doc/api/assert.md

# Assert

<!--introduced_in=v0.1.21-->

> Stability: 2 - Stable

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

The `assert` module provides a set of assertion functions for verifying
invariants.

## Strict assertion mode
<!-- YAML
added: v9.9.0
changes:
  - version:
      - v13.9.0
      - v12.16.2
    pr-url: https://github.com/nodejs/node/pull/31635
    description: Changed "strict mode" to "strict assertion mode" and "legacy
                 mode" to "legacy assertion mode" to avoid confusion with the
                 more usual meaning of "strict mode".
  - version: v9.9.0
    pr-url: https://github.com/nodejs/node/pull/17615
    description: Added error diffs to the strict assertion mode.
  - version: v9.9.0
    pr-url: https://github.com/nodejs/node/pull/17002
    description: Added strict assertion mode to the assert module.
-->

In strict assertion mode, non-strict methods behave like their corresponding
strict methods. For example, [`assert.deepEqual()`][] will behave like
[`assert.deepStrictEqual()`][].

In strict assertion mode, error messages for objects display a diff. In legacy
assertion mode, error messages for objects display the objects, often truncated.

To use strict assertion mode:

```js
const assert = require('assert').strict;
```

Example error diff:

```js
const assert = require('assert').strict;

assert.deepEqual([[[1, 2, 3]], 4, 5], [[[1, 2, '3']], 4, 5]);
// AssertionError: Expected inputs to be strictly deep-equal:
// + actual - expected ... Lines skipped
//
//   [
//     [
// ...
//       2,
// +     3
// -     '3'
//     ],
// ...
//     5
//   ]
```

To deactivate the colors, use the `NO_COLOR` or `NODE_DISABLE_COLORS`
environment variables. This will also deactivate the colors in the REPL. For
more on color support in terminal environments, read the tty
[getColorDepth()](tty.md#tty_writestream_getcolordepth_env) documentation.

## Legacy assertion mode

Legacy assertion mode uses the [Abstract Equality Comparison][] in:

* [`assert.deepEqual()`][]
* [`assert.equal()`][]
* [`assert.notDeepEqual()`][]
* [`assert.notEqual()`][]

To use legacy assertion mode:

```js
const assert = require('assert');
```

Whenever possible, use the [strict assertion mode][] instead. Otherwise, the
[Abstract Equality Comparison][] may cause surprising results. This is
especially true for [`assert.deepEqual()`][], where the comparison rules are
lax:

```js
// WARNING: This does not throw an AssertionError!
assert.deepEqual(/a/gi, new Date());
```

## Class: assert.AssertionError

* Extends: {errors.Error}

Indicates the failure of an assertion. All errors thrown by the `assert` module
will be instances of the `AssertionError` class.

### `new assert.AssertionError(options)`
<!-- YAML
added: v0.1.21
-->

* `options` {Object}
  * `message` {string} If provided, the error message is set to this value.
  * `actual` {any} The `actual` property on the error instance.
  * `expected` {any} The `expected` property on the error instance.
  * `operator` {string} The `operator` property on the error instance.
  * `stackStartFn` {Function} If provided, the generated stack trace omits
    frames before this function.

A subclass of `Error` that indicates the failure of an assertion.

All instances contain the built-in `Error` properties (`message` and `name`)
and:

* `actual` {any} Set to the `actual` argument for methods such as
  [`assert.strictEqual()`][].
* `expected` {any} Set to the `expected` value for methods such as
  [`assert.strictEqual()`][].
* `generatedMessage` {boolean} Indicates if the message was auto-generated
  (`true`) or not.
* `code` {string} Value is always `ERR_ASSERTION` to show that the error is an
  assertion error.
* `operator` {string} Set to the passed in operator value.

```js
const assert = require('assert');

// Generate an AssertionError to compare the error message later:
const { message } = new assert.AssertionError({
  actual: 1,
  expected: 2,
  operator: 'strictEqual'
});

// Verify error output:
try {
  assert.strictEqual(1, 2);
} catch (err) {
  assert(err instanceof assert.AssertionError);
  assert.strictEqual(err.message, message);
  assert.strictEqual(err.name, 'AssertionError');
  assert.strictEqual(err.actual, 1);
  assert.strictEqual(err.expected, 2);
  assert.strictEqual(err.code, 'ERR_ASSERTION');
  assert.strictEqual(err.operator, 'strictEqual');
  assert.strictEqual(err.generatedMessage, true);
}
```

## Class: `assert.CallTracker`
<!-- YAML
added: v14.2.0
-->

> Stability: 1 - Experimental

This feature is currently experimental and behavior might still change.

