Optionally choose what to mock
  context.mock.timers.enable({ apis: ['Date'], now: 100 });
  assert.strictEqual(Date.now(), 100);

  // Advance in time will also advance the date
  context.mock.timers.tick(200);
  assert.strictEqual(Date.now(), 300);
});
```

You can use the `.setTime()` method to manually move the mocked date to another
time. This method only accepts a positive integer.

**Note:** This method will execute any mocked timers that are in the past
from the new time.

In the below example we are setting a new time for the mocked date.

```mjs
import assert from 'node:assert';
import { test } from 'node:test';

test('sets the time of a date object', (context) => {
  // Optionally choose what to mock
  context.mock.timers.enable({ apis: ['Date'], now: 100 });
  assert.strictEqual(Date.now(), 100);

  // Advance in time will also advance the date
  context.mock.timers.setTime(1000);
  context.mock.timers.tick(200);
  assert.strictEqual(Date.now(), 1200);
});
```

```cjs
const assert = require('node:assert');
const { test } = require('node:test');

test('sets the time of a date object', (context) => {
  // Optionally choose what to mock
  context.mock.timers.enable({ apis: ['Date'], now: 100 });
  assert.strictEqual(Date.now(), 100);

  // Advance in time will also advance the date
  context.mock.timers.setTime(1000);
  context.mock.timers.tick(200);
  assert.strictEqual(Date.now(), 1200);
});
```

If you have any timer that's set to run in the past, it will be executed as if
the `.tick()` method has been called. This is useful if you want to test
time-dependent functionality that's already in the past.

```mjs
import assert from 'node:assert';
import { test } from 'node:test';

test('runs timers as setTime passes ticks', (context) => {
  // Optionally choose what to mock
  context.mock.timers.enable({ apis: ['setTimeout', 'Date'] });
  const fn = context.mock.fn();
  setTimeout(fn, 1000);

  context.mock.timers.setTime(800);
  // Timer is not executed as the time is not yet reached
  assert.strictEqual(fn.mock.callCount(), 0);
  assert.strictEqual(Date.now(), 800);

  context.mock.timers.setTime(1200);
  // Timer is executed as the time is now reached
  assert.strictEqual(fn.mock.callCount(), 1);
  assert.strictEqual(Date.now(), 1200);
});
```

```cjs
const assert = require('node:assert');
const { test } = require('node:test');

test('runs timers as setTime passes ticks', (context) => {
  // Optionally choose what to mock
  context.mock.timers.enable({ apis: ['setTimeout', 'Date'] });
  const fn = context.mock.fn();
  setTimeout(fn, 1000);

  context.mock.timers.setTime(800);
  // Timer is not executed as the time is not yet reached
  assert.strictEqual(fn.mock.callCount(), 0);
  assert.strictEqual(Date.now(), 800);

  context.mock.timers.setTime(1200);
  // Timer is executed as the time is now reached
  assert.strictEqual(fn.mock.callCount(), 1);
  assert.strictEqual(Date.now(), 1200);
});
```

Using `.runAll()` will execute all timers that are currently in the queue. This
will also advance the mocked date to the time of the last timer that was
executed as if the time has passed.

```mjs
import assert from 'node:assert';
import { test } from 'node:test';

test('runs timers as setTime passes ticks', (context) => {
  // Optionally choose what to mock
  context.mock.timers.enable({ apis: ['setTimeout', 'Date'] });
  const fn = context.mock.fn();
  setTimeout(fn, 1000);
  setTimeout(fn, 2000);
  setTimeout(fn, 3000);

  context.mock.timers.runAll();
  // All timers are executed as the time is now reached
  assert.strictEqual(fn.mock.callCount(), 3);
  assert.strictEqual(Date.now(), 3000);
});
```

```cjs
const assert = require('node:assert');
const { test } = require('node:test');

test('runs timers as setTime passes ticks', (context) => {
  // Optionally choose what to mock
  context.mock.timers.enable({ apis: ['setTimeout', 'Date'] });
  const fn = context.mock.fn();
  setTimeout(fn, 1000);
  setTimeout(fn, 2000);
  setTimeout(fn, 3000);

  context.mock.timers.runAll();
  // All timers are executed as the time is now reached
  assert.strictEqual(fn.mock.callCount(), 3);
  assert.strictEqual(Date.now(), 3000);
});
```

## Test reporters

<!-- YAML
added:
  - v19.6.0
  - v18.15.0
changes:
  - version: v19.9.0
    pr-url: https://github.com/nodejs/node/pull/47238
    description: Reporters are now exposed at `node:test/reporters`.
-->

The `node:test` module supports passing [`--test-reporter`][]
flags for the test runner to use a specific reporter.

The following built-reporters are supported:

* `tap`
  The `tap` reporter outputs the test results in the [TAP][] format.

* `spec`
  The `spec` reporter outputs the test results in a human-readable format.

* `dot`
  The `dot` reporter outputs the test results in a compact format,
  where each passing test is represented by a `.`,
  and each failing test is represented by a `X`.

* `junit`
  The junit reporter outputs test results in a jUnit XML format

* `lcov`
  The `lcov` reporter outputs test coverage when used with the
  [`--experimental-test-coverage`][] flag.

When `stdout` is a [TTY][], the `spec` reporter is used by default.
Otherwise, the `tap` reporter is used by default.

The exact output of these reporters is subject to change between versions of
Node.js, and should not be relied on programmatically. If programmatic access
to the test runner's output is required, use the events emitted by the
{TestsStream}.

The reporters are available via the `node:test/reporters` module:

```mjs
import { tap, spec, dot, junit, lcov } from 'node:test/reporters';
```

```cjs
const { tap, spec, dot, junit, lcov } = require('node:test/reporters');
```

### Custom reporters

[`--test-reporter`][] can be used to specify a path to custom reporter.
A custom reporter is a module that exports a value
accepted by [stream.compose][].
Reporters should transform events emitted by a {TestsStream}

Example of a custom reporter using {stream.Transform}:

```mjs
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;
      case 'test:watch:drained':
        callback(null, 'test watch queue drained');
        break;
      case 'test:start':
        callback(null, `test ${event.data.name} started`);
        break;
      case 'test:pass':
        callback(null, `test ${event.data.name} passed`);
        break;
      case 'test:fail':
        callback(null, `test ${event.data.name} failed`);
        break;
      case 'test:plan':
        callback(null, 'test plan');
        break;
      case 'test:diagnostic':
      case 'test:stderr':
      case 'test:stdout':
        callback(null, event.data.message);
        break;
      case 'test:coverage': {
        const { totalLineCount } = event.data.summary.totals;
        callback(null, `total line count: ${totalLineCount}\n`);
        break;
      }
    }
  },
});

export default customReporter;
```

```cjs
const { Transform } = require('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;
      case 'test:watch:drained':
        callback(null, 'test watch queue drained');
        break;
      case 'test:start':
        callback(null, `test ${event.data.name} started`);
        break;
      case 'test:pass':
        callback(null, `test ${event.data.name} passed`);
        break;
      case 'test:fail':
        callback(null, `test ${event.data.name} failed`);
        break;
      case 'test:plan':
        callback(null, 'test plan');
        break;
      case 'test:diagnostic':
      case 'test:stderr':
      case 'test:stdout':
        callback(null, event.data.message);
        break;
      case 'test:coverage': {
        const { totalLineCount } = event.data.summary.totals;
        callback(null, `total line count: ${totalLineCount}\n`);
        break;
      }
    }
  },
});

module.exports = customReporter;
```

Example of a custom reporter using a generator function:

```mjs
export default 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;
      case 'test:watch:drained':
        yield 'test watch queue drained';
        break;
      case 'test:start':
        yield `test ${event.data.name} started\n`;
        break;
      case 'test:pass':
        yield `test ${event.data.name} passed\n`;
        break;
      case 'test:fail':
        yield `test ${event.data.name} failed\n`;
        break;
      case 'test:plan':
        yield 'test plan';
        break;
      case 'test:diagnostic':
      case 'test:stderr':
      case 'test:stdout':
        yield `${event.data.message}\n`;
        break;
      case 'test:coverage': {
        const { totalLineCount } = event.data.summary.totals;
        yield `total line count: ${totalLineCount}\n`;
        break;
      }
    }
  }
}
```

```cjs
module.exports = 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;
      case 'test:watch:drained':
        yield 'test watch queue drained';
        break;
      case 'test:start':
        yield `test ${event.data.name} started\n`;
        break;
      case 'test:pass':
        yield `test ${event.data.name} passed\n`;
        break;
      case 'test:fail':
        yield `test ${event.data.name} failed\n`;
        break;
      case 'test:plan':
        yield 'test plan\n';
        break;
      case 'test:diagnostic':
      case 'test:stderr':
      case 'test:stdout':
        yield `${event.data.message}\n`;
        break;
      case 'test:coverage': {
        const { totalLineCount } = event.data.summary.totals;
        yield `total line count: ${totalLineCount}\n`;
        break;
      }
    }
  }
};
```

The value provided to `--test-reporter` should be a string like one used in an
`import()` in JavaScript code, or a value provided for [`--import`][].

### Multiple reporters

The [`--test-reporter`][] flag can be specified multiple times to report test
results in several formats. In this situation
it is required to specify a destination for each reporter
using [`--test-reporter-destination`][].
Destination can be `stdout`, `stderr`, or a file path.
Reporters and destinations are paired according
to the order they were specified.

In the following example, the `spec` reporter will output to `stdout`,
and the `dot` reporter will output to `file.txt`:

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

When a single reporter is specified, the destination will default to `stdout`,
unless a destination is explicitly provided.

## `run([options])`

<!-- YAML
added:
  - v18.9.0
  - v16.19.0
changes:
  - version: v20.14.0
    pr-url: https://github.com/nodejs/node/pull/52038
    description: Added the `forceExit` option.
  - version:
    - v20.1.0
    - v18.17.0
    pr-url: https://github.com/nodejs/node/pull/47628
    description: Add a testNamePatterns option.
-->

* `options` {Object} Configuration options for running tests. The following
  properties are supported:
  * `concurrency` {number|boolean} If a number is provided,
    then that many test processes would run in parallel, where each process
    corresponds to one test file.
    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][].
  * `forceExit`: {boolean} Configures the test runner to exit the process once
    all known tests have finished executing even if the event loop would
    otherwise remain active. **Default:** `false`.
  * `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}

**Note:** `shard` is used to horizontally parallelize test running across
machines or processes, ideal for large-scale executions across varied
environments. It's incompatible with `watch` mode, tailored for rapid
code iteration by automatically rerunning tests on file changes.

```mjs
import { tap } from 'node:test/reporters';
import { run } from 'node:test';
import process from 'node:process';
import path from 'node:path';

run({ files: [path.resolve('./tests/test.js')] })
 .on('test:fail', () => {
   process.exitCode = 1;
 })
 .compose(tap)
 .pipe(process.stdout);
```

```cjs
const { tap } = require('node:test/reporters');
const { run } = require('node:test');
const path = require('node:path');

run({ files: [path.resolve('./tests/test.js')] })
 .on('test:fail', () => {
   process.exitCode = 1;
 })
 .compose(tap)
 .pipe(process.stdout);
```

## `suite([name][, options][, fn])`

<!-- YAML
added: v20.13.0
-->

* `name` {string} The name of the suite, which is displayed when reporting test
  results. **Default:** The `name` property of `fn`, or `'<anonymous>'` if `fn`
  does not have a name.
* `options` {Object} Optional configuration options for the suite.
  This supports the same options as `test([name][, options][, fn])`.
* `fn` {Function|AsyncFunction} The suite function declaring nested tests and
  suites. The first argument to this function is a [`SuiteContext`][] object.
  **Default:** A no-op function.
* Returns: {Promise} Immediately fulfilled with `undefined`.

The `suite()` function is imported from the `node:test` module.

## `suite.skip([name][, options][, fn])`

<!-- YAML
added: v20.13.0
-->

Shorthand for skipping a suite. This is the same as
[`suite([name], { skip: true }[, fn])`][suite options].

## `suite.todo([name][, options][, fn])`

<!-- YAML
added: v20.13.0
-->

Shorthand for marking a suite as `TODO`. This is the same as
[`suite([name], { todo: true }[, fn])`][suite options].

## `suite.only([name][, options][, fn])`

<!-- YAML
added: v20.13.0
-->

Shorthand for marking a suite as `only`. This is the same as
[`suite([name], { only: true }[, fn])`][suite options].

## `test([name][, options][, fn])`

<!-- YAML
added:
  - v18.0.0
  - v16.17.0
changes:
  - version: v20.2.0
    pr-url: https://github.com/nodejs/node/pull/47909
    description: Added the `skip`, `todo`, and `only` shorthands.
  - version:
    - v18.8.0
    - v16.18.0
    pr-url: https://github.com/nodejs/node/pull/43554
    description: Add a `signal` option.
  - version:
    - v18.7.0
    - v16.17.0
    pr-url: https://github.com/nodejs/node/pull/43505
    description: Add a `timeout` option.
-->

* `name` {string} The name of the test, 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 test. The following
  properties are supported:
  * `concurrency` {number|boolean} If a number is provided,
    then that many tests would run in parallel within the application thread.
    If `true`, all scheduled asynchronous tests run concurrently within the
    thread. If `false`, only one test runs at a time.
    If unspecified, subtests inherit this value from their parent.
    **Default:** `false`.
  * `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`.
  * `plan` {number} The number of assertions and subtests expected to be run in the test.
    If the number of assertions run in the test does not match the number
    specified in the plan, the test will fail.
    **Default:** `undefined`.
* `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: {Promise} Fulfilled with `undefined` once
  the test completes, or immediately if the test runs within a suite.

The `test()` function is the value imported from the `test` module. Each
invocation of this function results in reporting the test to the {TestsStream}.

The `TestContext` object passed to the `fn` argument can be used to perform
actions related to the current test. Examples include skipping the test, adding
additional diagnostic information, or creating subtests.

`test()` returns a `Promise` that fulfills once the test completes.
if `test()` is called within a suite, it fulfills immediately.
The return value can usually be discarded for top level tests.
However, the return value from subtests should be used to prevent the parent
test from finishing first and cancelling the subtest
as shown in the following example.

