# The `test` npm package

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This is a user-land port of [`node:test`](https://nodejs.org/api/test.html),
the experimental test runner introduced in Node.js 18. This module makes it
available in Node.js 14 and later.

Minimal dependencies, with full test suite.

Differences from the core implementation:

- Doesn't hide its own stack frames.
- Some features require the use of `--experimental-abortcontroller` CLI flag to
  work on Node.js v14.x. It's recommended to pass
  `NODE_OPTIONS='--experimental-abortcontroller --no-warnings'` in your env if
  you are testing on v14.x.

## Docs

### Test runner

> Stability: 1 - Experimental

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

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

```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` resolves.
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 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. 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)
  })
})
```

In this example, `await` is used to ensure that both subtests have completed.
This is necessary because parent tests do not wait for their subtests to
complete. 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')
})
```

## `describe`/`it` syntax

Running tests can also be done using `describe` to declare a suite
and `it` to declare a test.
A suite is used to organize and group related tests together.
`it` is an alias for `test`, except there is no test context passed,
since nesting is done using suites.

```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 `test` module.

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

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

### `only` tests

If `node--test` 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--test 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.
})
```

## Running tests from the command line

The Node.js test runner can be invoked from the command line:

```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/
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. 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.

## 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 '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('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);
});
```


## Test reporters

<!-- YAML
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-->

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 is the default reporter used by the test runner. It 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`.

### 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: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':
        callback(null, event.data.message);
        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: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':
        callback(null, event.data.message);
        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: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 ${event.data.name} failed\n`;
        break;
      case 'test:plan':
        yield 'test plan';
        break;
      case 'test:diagnostic':
        yield `${event.data.message}\n`;
        break;
    }
  }
}
```

```cjs
module.exports = async function * customReporter(source) {
  for await (const event of source) {
    switch (event.type) {
      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 ${event.data.name} failed\n`;
        break;
      case 'test:plan':
        yield 'test plan\n';
        break;
      case 'test:diagnostic':
        yield `${event.data.message}\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: REPLACEME
-->

* `options` {Object} Configuration options for running tests. The following
  properties are supported:
  * `concurrency` {number|boolean} If a number is provided,
    then that many files would run in parallel.
    If `true`, it would run `os.availableParallelism() - 1` test files in
    parallel.
    If `false`, it would only run one test file at a time.
    **Default:** `false`.
  * `files`: {Array} An array containing the list of files to run.
    **Default** matching files from [test runner execution model][].
  * `signal` {AbortSignal} Allows aborting an in-progress test execution.
  * `timeout` {number} A number of milliseconds the test execution will
    fail after.
    If unspecified, subtests inherit this value from their parent.
    **Default:** `Infinity`.
  * `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`.
    
* Returns: {TestsStream}

```js
run({ files: [path.resolve('./tests/test.js')] })
  .pipe(process.stdout);
```

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

- `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.
    If `true`, it would run `os.availableParallelism() - 1` tests in parallel.
    For subtests, it will be `Infinity` tests in parallel.
    If `false`, it would only run one test 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`.
- `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} Resolved with `undefined` once the test completes.

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 resolves once the test completes. 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.

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

* `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} Configuration options for the suite.
  supports the same options as `test([name][, options][, fn])`.
* `fn` {Function|AsyncFunction} The function under suite
  declaring all subtests and subsuites.
  The first argument to this function is a [`SuiteContext`][] object.
  **Default:** A no-op function.
* Returns: `undefined`.

The `describe()` function imported from the `test` module. Each
invocation of this function results in the creation of a Subtest.
After invocation of top level `describe` functions,
all top level tests and suites will execute.

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

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

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

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

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

* `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 suite.
  supports the same options as `test([name][, options][, fn])`.
* `fn` {Function|AsyncFunction} The function under test.
  If the test uses callbacks, the callback function is passed as an argument.
  **Default:** A no-op function.
* Returns: `undefined`.

The `it()` function is the value imported from the `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].

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

* `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 is used to create a hook running before running 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])`

* `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 is used to create a hook running after  running a suite.

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

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

* `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 is used to create a hook running
before each subtest of the current suite.

```js
describe('tests', async () => {
  beforeEach(() => t.diagnostics('about to run a test'));
  it('is a subtest', () => {
    assert.ok('some relevant assertion here');
  });
});
```

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

* `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 is used to create a hook running
after each subtest of the current test.