### `new assert.CallTracker()`
<!-- YAML
added: v14.2.0
-->

Creates a new [`CallTracker`][] object which can be used to track if functions
were called a specific number of times. The `tracker.verify()` must be called
for the verification to take place. The usual pattern would be to call it in a
[`process.on('exit')`][] handler.

```js
const assert = require('assert');

const tracker = new assert.CallTracker();

function func() {}

// callsfunc() must be called exactly 1 time before tracker.verify().
const callsfunc = tracker.calls(func, 1);

callsfunc();

// Calls tracker.verify() and verifies if all tracker.calls() functions have
// been called exact times.
process.on('exit', () => {
  tracker.verify();
});
```

### `tracker.calls([fn][, exact])`
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added: v14.2.0
-->

* `fn` {Function} **Default** A no-op function.
* `exact` {number} **Default** `1`.
* Returns: {Function} that wraps `fn`.

The wrapper function is expected to be called exactly `exact` times. If the
function has not been called exactly `exact` times when
[`tracker.verify()`][] is called, then [`tracker.verify()`][] will throw an
error.

```js
const assert = require('assert');

// Creates call tracker.
const tracker = new assert.CallTracker();

function func() {}

// Returns a function that wraps func() that must be called exact times
// before tracker.verify().
const callsfunc = tracker.calls(func);
```

### `tracker.report()`
<!-- YAML
added: v14.2.0
-->

* Returns: {Array} of objects containing information about the wrapper functions
  returned by [`tracker.calls()`][].
* Object {Object}
  * `message` {string}
  * `actual` {number} The actual number of times the function was called.
  * `expected` {number} The number of times the function was expected to be
    called.
  * `operator` {string} The name of the function that is wrapped.
  * `stack` {Object} A stack trace of the function.

The arrays contains information about the expected and actual number of calls of
the functions that have not been called the expected number of times.

```js
const assert = require('assert');

// Creates call tracker.
const tracker = new assert.CallTracker();

function func() {}

function foo() {}

// Returns a function that wraps func() that must be called exact times
// before tracker.verify().
const callsfunc = tracker.calls(func, 2);

// Returns an array containing information on callsfunc()
tracker.report();
// [
//  {
//    message: 'Expected the func function to be executed 2 time(s) but was
//    executed 0 time(s).',
//    actual: 0,
//    expected: 2,
//    operator: 'func',
//    stack: stack trace
//  }
// ]
```

### `tracker.verify()`
<!-- YAML
added: v14.2.0
-->

Iterates through the list of functions passed to
[`tracker.calls()`][] and will throw an error for functions that
have not been called the expected number of times.

```js
const assert = require('assert');

// Creates call tracker.
const tracker = new assert.CallTracker();

function func() {}

// Returns a function that wraps func() that must be called exact times
// before tracker.verify().
const callsfunc = tracker.calls(func, 2);

callsfunc();

// Will throw an error since callsfunc() was only called once.
tracker.verify();
```

## `assert(value[, message])`
<!-- YAML
added: v0.5.9
-->

* `value` {any} The input that is checked for being truthy.
* `message` {string|Error}

An alias of [`assert.ok()`][].

## `assert.deepEqual(actual, expected[, message])`
<!-- YAML
added: v0.1.21
changes:
  - version: v14.18.0
    pr-url: https://github.com/nodejs/node/pull/38113
    description: In Legacy assertion mode, changed status from Deprecated to
                 Legacy.
  - version: v14.0.0
    pr-url: https://github.com/nodejs/node/pull/30766
    description: NaN is now treated as being identical in case both sides are
                 NaN.
  - version: v12.0.0
    pr-url: https://github.com/nodejs/node/pull/25008
    description: The type tags are now properly compared and there are a couple
                 minor comparison adjustments to make the check less surprising.
  - version: v9.0.0
    pr-url: https://github.com/nodejs/node/pull/15001
    description: The `Error` names and messages are now properly compared.
  - version: v8.0.0
    pr-url: https://github.com/nodejs/node/pull/12142
    description: The `Set` and `Map` content is also compared.
  - version:
    - v6.4.0
    - v4.7.1
    pr-url: https://github.com/nodejs/node/pull/8002
    description: Typed array slices are handled correctly now.
  - version:
    - v6.1.0
    - v4.5.0
    pr-url: https://github.com/nodejs/node/pull/6432
    description: Objects with circular references can be used as inputs now.
  - version:
    - v5.10.1
    - v4.4.3
    pr-url: https://github.com/nodejs/node/pull/5910
    description: Handle non-`Uint8Array` typed arrays correctly.
-->