```js
test('top level test', async (t) => {
  // The setTimeout() in the following subtest would cause it to outlive its
  // parent test if 'await' is removed on the next line. Once the parent test
  // completes, it will cancel any outstanding subtests.
  await t.test('longer running subtest', async (t) => {
    return new Promise((resolve, reject) => {
      setTimeout(resolve, 1000);
    });
  });
});
```

The `timeout` option can be used to fail the test if it takes longer than
`timeout` milliseconds to complete. However, it is not a reliable mechanism for
canceling tests because a running test might block the application thread and
thus prevent the scheduled cancellation.

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

Shorthand for skipping a test,
same as [`test([name], { skip: true }[, fn])`][it options].

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

Shorthand for marking a test as `TODO`,
same as [`test([name], { todo: true }[, fn])`][it options].

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

Shorthand for marking a test as `only`,
same as [`test([name], { only: true }[, fn])`][it options].

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

Alias for [`suite()`][].

The `describe()` function is imported from the `node:test` module.

## `describe.skip([name][, options][, fn])`

Shorthand for skipping a suite. This is the same as
[`describe([name], { skip: true }[, fn])`][describe options].

## `describe.todo([name][, options][, fn])`

Shorthand for marking a suite as `TODO`. This is the same as
[`describe([name], { todo: true }[, fn])`][describe options].

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

<!-- YAML
added:
  - v19.8.0
  - v18.15.0
-->

Shorthand for marking a suite as `only`. This is the same as
[`describe([name], { only: true }[, fn])`][describe options].

## `it([name][, options][, fn])`

<!-- YAML
added:
  - v18.6.0
  - v16.17.0
changes:
  - version: v19.8.0
    pr-url: https://github.com/nodejs/node/pull/46889
    description: Calling `it()` is now equivalent to calling `test()`.
-->

Alias for [`test()`][].

The `it()` function is imported from the `node:test` module.

## `it.skip([name][, options][, fn])`

Shorthand for skipping a test,
same as [`it([name], { skip: true }[, fn])`][it options].

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

Shorthand for marking a test as `TODO`,
same as [`it([name], { todo: true }[, fn])`][it options].

## `it.only([name][, options][, fn])`

<!-- YAML
added:
  - v19.8.0
  - v18.15.0
-->

Shorthand for marking a test as `only`,
same as [`it([name], { only: true }[, fn])`][it options].

## `before([fn][, options])`

<!-- YAML
added:
  - v18.8.0
  - v16.18.0
-->

* `fn` {Function|AsyncFunction} The hook function.
  If the hook uses callbacks,
  the callback function is passed as the second argument. **Default:** A no-op
  function.
* `options` {Object} 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.
    **Default:** `Infinity`.

This function creates a hook that runs before executing a suite.

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

## `after([fn][, options])`

<!-- YAML
added:
 - v18.8.0
 - v16.18.0
-->

* `fn` {Function|AsyncFunction} The hook function.
  If the hook uses callbacks,
  the callback function is passed as the second argument. **Default:** A no-op
  function.
* `options` {Object} 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.
    **Default:** `Infinity`.

This function creates a hook that runs after executing a suite.

```js
describe('tests', async () => {
  after(() => console.log('finished running tests'));
  it('is a subtest', () => {
    assert.ok('some relevant assertion here');
  });
});
```

**Note:** The `after` hook is guaranteed to run,
even if tests within the suite fail.

## `beforeEach([fn][, options])`

<!-- YAML
added:
  - v18.8.0
  - v16.18.0
-->

* `fn` {Function|AsyncFunction} The hook function.
  If the hook uses callbacks,
  the callback function is passed as the second argument. **Default:** A no-op
  function.
* `options` {Object} 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.
    **Default:** `Infinity`.

This function creates a hook that runs before each test in the current suite.

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

## `afterEach([fn][, options])`

<!-- YAML
added:
  - v18.8.0
  - v16.18.0
-->

* `fn` {Function|AsyncFunction} The hook function.
  If the hook uses callbacks,
  the callback function is passed as the second argument. **Default:** A no-op
  function.
* `options` {Object} 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.
    **Default:** `Infinity`.

This function creates a hook that runs after each test in the current suite.
The `afterEach()` hook is run even if the test fails.

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

## Class: `MockFunctionContext`

<!-- YAML
added:
  - v19.1.0
  - v18.13.0
-->

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

### `ctx.calls`

<!-- YAML
added:
  - v19.1.0
  - v18.13.0
-->

* {Array}

A getter that returns a copy of the internal array used to track calls to the
mock. Each entry in the array is an object with the following properties.

* `arguments` {Array} An array of the arguments passed to the mock function.
* `error` {any} If the mocked function threw then this property contains the
  thrown value. **Default:** `undefined`.
* `result` {any} The value returned by the mocked function.
* `stack` {Error} An `Error` object whose stack can be used to determine the
  callsite of the mocked function invocation.
* `target` {Function|undefined} If the mocked function is a constructor, this
  field contains the class being constructed. Otherwise this will be
  `undefined`.
* `this` {any} The mocked function's `this` value.

### `ctx.callCount()`

<!-- YAML
added:
  - v19.1.0
  - v18.13.0
-->

* Returns: {integer} The number of times that this mock has been invoked.

This function returns the number of times that this mock has been invoked. This
function is more efficient than checking `ctx.calls.length` because `ctx.calls`
is a getter that creates a copy of the internal call tracking array.

### `ctx.mockImplementation(implementation)`

<!-- YAML
added:
  - v19.1.0
  - v18.13.0
-->

* `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.

The following example creates a mock function using `t.mock.fn()`, calls the
mock function, and then changes the mock implementation to a different function.