```js
describe('tests', async () => {
  afterEach(() => t.diagnostics('about to run a test'));
  it('is a subtest', () => {
    assert.ok('some relevant assertion here');
  });
});
```

## Class: `MockFunctionContext`

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

### `ctx.calls`

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* {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()`

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* 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)`

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* `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])`

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* `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.restore()`

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

## Class: `MockTracker`

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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])`

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* `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])`

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

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

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* `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()`

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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()`

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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])`

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

## Class: `TestsStream`

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* Extends {ReadableStream}

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

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

* `data` {Object}
  * `file` {string|undefined} The path of the test file,
    undefined if test is not ran through a file.
  * `message` {string} The diagnostic message.
  * `nesting` {number} The nesting level of the test.

Emitted when [`context.diagnostic`][] is called.

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

* `data` {Object}
  * `details` {Object} Additional execution metadata.
    * `duration` {number} The duration of the test in milliseconds.
    * `error` {Error} The error thrown by the test.
  * `file` {string|undefined} The path of the test file,
    undefined if test is not ran through a file.
  * `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.

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

* `data` {Object}
  * `details` {Object} Additional execution metadata.
    * `duration` {number} The duration of the test in milliseconds.
  * `file` {string|undefined} The path of the test file,
    undefined if test is not ran through a file.
  * `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.

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

* `data` {Object}
  * `file` {string|undefined} The path of the test file,
    undefined if test is not ran through a file.
  * `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.

### Event: `'test:start'`

* `data` {Object}
  * `file` {string|undefined} The path of the test file,
    undefined if test is not ran through a file.
  * `name` {string} The test name.
  * `nesting` {number} The nesting level of the test.

Emitted when a test starts.

## Class: `TestContext`

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

* `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.diagnostics(`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])`

* `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.diagnostics(`finished running ${t.name}`));
  await t.test(
    'This is a subtest',
    (t) => {
      assert.ok('some relevant assertion here');
    }
  );
});
```

### `context.diagnostic(message)`

- `message` {string}Message to be reported.

This function is used to write diagnostics to the output. Any diagnostic
information is included at the end of the test's results. This function does
not return a value.

 `context.name`

The name of the test.

### `context.runOnly(shouldRunOnlyTests)`

- `shouldRunOnlyTests` {boolean} Whether or not to run `only` tests.

If `shouldRunOnlyTests` is truthy, the test context will only run tests that
have the `only` option set. Otherwise, all tests are run. If Node.js was not
started with the [`--test-only`][] command-line option, this function is a
no-op.

### `context.signal`

* [`AbortSignal`][] Can be used to abort test subtasks when the test has been aborted.

> **Warning**
> On Node.js v14.x, this feature won't be available unless you pass the
> `--experimental-abortcontroller` CLI flag or added an external global polyfill
> for `AbortController`.

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

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

* `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.

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

* `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.

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

- `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.
    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`.
  - `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`.
  - `signal` {AbortSignal} Allows aborting an in-progress test.
  - `todo` {boolean|string} If truthy, the test marked as `TODO`. If a string
    is provided, that string is displayed in the test results as the reason why
    the test is `TODO`. **Default:** `false`.
  - `timeout` {number} A number of milliseconds the test will fail after.
    If unspecified, subtests inherit this value from their parent.
    **Default:** `Infinity`.
- `fn` {Function|AsyncFunction} The function under test. The first argument
  to this function is a [`TestContext`][] object. If the test uses callbacks,
  the callback function is passed as the second argument. **Default:** A no-op
  function.
- Returns: {Promise} Resolved 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.

## Class: `SuiteContext`

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`

The name of the suite.

### `context.signal`

* [`AbortSignal`][] Can be used to abort test subtasks when the test has been aborted.

> **Warning**
> On Node.js v14.x, this feature won't be available unless you pass the
> `--experimental-abortcontroller` CLI flag or added an external global polyfill
> for `AbortController`.


[`AbortSignal`]: https://developer.mozilla.org/en-US/docs/Web/API/AbortSignal
[TAP]: https://testanything.org/
[`MockFunctionContext`]: #class-mockfunctioncontext
[`MockTracker.method`]: #mockmethodobject-methodname-implementation-options
[`MockTracker`]: #class-mocktracke
[`SuiteContext`]: #class-suitecontext
[`TestContext`]: #class-testcontext
[`context.diagnostic`]: #contextdiagnosticmessage
[`context.skip`]: #contextskipmessage
[`context.todo`]: #contexttodomessage
[`run()`]: #runoptions
[`test()`]: #testname-options-fn
[describe options]: #describename-options-fn
[it options]: #testname-options-fn
[test runner execution model]: #test-runner-execution-model

## License

[MIT](./LICENSE)