* `actual` {any}
* `expected` {any}
* `message` {string|Error}

**Strict assertion mode**

An alias of [`assert.deepStrictEqual()`][].

**Legacy assertion mode**

> Stability: 3 - Legacy: Use [`assert.deepStrictEqual()`][] instead.

Tests for deep equality between the `actual` and `expected` parameters. Consider
using [`assert.deepStrictEqual()`][] instead. [`assert.deepEqual()`][] can have
surprising results.

_Deep equality_ means that the enumerable "own" properties of child objects
are also recursively evaluated by the following rules.

### Comparison details

* Primitive values are compared with the [Abstract Equality Comparison][]
  ( `==` ) with the exception of `NaN`. It is treated as being identical in case
  both sides are `NaN`.
* [Type tags][Object.prototype.toString()] of objects should be the same.
* Only [enumerable "own" properties][] are considered.
* [`Error`][] names and messages are always compared, even if these are not
  enumerable properties.
* [Object wrappers][] are compared both as objects and unwrapped values.
* `Object` properties are compared unordered.
* [`Map`][] keys and [`Set`][] items are compared unordered.
* Recursion stops when both sides differ or both sides encounter a circular
  reference.
* Implementation does not test the [`[[Prototype]]`][prototype-spec] of
  objects.
* [`Symbol`][] properties are not compared.
* [`WeakMap`][] and [`WeakSet`][] comparison does not rely on their values.

The following example does not throw an [`AssertionError`][] because the
primitives are considered equal by the [Abstract Equality Comparison][]
( `==` ).

```js
// WARNING: This does not throw an AssertionError!
assert.deepEqual('+00000000', false);
```

"Deep" equality means that the enumerable "own" properties of child objects
are evaluated also:

```js
const assert = require('assert');

const obj1 = {
  a: {
    b: 1
  }
};
const obj2 = {
  a: {
    b: 2
  }
};
const obj3 = {
  a: {
    b: 1
  }
};
const obj4 = Object.create(obj1);

assert.deepEqual(obj1, obj1);
// OK

// Values of b are different:
assert.deepEqual(obj1, obj2);
// AssertionError: { a: { b: 1 } } deepEqual { a: { b: 2 } }

assert.deepEqual(obj1, obj3);
// OK

// Prototypes are ignored:
assert.deepEqual(obj1, obj4);
// AssertionError: { a: { b: 1 } } deepEqual {}
```

If the values are not equal, an [`AssertionError`][] is thrown with a `message`
property set equal to the value of the `message` parameter. If the `message`
parameter is undefined, a default error message is assigned. If the `message`
parameter is an instance of an [`Error`][] then it will be thrown instead of the
[`AssertionError`][].

## `assert.deepStrictEqual(actual, expected[, message])`
<!-- YAML
added: v1.2.0
changes:
  - version: v9.0.0
    pr-url: https://github.com/nodejs/node/pull/15169
    description: Enumerable symbol properties are now compared.
  - version: v9.0.0
    pr-url: https://github.com/nodejs/node/pull/15036
    description: The `NaN` is now compared using the
              [SameValueZero](https://tc39.github.io/ecma262/#sec-samevaluezero)
              comparison.
  - version: v8.5.0
    pr-url: https://github.com/nodejs/node/pull/15001
    description: The `Error` names and messages are now properly compared.
  - version: v8.0.0
    pr-url: https://github.com/nodejs/node/pull/12142
    description: The `Set` and `Map` content is also compared.
  - version:
    - v6.4.0
    - v4.7.1
    pr-url: https://github.com/nodejs/node/pull/8002
    description: Typed array slices are handled correctly now.
  - version: v6.1.0
    pr-url: https://github.com/nodejs/node/pull/6432
    description: Objects with circular references can be used as inputs now.
  - version:
    - v5.10.1
    - v4.4.3
    pr-url: https://github.com/nodejs/node/pull/5910
    description: Handle non-`Uint8Array` typed arrays correctly.
-->

* `actual` {any}
* `expected` {any}
* `message` {string|Error}

Tests for deep equality between the `actual` and `expected` parameters.
"Deep" equality means that the enumerable "own" properties of child objects
are recursively evaluated also by the following rules.