```js
test('changes a mock behavior', (t) => {
  let cnt = 0;

  function addOne() {
    cnt++;
    return cnt;
  }

  function addTwo() {
    cnt += 2;
    return cnt;
  }

  const fn = t.mock.fn(addOne);

  assert.strictEqual(fn(), 1);
  fn.mock.mockImplementation(addTwo);
  assert.strictEqual(fn(), 3);
  assert.strictEqual(fn(), 5);
});
```

### `ctx.mockImplementationOnce(implementation[, onCall])`

<!-- YAML
added:
  - v19.1.0
  - v18.13.0
-->

* `implementation` {Function|AsyncFunction} The function to be used as the
  mock's implementation for the invocation number specified by `onCall`.
* `onCall` {integer} 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.

This function is used to change the behavior of an existing mock for a single
invocation. Once invocation `onCall` has occurred, the mock will revert to
whatever behavior it would have used had `mockImplementationOnce()` not been
called.

The following example creates a mock function using `t.mock.fn()`, calls the
mock function, changes the mock implementation to a different function for the
next invocation, and then resumes its previous behavior.

```js
test('changes a mock behavior once', (t) => {
  let cnt = 0;

  function addOne() {
    cnt++;
    return cnt;
  }

  function addTwo() {
    cnt += 2;
    return cnt;
  }

  const fn = t.mock.fn(addOne);

  assert.strictEqual(fn(), 1);
  fn.mock.mockImplementationOnce(addTwo);
  assert.strictEqual(fn(), 3);
  assert.strictEqual(fn(), 4);
});
```

### `ctx.resetCalls()`

<!-- YAML
added:
  - v19.3.0
  - v18.13.0
-->

Resets the call history of the mock function.

### `ctx.restore()`

<!-- YAML
added:
  - v19.1.0
  - v18.13.0
-->

Resets the implementation of the mock function to its original behavior. The
mock can still be used after calling this function.

## Class: `MockTracker`

<!-- YAML
added:
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  - v18.13.0
-->

The `MockTracker` class is used to manage mocking functionality. The test runner
module provides a top level `mock` export which is a `MockTracker` instance.
Each test also provides its own `MockTracker` instance via the test context's
`mock` property.

### `mock.fn([original[, implementation]][, options])`

<!-- YAML
added:
  - v19.1.0
  - v18.13.0
-->

* `original` {Function|AsyncFunction} An optional function to create a mock on.
  **Default:** A no-op function.
* `implementation` {Function|AsyncFunction} An optional function used as the
  mock implementation for `original`. This is useful for creating mocks that
  exhibit one behavior for a specified number of calls and then restore the
  behavior of `original`. **Default:** The function specified by `original`.
* `options` {Object} Optional configuration options for the mock function. The
  following properties are supported:
  * `times` {integer} The number of times that the mock will use the behavior of
    `implementation`. Once the mock function has been called `times` times, it
    will automatically restore the behavior of `original`. This value must be an
    integer greater than zero. **Default:** `Infinity`.
* Returns: {Proxy} The mocked function. The mocked function contains a special
  `mock` property, which is an instance of [`MockFunctionContext`][], and can
  be used for inspecting and changing the behavior of the mocked function.

This function is used to create a mock function.

The following example creates a mock function that increments a counter by one
on each invocation. The `times` option is used to modify the mock behavior such
that the first two invocations add two to the counter instead of one.

```js
test('mocks a counting function', (t) => {
  let cnt = 0;

  function addOne() {
    cnt++;
    return cnt;
  }

  function addTwo() {
    cnt += 2;
    return cnt;
  }

  const fn = t.mock.fn(addOne, addTwo, { times: 2 });

  assert.strictEqual(fn(), 2);
  assert.strictEqual(fn(), 4);
  assert.strictEqual(fn(), 5);
  assert.strictEqual(fn(), 6);
});
```

### `mock.getter(object, methodName[, implementation][, options])`

<!-- YAML
added:
  - v19.3.0
  - v18.13.0
-->

This function is syntax sugar for [`MockTracker.method`][] with `options.getter`
set to `true`.

### `mock.method(object, methodName[, implementation][, options])`

<!-- YAML
added:
  - v19.1.0
  - v18.13.0
-->

* `object` {Object} The object whose method is being mocked.
* `methodName` {string|symbol} The identifier of the method on `object` to mock.
  If `object[methodName]` is not a function, an error is thrown.
* `implementation` {Function|AsyncFunction} An optional function used as the
  mock implementation for `object[methodName]`. **Default:** The original method
  specified by `object[methodName]`.
* `options` {Object} Optional configuration options for the mock method. The
  following properties are supported:
  * `getter` {boolean} If `true`, `object[methodName]` is treated as a getter.
    This option cannot be used with the `setter` option. **Default:** false.
  * `setter` {boolean} If `true`, `object[methodName]` is treated as a setter.
    This option cannot be used with the `getter` option. **Default:** false.
  * `times` {integer} The number of times that the mock will use the behavior of
    `implementation`. Once the mocked method has been called `times` times, it
    will automatically restore the original behavior. This value must be an
    integer greater than zero. **Default:** `Infinity`.
* Returns: {Proxy} The mocked method. The mocked method contains a special
  `mock` property, which is an instance of [`MockFunctionContext`][], and can
  be used for inspecting and changing the behavior of the mocked method.

This function is used to create a mock on an existing object method. The
following example demonstrates how a mock is created on an existing object
method.

```js
test('spies on an object method', (t) => {
  const number = {
    value: 5,
    subtract(a) {
      return this.value - a;
    },
  };

  t.mock.method(number, 'subtract');
  assert.strictEqual(number.subtract.mock.calls.length, 0);
  assert.strictEqual(number.subtract(3), 2);
  assert.strictEqual(number.subtract.mock.calls.length, 1);

  const call = number.subtract.mock.calls[0];