### Comparison details

* Primitive values are compared using the [SameValue Comparison][], used by
  [`Object.is()`][].
* [Type tags][Object.prototype.toString()] of objects should be the same.
* [`[[Prototype]]`][prototype-spec] of objects are compared using
  the [Strict Equality Comparison][].
* Only [enumerable "own" properties][] are considered.
* [`Error`][] names and messages are always compared, even if these are not
  enumerable properties.
* Enumerable own [`Symbol`][] properties are compared as well.
* [Object wrappers][] are compared both as objects and unwrapped values.
* `Object` properties are compared unordered.
* [`Map`][] keys and [`Set`][] items are compared unordered.
* Recursion stops when both sides differ or both sides encounter a circular
  reference.
* [`WeakMap`][] and [`WeakSet`][] comparison does not rely on their values. See
  below for further details.

```js
const assert = require('assert').strict;

// This fails because 1 !== '1'.
assert.deepStrictEqual({ a: 1 }, { a: '1' });
// AssertionError: Expected inputs to be strictly deep-equal:
// + actual - expected
//
//   {
// +   a: 1
// -   a: '1'
//   }

// The following objects don't have own properties
const date = new Date();
const object = {};
const fakeDate = {};
Object.setPrototypeOf(fakeDate, Date.prototype);

// Different [[Prototype]]:
assert.deepStrictEqual(object, fakeDate);
// AssertionError: Expected inputs to be strictly deep-equal:
// + actual - expected
//
// + {}
// - Date {}

// Different type tags:
assert.deepStrictEqual(date, fakeDate);
// AssertionError: Expected inputs to be strictly deep-equal:
// + actual - expected
//
// + 2018-04-26T00:49:08.604Z
// - Date {}

assert.deepStrictEqual(NaN, NaN);
// OK, because of the SameValue comparison

// Different unwrapped numbers:
assert.deepStrictEqual(new Number(1), new Number(2));
// AssertionError: Expected inputs to be strictly deep-equal:
// + actual - expected
//
// + [Number: 1]
// - [Number: 2]

assert.deepStrictEqual(new String('foo'), Object('foo'));
// OK because the object and the string are identical when unwrapped.

assert.deepStrictEqual(-0, -0);
// OK

// Different zeros using the SameValue Comparison:
assert.deepStrictEqual(0, -0);
// AssertionError: Expected inputs to be strictly deep-equal:
// + actual - expected
//
// + 0
// - -0

const symbol1 = Symbol();
const symbol2 = Symbol();
assert.deepStrictEqual({ [symbol1]: 1 }, { [symbol1]: 1 });
// OK, because it is the same symbol on both objects.

assert.deepStrictEqual({ [symbol1]: 1 }, { [symbol2]: 1 });
// AssertionError [ERR_ASSERTION]: Inputs identical but not reference equal:
//
// {
//   [Symbol()]: 1
// }

const weakMap1 = new WeakMap();
const weakMap2 = new WeakMap([[{}, {}]]);
const weakMap3 = new WeakMap();
weakMap3.unequal = true;

assert.deepStrictEqual(weakMap1, weakMap2);
// OK, because it is impossible to compare the entries

// Fails because weakMap3 has a property that weakMap1 does not contain:
assert.deepStrictEqual(weakMap1, weakMap3);
// AssertionError: Expected inputs to be strictly deep-equal:
// + actual - expected
//
//   WeakMap {
// +   [items unknown]
// -   [items unknown],
// -   unequal: true
//   }
```

If the values are not equal, an [`AssertionError`][] is thrown with a `message`
property set equal to the value of the `message` parameter. If the `message`
parameter is undefined, a default error message is assigned. If the `message`
parameter is an instance of an [`Error`][] then it will be thrown instead of the
`AssertionError`.