  assert.deepStrictEqual(call.arguments, [3]);
  assert.strictEqual(call.result, 2);
  assert.strictEqual(call.error, undefined);
  assert.strictEqual(call.target, undefined);
  assert.strictEqual(call.this, number);
});
```

### `mock.reset()`

<!-- YAML
added:
  - v19.1.0
  - v18.13.0
-->

This function restores the default behavior of all mocks that were previously
created by this `MockTracker` and disassociates the mocks from the
`MockTracker` instance. Once disassociated, the mocks can still be used, but the
`MockTracker` instance can no longer be used to reset their behavior or
otherwise interact with them.

After each test completes, this function is called on the test context's
`MockTracker`. If the global `MockTracker` is used extensively, calling this
function manually is recommended.

### `mock.restoreAll()`

<!-- YAML
added:
  - v19.1.0
  - v18.13.0
-->

This function restores the default behavior of all mocks that were previously
created by this `MockTracker`. Unlike `mock.reset()`, `mock.restoreAll()` does
not disassociate the mocks from the `MockTracker` instance.

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

<!-- YAML
added:
  - v19.3.0
  - v18.13.0
-->

This function is syntax sugar for [`MockTracker.method`][] with `options.setter`
set to `true`.

## Class: `MockTimers`

<!-- YAML
added:
  - v20.4.0
-->

> Stability: 1 - Experimental

Mocking timers is a technique commonly used in software testing to simulate and
control the behavior of timers, such as `setInterval` and `setTimeout`,
without actually waiting for the specified time intervals.

MockTimers is also able to mock the `Date` object.

The [`MockTracker`][] provides a top-level `timers` export
which is a `MockTimers` instance.

### `timers.enable([enableOptions])`

<!-- YAML
added:
  - v20.4.0
changes:
  - version: v20.11.0
    pr-url: https://github.com/nodejs/node/pull/48638
    description: Updated parameters to be an option object with available APIs
                 and the default initial epoch.
-->

Enables timer mocking for the specified timers.

* `enableOptions` {Object} Optional configuration options for enabling timer
  mocking. The following properties are supported:
  * `apis` {Array} An optional array containing the timers to mock.
    The currently supported timer values are `'setInterval'`, `'setTimeout'`, `'setImmediate'`,
    and `'Date'`. **Default:** `['setInterval', 'setTimeout', 'setImmediate', 'Date']`.
    If no array is provided, all time related APIs (`'setInterval'`, `'clearInterval'`,
    `'setTimeout'`, `'clearTimeout'`, and `'Date'`) will be mocked by default.
  * `now` {number | Date} An optional number or Date object representing the
    initial time (in milliseconds) to use as the value
    for `Date.now()`. **Default:** `0`.

**Note:** When you enable mocking for a specific timer, its associated
clear function will also be implicitly mocked.

**Note:** Mocking `Date` will affect the behavior of the mocked timers
as they use the same internal clock.

Example usage without setting initial time:

```mjs
import { mock } from 'node:test';
mock.timers.enable({ apis: ['setInterval'] });
```

```cjs
const { mock } = require('node:test');
mock.timers.enable({ apis: ['setInterval'] });
```

The above example enables mocking for the `setInterval` timer and
implicitly mocks the `clearInterval` function. Only the `setInterval`
and `clearInterval` functions from [node:timers](./timers.md),
[node:timers/promises](./timers.md#timers-promises-api), and
`globalThis` will be mocked.

Example usage with initial time set

```mjs
import { mock } from 'node:test';
mock.timers.enable({ apis: ['Date'], now: 1000 });
```

```cjs
const { mock } = require('node:test');
mock.timers.enable({ apis: ['Date'], now: 1000 });
```

Example usage with initial Date object as time set

```mjs
import { mock } from 'node:test';
mock.timers.enable({ apis: ['Date'], now: new Date() });
```

```cjs
const { mock } = require('node:test');
mock.timers.enable({ apis: ['Date'], now: new Date() });
```

Alternatively, if you call `mock.timers.enable()` without any parameters:

All timers (`'setInterval'`, `'clearInterval'`, `'setTimeout'`, and `'clearTimeout'`)
will be mocked. The `setInterval`, `clearInterval`, `setTimeout`, and `clearTimeout`
functions from `node:timers`, `node:timers/promises`,
and `globalThis` will be mocked. As well as the global `Date` object.

### `timers.reset()`

<!-- YAML
added:
  - v20.4.0
-->

This function restores the default behavior of all mocks that were previously
created by this  `MockTimers` instance and disassociates the mocks
from the  `MockTracker` instance.

**Note:** After each test completes, this function is called on
the test context's  `MockTracker`.

```mjs
import { mock } from 'node:test';
mock.timers.reset();
```

```cjs
const { mock } = require('node:test');
mock.timers.reset();
```

### `timers[Symbol.dispose]()`

Calls `timers.reset()`.

### `timers.tick(milliseconds)`

<!-- YAML
added:
  - v20.4.0
-->

Advances time for all mocked timers.

* `milliseconds` {number} The amount of time, in milliseconds,
  to advance the timers.

**Note:** This diverges from how `setTimeout` in Node.js behaves and accepts
only positive numbers. In Node.js, `setTimeout` with negative numbers is
only supported for web compatibility reasons.

The following example mocks a `setTimeout` function and
by using `.tick` advances in
time triggering all pending timers.

```mjs
import assert from 'node:assert';
import { test } from '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);
});
```

```cjs
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);
});
```

Alternativelly, the `.tick` function can be called many times

```mjs
import assert from 'node:assert';
import { test } from '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'] });
  const nineSecs = 9000;
  setTimeout(fn, nineSecs);

  const twoSeconds = 3000;
  context.mock.timers.tick(twoSeconds);
  context.mock.timers.tick(twoSeconds);
  context.mock.timers.tick(twoSeconds);

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

```cjs
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'] });
  const nineSecs = 9000;
  setTimeout(fn, nineSecs);

  const twoSeconds = 3000;
  context.mock.timers.tick(twoSeconds);
  context.mock.timers.tick(twoSeconds);
  context.mock.timers.tick(twoSeconds);

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

Advancing time using `.tick` will also advance the time for any `Date` object
created after the mock was enabled (if `Date` was also set to be mocked).

```mjs
import assert from 'node:assert';
import { test } from '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', 'Date'] });
  setTimeout(fn, 9999);

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

  // Advance in time
  context.mock.timers.tick(9999);
  assert.strictEqual(fn.mock.callCount(), 1);
  assert.strictEqual(Date.now(), 9999);
});
```

```cjs
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', 'Date'] });

  setTimeout(fn, 9999);
  assert.strictEqual(fn.mock.callCount(), 0);
  assert.strictEqual(Date.now(), 0);

  // Advance in time
  context.mock.timers.tick(9999);
  assert.strictEqual(fn.mock.callCount(), 1);
  assert.strictEqual(Date.now(), 9999);
});
```

#### Using clear functions

As mentioned, all clear functions from timers (`clearTimeout` and `clearInterval`)
are implicity mocked. Take a look at this example using `setTimeout`:

```mjs
import assert from 'node:assert';
import { test } from 'node:test';

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

  // Optionally choose what to mock
  context.mock.timers.enable({ apis: ['setTimeout'] });
  const id = setTimeout(fn, 9999);

  // Implicity mocked as well
  clearTimeout(id);
  context.mock.timers.tick(9999);

  // As that setTimeout was cleared the mock function will never be called
  assert.strictEqual(fn.mock.callCount(), 0);
});
```

```cjs
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();

  // Optionally choose what to mock
  context.mock.timers.enable({ apis: ['setTimeout'] });
  const id = setTimeout(fn, 9999);

  // Implicity mocked as well
  clearTimeout(id);
  context.mock.timers.tick(9999);

  // As that setTimeout was cleared the mock function will never be called
  assert.strictEqual(fn.mock.callCount(), 0);
});
```

#### Working with Node.js timers modules

Once you enable mocking timers, [node:timers](./timers.md),
[node:timers/promises](./timers.md#timers-promises-api) modules,
and timers from the Node.js global context are enabled:

**Note:** Destructuring functions such as
`import { setTimeout } from 'node:timers'` is currently
not supported by this API.