## `assert.doesNotMatch(string, regexp[, message])`
<!-- YAML
added:
  - v13.6.0
  - v12.16.0
-->

* `string` {string}
* `regexp` {RegExp}
* `message` {string|Error}

> Stability: 1 - Experimental

Expects the `string` input not to match the regular expression.

This feature is currently experimental and the name might change or it might be
completely removed again.

```js
const assert = require('assert').strict;

assert.doesNotMatch('I will fail', /fail/);
// AssertionError [ERR_ASSERTION]: The input was expected to not match the ...

assert.doesNotMatch(123, /pass/);
// AssertionError [ERR_ASSERTION]: The "string" argument must be of type string.

assert.doesNotMatch('I will pass', /different/);
// OK
```

If the values do match, or if the `string` argument is of another type than
`string`, an [`AssertionError`][] is thrown with a `message` property set equal
to the value of the `message` parameter. If the `message` parameter is
undefined, a default error message is assigned. If the `message` parameter is an
instance of an [`Error`][] then it will be thrown instead of the
[`AssertionError`][].

## `assert.doesNotReject(asyncFn[, error][, message])`
<!-- YAML
added: v10.0.0
-->

* `asyncFn` {Function|Promise}
* `error` {RegExp|Function}
* `message` {string}

Awaits the `asyncFn` promise or, if `asyncFn` is a function, immediately
calls the function and awaits the returned promise to complete. It will then
check that the promise is not rejected.

If `asyncFn` is a function and it throws an error synchronously,
`assert.doesNotReject()` will return a rejected `Promise` with that error. If
the function does not return a promise, `assert.doesNotReject()` will return a
rejected `Promise` with an [`ERR_INVALID_RETURN_VALUE`][] error. In both cases
the error handler is skipped.

Using `assert.doesNotReject()` is actually not useful because there is little
benefit in catching a rejection and then rejecting it again. Instead, consider
adding a comment next to the specific code path that should not reject and keep
error messages as expressive as possible.

If specified, `error` can be a [`Class`][], [`RegExp`][] or a validation
function. See [`assert.throws()`][] for more details.

Besides the async nature to await the completion behaves identically to
[`assert.doesNotThrow()`][].

<!-- eslint-disable no-restricted-syntax -->
```js
(async () => {
  await assert.doesNotReject(
    async () => {
      throw new TypeError('Wrong value');
    },
    SyntaxError
  );
})();
```

<!-- eslint-disable no-restricted-syntax -->
```js
assert.doesNotReject(Promise.reject(new TypeError('Wrong value')))
  .then(() => {
    // ...
  });
```

## `assert.doesNotThrow(fn[, error][, message])`
<!-- YAML
added: v0.1.21
changes:
  - version:
    - v5.11.0
    - v4.4.5
    pr-url: https://github.com/nodejs/node/pull/2407
    description: The `message` parameter is respected now.
  - version: v4.2.0
    pr-url: https://github.com/nodejs/node/pull/3276
    description: The `error` parameter can now be an arrow function.
-->

* `fn` {Function}
* `error` {RegExp|Function}
* `message` {string}

Asserts that the function `fn` does not throw an error.

Using `assert.doesNotThrow()` is actually not useful because there
is no benefit in catching an error and then rethrowing it. Instead, consider
adding a comment next to the specific code path that should not throw and keep
error messages as expressive as possible.

When `assert.doesNotThrow()` is called, it will immediately call the `fn`
function.

If an error is thrown and it is the same type as that specified by the `error`
parameter, then an [`AssertionError`][] is thrown. If the error is of a
different type, or if the `error` parameter is undefined, the error is
propagated back to the caller.

If specified, `error` can be a [`Class`][], [`RegExp`][] or a validation
function. See [`assert.throws()`][] for more details.

The following, for instance, will throw the [`TypeError`][] because there is no
matching error type in the assertion:

<!-- eslint-disable no-restricted-syntax -->
```js
assert.doesNotThrow(
  () => {
    throw new TypeError('Wrong value');
  },
  SyntaxError
);
```

However, the following will result in an [`AssertionError`][] with the message
'Got unwanted exception...':

<!-- eslint-disable no-restricted-syntax -->
```js
assert.doesNotThrow(
  () => {
    throw new TypeError('Wrong value');
  },
  TypeError
);
```

If an [`AssertionError`][] is thrown and a value is provided for the `message`
parameter, the value of `message` will be appended to the [`AssertionError`][]
message:

<!-- eslint-disable no-restricted-syntax -->
```js
assert.doesNotThrow(
  () => {
    throw new TypeError('Wrong value');
  },
  /Wrong value/,
  'Whoops'
);
// Throws: AssertionError: Got unwanted exception: Whoops
```

## `assert.equal(actual, expected[, message])`
<!-- YAML
added: v0.1.21
changes:
  - version: v14.18.0
    pr-url: https://github.com/nodejs/node/pull/38113
    description: In Legacy assertion mode, changed status from Deprecated to
                 Legacy.
  - version: v14.0.0
    pr-url: https://github.com/nodejs/node/pull/30766
    description: NaN is now treated as being identical in case both sides are
                 NaN.
-->

* `actual` {any}
* `expected` {any}
* `message` {string|Error}

**Strict assertion mode**

An alias of [`assert.strictEqual()`][].

**Legacy assertion mode**

> Stability: 3 - Legacy: Use [`assert.strictEqual()`][] instead.

Tests shallow, coercive equality between the `actual` and `expected` parameters
using the [Abstract Equality Comparison][] ( `==` ). `NaN` is special handled
and treated as being identical in case both sides are `NaN`.

```js
const assert = require('assert');

assert.equal(1, 1);
// OK, 1 == 1
assert.equal(1, '1');
// OK, 1 == '1'
assert.equal(NaN, NaN);
// OK

assert.equal(1, 2);
// AssertionError: 1 == 2
assert.equal({ a: { b: 1 } }, { a: { b: 1 } });
// AssertionError: { a: { b: 1 } } == { a: { b: 1 } }
```

If the values are not equal, an [`AssertionError`][] is thrown with a `message`
property set equal to the value of the `message` parameter. If the `message`
parameter is undefined, a default error message is assigned. If the `message`
parameter is an instance of an [`Error`][] then it will be thrown instead of the
`AssertionError`.

## `assert.fail([message])`
<!-- YAML
added: v0.1.21
-->

* `message` {string|Error} **Default:** `'Failed'`

Throws an [`AssertionError`][] with the provided error message or a default
error message. If the `message` parameter is an instance of an [`Error`][] then
it will be thrown instead of the [`AssertionError`][].

```js
const assert = require('assert').strict;

assert.fail();
// AssertionError [ERR_ASSERTION]: Failed

assert.fail('boom');
// AssertionError [ERR_ASSERTION]: boom

assert.fail(new TypeError('need array'));
// TypeError: need array
```

Using `assert.fail()` with more than two arguments is possible but deprecated.
See below for further details.

## `assert.fail(actual, expected[, message[, operator[, stackStartFn]]])`
<!-- YAML
added: v0.1.21
changes:
  - version: v10.0.0
    pr-url: https://github.com/nodejs/node/pull/18418
    description: Calling `assert.fail()` with more than one argument is
                 deprecated and emits a warning.
-->

> Stability: 0 - Deprecated: Use `assert.fail([message])` or other assert
> functions instead.

* `actual` {any}
* `expected` {any}
* `message` {string|Error}
* `operator` {string} **Default:** `'!='`
* `stackStartFn` {Function} **Default:** `assert.fail`

If `message` is falsy, the error message is set as the values of `actual` and
`expected` separated by the provided `operator`. If just the two `actual` and
`expected` arguments are provided, `operator` will default to `'!='`. If
`message` is provided as third argument it will be used as the error message and
the other arguments will be stored as properties on the thrown object. If
`stackStartFn` is provided, all stack frames above that function will be
removed from stacktrace (see [`Error.captureStackTrace`][]). If no arguments are
given, the default message `Failed` will be used.

```js
const assert = require('assert').strict;

assert.fail('a', 'b');
// AssertionError [ERR_ASSERTION]: 'a' != 'b'

assert.fail(1, 2, undefined, '>');
// AssertionError [ERR_ASSERTION]: 1 > 2

assert.fail(1, 2, 'fail');
// AssertionError [ERR_ASSERTION]: fail

assert.fail(1, 2, 'whoops', '>');
// AssertionError [ERR_ASSERTION]: whoops

assert.fail(1, 2, new TypeError('need array'));
// TypeError: need array
```

In the last three cases `actual`, `expected`, and `operator` have no
influence on the error message.