```mjs
import assert from 'node:assert';
import { test } from 'node:test';
import nodeTimers from 'node:timers';
import nodeTimersPromises from 'node:timers/promises';

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

  // Optionally choose what to mock
  context.mock.timers.enable({ apis: ['setTimeout'] });
  setTimeout(globalTimeoutObjectSpy, 9999);
  nodeTimers.setTimeout(nodeTimerSpy, 9999);

  const promise = nodeTimersPromises.setTimeout(9999).then(nodeTimerPromiseSpy);

  // Advance in time
  context.mock.timers.tick(9999);
  assert.strictEqual(globalTimeoutObjectSpy.mock.callCount(), 1);
  assert.strictEqual(nodeTimerSpy.mock.callCount(), 1);
  await promise;
  assert.strictEqual(nodeTimerPromiseSpy.mock.callCount(), 1);
});
```

```cjs
const assert = require('node:assert');
const { test } = require('node:test');
const nodeTimers = require('node:timers');
const nodeTimersPromises = require('node:timers/promises');

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

  // Optionally choose what to mock
  context.mock.timers.enable({ apis: ['setTimeout'] });
  setTimeout(globalTimeoutObjectSpy, 9999);
  nodeTimers.setTimeout(nodeTimerSpy, 9999);

  const promise = nodeTimersPromises.setTimeout(9999).then(nodeTimerPromiseSpy);

  // Advance in time
  context.mock.timers.tick(9999);
  assert.strictEqual(globalTimeoutObjectSpy.mock.callCount(), 1);
  assert.strictEqual(nodeTimerSpy.mock.callCount(), 1);
  await promise;
  assert.strictEqual(nodeTimerPromiseSpy.mock.callCount(), 1);
});
```

In Node.js, `setInterval` from [node:timers/promises](./timers.md#timers-promises-api)
is an `AsyncGenerator` and is also supported by this API:

```mjs
import assert from 'node:assert';
import { test } from 'node:test';
import nodeTimersPromises from 'node:timers/promises';
test('should tick five times testing a real use case', async (context) => {
  context.mock.timers.enable({ apis: ['setInterval'] });

  const expectedIterations = 3;
  const interval = 1000;
  const startedAt = Date.now();
  async function run() {
    const times = [];
    for await (const time of nodeTimersPromises.setInterval(interval, startedAt)) {
      times.push(time);
      if (times.length === expectedIterations) break;
    }
    return times;
  }

  const r = run();
  context.mock.timers.tick(interval);
  context.mock.timers.tick(interval);
  context.mock.timers.tick(interval);

  const timeResults = await r;
  assert.strictEqual(timeResults.length, expectedIterations);
  for (let it = 1; it < expectedIterations; it++) {
    assert.strictEqual(timeResults[it - 1], startedAt + (interval * it));
  }
});
```

```cjs
const assert = require('node:assert');
const { test } = require('node:test');
const nodeTimersPromises = require('node:timers/promises');
test('should tick five times testing a real use case', async (context) => {
  context.mock.timers.enable({ apis: ['setInterval'] });

  const expectedIterations = 3;
  const interval = 1000;
  const startedAt = Date.now();
  async function run() {
    const times = [];
    for await (const time of nodeTimersPromises.setInterval(interval, startedAt)) {
      times.push(time);
      if (times.length === expectedIterations) break;
    }
    return times;
  }

  const r = run();
  context.mock.timers.tick(interval);
  context.mock.timers.tick(interval);
  context.mock.timers.tick(interval);

  const timeResults = await r;
  assert.strictEqual(timeResults.length, expectedIterations);
  for (let it = 1; it < expectedIterations; it++) {
    assert.strictEqual(timeResults[it - 1], startedAt + (interval * it));
  }
});
```

### `timers.runAll()`

<!-- YAML
added:
  - v20.4.0
-->

Triggers 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.

The example below triggers all pending timers immediately,
causing them to execute without any delay.

```mjs
import assert from 'node:assert';
import { test } from 'node:test';

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

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

  assert.deepStrictEqual(results, []);

  context.mock.timers.runAll();
  assert.deepStrictEqual(results, [3, 2, 1]);
  // The Date object is also advanced to the furthest timer's time
  assert.strictEqual(Date.now(), 9999);
});
```

```cjs
const assert = require('node: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);

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

  assert.deepStrictEqual(results, []);

  context.mock.timers.runAll();
  assert.deepStrictEqual(results, [3, 2, 1]);
  // The Date object is also advanced to the furthest timer's time
  assert.strictEqual(Date.now(), 9999);
});
```

**Note:** The `runAll()` function is specifically designed for
triggering timers in the context of timer mocking.
It does not have any effect on real-time system
clocks or actual timers outside of the mocking environment.

### `timers.setTime(milliseconds)`

<!-- YAML
added:
  - v20.11.0
-->

Sets the current Unix timestamp that will be used as reference for any mocked
`Date` objects.

```mjs
import assert from 'node:assert';
import { test } from 'node:test';

test('runAll functions following the given order', (context) => {
  const now = Date.now();
  const setTime = 1000;
  // Date.now is not mocked
  assert.deepStrictEqual(Date.now(), now);

  context.mock.timers.enable({ apis: ['Date'] });
  context.mock.timers.setTime(setTime);
  // Date.now is now 1000
  assert.strictEqual(Date.now(), setTime);
});
```

```cjs
const assert = require('node:assert');
const { test } = require('node:test');

test('setTime replaces current time', (context) => {
  const now = Date.now();
  const setTime = 1000;
  // Date.now is not mocked
  assert.deepStrictEqual(Date.now(), now);

  context.mock.timers.enable({ apis: ['Date'] });
  context.mock.timers.setTime(setTime);
  // Date.now is now 1000
  assert.strictEqual(Date.now(), setTime);
});
```

#### Dates and Timers working together

Dates and timer objects are dependent on each other. If you use `setTime()` to
pass the current time to the mocked `Date` object, the set timers with
`setTimeout` and `setInterval` will **not** be affected.

However, the `tick` method **will** advanced the mocked `Date` object.

```mjs
import assert from 'node:assert';
import { test } from '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);
});
```

```cjs
const assert = require('node: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);
});
```

## Class: `TestsStream`

<!-- YAML
added:
  - v18.9.0
  - v16.19.0
changes:
  - version:
    - v20.0.0
    - v19.9.0
    - v18.17.0
    pr-url: https://github.com/nodejs/node/pull/47094
    description: added type to test:pass and test:fail events for when the test is a suite.
-->

* Extends {Readable}

A successful call to [`run()`][] method will return a new {TestsStream}
object, streaming a series of events representing the execution of the tests.
`TestsStream` will emit events, in the order of the tests definition

Some of the events are guaranteed to be emitted in the same order as the tests
are defined, while others are emitted in the order that the tests execute.

### Event: `'test:coverage'`

* `data` {Object}
  * `summary` {Object} An object containing the coverage report.
    * `files` {Array} An array of coverage reports for individual files. Each
      report is an object with the following schema:
      * `path` {string} The absolute path of the file.
      * `totalLineCount` {number} The total number of lines.
      * `totalBranchCount` {number} The total number of branches.
      * `totalFunctionCount` {number} The total number of functions.
      * `coveredLineCount` {number} The number of covered lines.
      * `coveredBranchCount` {number} The number of covered branches.
      * `coveredFunctionCount` {number} The number of covered functions.
      * `coveredLinePercent` {number} The percentage of lines covered.
      * `coveredBranchPercent` {number} The percentage of branches covered.
      * `coveredFunctionPercent` {number} The percentage of functions covered.
      * `functions` {Array} An array of functions representing function
        coverage.
        * `name` {string} The name of the function.
        * `line` {number} The line number where the function is defined.
        * `count` {number} The number of times the function was called.
      * `branches` {Array} An array of branches representing branch coverage.
        * `line` {number} The line number where the branch is defined.
        * `count` {number} The number of times the branch was taken.
      * `lines` {Array} An array of lines representing line
        numbers and the number of times they were covered.
        * `line` {number} The line number.
        * `count` {number} The number of times the line was covered.
    * `totals` {Object} An object containing a summary of coverage for all
      files.
      * `totalLineCount` {number} The total number of lines.
      * `totalBranchCount` {number} The total number of branches.
      * `totalFunctionCount` {number} The total number of functions.
      * `coveredLineCount` {number} The number of covered lines.
      * `coveredBranchCount` {number} The number of covered branches.
      * `coveredFunctionCount` {number} The number of covered functions.
      * `coveredLinePercent` {number} The percentage of lines covered.
      * `coveredBranchPercent` {number} The percentage of branches covered.
      * `coveredFunctionPercent` {number} The percentage of functions covered.
    * `workingDirectory` {string} The working directory when code coverage
      began. This is useful for displaying relative path names in case the tests
      changed the working directory of the Node.js process.
  * `nesting` {number} The nesting level of the test.

Emitted when code coverage is enabled and all tests have completed.

### Event: `'test:complete'`

* `data` {Object}
  * `column` {number|undefined} The column number where the test is defined, or
    `undefined` if the test was run through the REPL.
  * `details` {Object} Additional execution metadata.
    * `passed` {boolean} Whether the test passed or not.
    * `duration_ms` {number} The duration of the test in milliseconds.
    * `error` {Error|undefined} An error wrapping the error thrown by the test
      if it did not pass.
      * `cause` {Error} The actual error thrown by the test.
    * `type` {string|undefined} The type of the test, used to denote whether
      this is a suite.
  * `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.
  * `testNumber` {number} The ordinal number of the test.
  * `todo` {string|boolean|undefined} Present if [`context.todo`][] is called
  * `skip` {string|boolean|undefined} Present if [`context.skip`][] is called

Emitted when a test completes its execution.
This event is not emitted in the same order as the tests are
defined.
The corresponding declaration ordered events are `'test:pass'` and `'test:fail'`.

### Event: `'test:dequeue'`

* `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 is dequeued, right before it is executed.
This event is not guaranteed to be emitted in the same order as the tests are
defined. The corresponding declaration ordered event is `'test:start'`.

### Event: `'test:diagnostic'`

* `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.
  * `message` {string} The diagnostic message.
  * `nesting` {number} The nesting level of the test.

Emitted when [`context.diagnostic`][] is called.
This event is guaranteed to be emitted in the same order as the tests are
defined.

### Event: `'test:enqueue'`

* `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 is enqueued for execution.

### Event: `'test:fail'`

* `data` {Object}
  * `column` {number|undefined} The column number where the test is defined, or
    `undefined` if the test was run through the REPL.
  * `details` {Object} Additional execution metadata.
    * `duration_ms` {number} The duration of the test in milliseconds.
    * `error` {Error} An error wrapping the error thrown by the test.
      * `cause` {Error} The actual error thrown by the test.
    * `type` {string|undefined} The type of the test, used to denote whether
      this is a suite.
  * `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.
  * `testNumber` {number} The ordinal number of the test.
  * `todo` {string|boolean|undefined} Present if [`context.todo`][] is called
  * `skip` {string|boolean|undefined} Present if [`context.skip`][] is called

Emitted when a test fails.
This event is guaranteed to be emitted in the same order as the tests are
defined.
The corresponding execution ordered event is `'test:complete'`.

### Event: `'test:pass'`

* `data` {Object}
  * `column` {number|undefined} The column number where the test is defined, or
    `undefined` if the test was run through the REPL.
  * `details` {Object} Additional execution metadata.
    * `duration_ms` {number} The duration of the test in milliseconds.
    * `type` {string|undefined} The type of the test, used to denote whether
      this is a suite.
  * `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.
  * `testNumber` {number} The ordinal number of the test.
  * `todo` {string|boolean|undefined} Present if [`context.todo`][] is called
  * `skip` {string|boolean|undefined} Present if [`context.skip`][] is called

Emitted when a test passes.
This event is guaranteed to be emitted in the same order as the tests are
defined.
The corresponding execution ordered event is `'test:complete'`.

### Event: `'test:plan'`

* `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.
  * `nesting` {number} The nesting level of the test.
  * `count` {number} The number of subtests that have ran.

Emitted when all subtests have completed for a given test.
This event is guaranteed to be emitted in the same order as the tests are
defined.

### 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.
The corresponding execution ordered event is `'test:dequeue'`.

### Event: `'test:stderr'`

* `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} The path of the test file.
  * `line` {number|undefined} The line number where the test is defined, or
    `undefined` if the test was run through the REPL.
  * `message` {string} The message written to `stderr`.

Emitted when a running test writes to `stderr`.
This event is only emitted if `--test` flag is passed.
This event is not guaranteed to be emitted in the same order as the tests are
defined.

### Event: `'test:stdout'`

* `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} The path of the test file.
  * `line` {number|undefined} The line number where the test is defined, or
    `undefined` if the test was run through the REPL.
  * `message` {string} The message written to `stdout`.

Emitted when a running test writes to `stdout`.
This event is only emitted if `--test` flag is passed.
This event is not guaranteed to be emitted in the same order as the tests are
defined.

### Event: `'test:watch:drained'`

Emitted when no more tests are queued for execution in watch mode.

## Class: `TestContext`

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An instance of `TestContext` is passed to each test function in order to
interact with the test runner. However, the `TestContext` constructor is not
exposed as part of the API.

### `context.before([fn][, options])`

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* `fn` {Function|AsyncFunction} The hook function. The first argument
  to this function is a [`TestContext`][] object. If the hook uses callbacks,
  the callback function is passed as the second argument. **Default:** A no-op
  function.
* `options` {Object} 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.
    **Default:** `Infinity`.

This function is used to create a hook running before
subtest of the current test.

### `context.beforeEach([fn][, options])`

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* `fn` {Function|AsyncFunction} The hook function. The first argument
  to this function is a [`TestContext`][] object. If the hook uses callbacks,
  the callback function is passed as the second argument. **Default:** A no-op
  function.
* `options` {Object} 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.
    **Default:** `Infinity`.

This function is used to create a hook running
before each subtest of the current test.

```js
test('top level test', async (t) => {
  t.beforeEach((t) => t.diagnostic(`about to run ${t.name}`));
  await t.test(
    'This is a subtest',
    (t) => {
      assert.ok('some relevant assertion here');
    },
  );
});
```

### `context.after([fn][, options])`

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* `fn` {Function|AsyncFunction} The hook function. The first argument
  to this function is a [`TestContext`][] object. If the hook uses callbacks,
  the callback function is passed as the second argument. **Default:** A no-op
  function.
* `options` {Object} 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.
    **Default:** `Infinity`.

This function is used to create a hook that runs after the current test
finishes.

```js
test('top level test', async (t) => {
  t.after((t) => t.diagnostic(`finished running ${t.name}`));
  assert.ok('some relevant assertion here');
});
```

### `context.afterEach([fn][, options])`

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* `fn` {Function|AsyncFunction} The hook function. The first argument
  to this function is a [`TestContext`][] object. If the hook uses callbacks,
  the callback function is passed as the second argument. **Default:** A no-op
  function.
* `options` {Object} 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.
    **Default:** `Infinity`.

This function is used to create a hook running
after each subtest of the current test.

```js
test('top level test', async (t) => {
  t.afterEach((t) => t.diagnostic(`finished running ${t.name}`));
  await t.test(
    'This is a subtest',
    (t) => {
      assert.ok('some relevant assertion here');
    },
  );
});
```

### `context.diagnostic(message)`

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* `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.

```js
test('top level test', (t) => {
  t.diagnostic('A diagnostic message');
});
```

### `context.name`

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The name of the test.

### `context.plan(count)`

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> Stability: 1 - Experimental

* `count` {number} The number of assertions and subtests that are expected to run.

This function is used to set the number of assertions and subtests that are expected to run
within the test. If the number of assertions and subtests that run does not match the
expected count, the test will fail.

> Note: To make sure assertions are tracked, `t.assert` must be used instead of `assert` directly.

```js
test('top level test', (t) => {
  t.plan(2);
  t.assert.ok('some relevant assertion here');
  t.subtest('subtest', () => {});
});
```

When working with asynchronous code, the `plan` function can be used to ensure that the
correct number of assertions are run:

```js
test('planning with streams', (t, done) => {
  function* generate() {
    yield 'a';
    yield 'b';
    yield 'c';
  }
  const expected = ['a', 'b', 'c'];
  t.plan(expected.length);
  const stream = Readable.from(generate());
  stream.on('data', (chunk) => {
    t.assert.strictEqual(chunk, expected.shift());
  });

  stream.on('end', () => {
    done();
  });
});
```

### `context.runOnly(shouldRunOnlyTests)`

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* `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.

```js
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 }),
  ]);
});
```

### `context.signal`

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* Type: {AbortSignal}

Can be used to abort test subtasks when the test has been aborted.

```js
test('top level test', async (t) => {
  await fetch('some/uri', { signal: t.signal });
});
```

### `context.skip([message])`

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* `message` {string} Optional skip message.

This function causes the test's output to indicate the test as skipped. If
`message` is provided, it is included in the output. Calling `skip()` does
not terminate execution of the test function. This function does not return a
value.

```js
test('top level test', (t) => {
  // Make sure to return here as well if the test contains additional logic.
  t.skip('this is skipped');
});
```

### `context.todo([message])`

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* `message` {string} Optional `TODO` message.

This function adds a `TODO` directive to the test's output. If `message` is
provided, it is included in the output. Calling `todo()` does not terminate
execution of the test function. This function does not return a value.

```js
test('top level test', (t) => {
  // This test is marked as `TODO`
  t.todo('this is a todo');
});
```

### `context.test([name][, options][, fn])`

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    description: Add a `timeout` option.
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* `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`.
  * `plan` {number} The number of assertions and subtests expected to be run in the test.
    If the number of assertions run in the test does not match the number
    specified in the plan, the test will fail.
    **Default:** `undefined`.
* `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: {Promise} Fulfilled with `undefined` once the test completes.

This function is used to create subtests under the current test. This function
behaves in the same fashion as the top level [`test()`][] function.

```js
test('top level test', async (t) => {
  await t.test(
    'This is a subtest',
    { only: false, skip: false, concurrency: 1, todo: false, plan: 4 },
    (t) => {
      assert.ok('some relevant assertion here');
    },
  );
});
```

## Class: `SuiteContext`

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An instance of `SuiteContext` is passed to each suite function in order to
interact with the test runner. However, the `SuiteContext` constructor is not
exposed as part of the API.

### `context.name`

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The name of the suite.

### `context.signal`

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* Type: {AbortSignal}

Can be used to abort test subtasks when the test has been aborted.

[TAP]: https://testanything.org/
[TTY]: tty.md
[`--experimental-test-coverage`]: cli.md#--experimental-test-coverage
[`--import`]: cli.md#--importmodule
[`--test-concurrency`]: cli.md#--test-concurrency
[`--test-name-pattern`]: cli.md#--test-name-pattern
[`--test-only`]: cli.md#--test-only
[`--test-reporter-destination`]: cli.md#--test-reporter-destination
[`--test-reporter`]: cli.md#--test-reporter
[`--test`]: cli.md#--test
[`MockFunctionContext`]: #class-mockfunctioncontext
[`MockTimers`]: #class-mocktimers
[`MockTracker.method`]: #mockmethodobject-methodname-implementation-options
[`MockTracker`]: #class-mocktracker
[`NODE_V8_COVERAGE`]: cli.md#node_v8_coveragedir
[`SuiteContext`]: #class-suitecontext
[`TestContext`]: #class-testcontext
[`context.diagnostic`]: #contextdiagnosticmessage
[`context.skip`]: #contextskipmessage
[`context.todo`]: #contexttodomessage
[`describe()`]: #describename-options-fn
[`it()`]: #itname-options-fn
[`run()`]: #runoptions
[`suite()`]: #suitename-options-fn
[`test()`]: #testname-options-fn
[describe options]: #describename-options-fn
[it options]: #testname-options-fn
[stream.compose]: stream.md#streamcomposestreams
[suite options]: #suitename-options-fn
[test reporters]: #test-reporters
[test runner execution model]: #test-runner-execution-model