Example use of `stackStartFn` for truncating the exception's stacktrace:

```js
function suppressFrame() {
  assert.fail('a', 'b', undefined, '!==', suppressFrame);
}
suppressFrame();
// AssertionError [ERR_ASSERTION]: 'a' !== 'b'
//     at repl:1:1
//     at ContextifyScript.Script.runInThisContext (vm.js:44:33)
//     ...
```

## `assert.ifError(value)`
<!-- YAML
added: v0.1.97
changes:
  - version: v10.0.0
    pr-url: https://github.com/nodejs/node/pull/18247
    description: Instead of throwing the original error it is now wrapped into
                 an [`AssertionError`][] that contains the full stack trace.
  - version: v10.0.0
    pr-url: https://github.com/nodejs/node/pull/18247
    description: Value may now only be `undefined` or `null`. Before all falsy
                 values were handled the same as `null` and did not throw.
-->

* `value` {any}

Throws `value` if `value` is not `undefined` or `null`. This is useful when
testing the `error` argument in callbacks. The stack trace contains all frames
from the error passed to `ifError()` including the potential new frames for
`ifError()` itself.

```js
const assert = require('assert').strict;

assert.ifError(null);
// OK
assert.ifError(0);
// AssertionError [ERR_ASSERTION]: ifError got unwanted exception: 0
assert.ifError('error');
// AssertionError [ERR_ASSERTION]: ifError got unwanted exception: 'error'
assert.ifError(new Error());
// AssertionError [ERR_ASSERTION]: ifError got unwanted exception: Error

// Create some random error frames.
let err;
(function errorFrame() {
  err = new Error('test error');
})();

(function ifErrorFrame() {
  assert.ifError(err);
})();
// AssertionError [ERR_ASSERTION]: ifError got unwanted exception: test error
//     at ifErrorFrame
//     at errorFrame
```

## `assert.match(string, regexp[, message])`
<!-- YAML
added:
  - v13.6.0
  - v12.16.0
-->

* `string` {string}
* `regexp` {RegExp}
* `message` {string|Error}

> Stability: 1 - Experimental

Expects the `string` input to match the regular expression.

This feature is currently experimental and the name might change or it might be
completely removed again.

```js
const assert = require('assert').strict;

assert.match('I will fail', /pass/);
// AssertionError [ERR_ASSERTION]: The input did not match the regular ...

assert.match(123, /pass/);
// AssertionError [ERR_ASSERTION]: The "string" argument must be of type string.

assert.match('I will pass', /pass/);
// OK
```

If the values do not match, or if the `string` argument is of another type than
`string`, an [`AssertionError`][] is thrown with a `message` property set equal
to the value of the `message` parameter. If the `message` parameter is
undefined, a default error message is assigned. If the `message` parameter is an
instance of an [`Error`][] then it will be thrown instead of the
[`AssertionError`][].

## `assert.notDeepEqual(actual, expected[, message])`
<!-- YAML
added: v0.1.21
changes:
  - version: v14.18.0
    pr-url: https://github.com/nodejs/node/pull/38113
    description: In Legacy assertion mode, changed status from Deprecated to
                 Legacy.
  - version: v14.0.0
    pr-url: https://github.com/nodejs/node/pull/30766
    description: NaN is now treated as being identical in case both sides are
                 NaN.
  - version: v9.0.0
    pr-url: https://github.com/nodejs/node/pull/15001
    description: The `Error` names and messages are now properly compared.
  - version: v8.0.0
    pr-url: https://github.com/nodejs/node/pull/12142
    description: The `Set` and `Map` content is also compared.
  - version:
    - v6.4.0
    - v4.7.1
    pr-url: https://github.com/nodejs/node/pull/8002
    description: Typed array slices are handled correctly now.
  - version:
    - v6.1.0
    - v4.5.0
    pr-url: https://github.com/nodejs/node/pull/6432
    description: Objects with circular references can be used as inputs now.
  - version:
    - v5.10.1
    - v4.4.3
    pr-url: https://github.com/nodejs/node/pull/5910
    description: Handle non-`Uint8Array` typed arrays correctly.
-->

* `actual` {any}
* `expected` {any}
* `message` {string|Error}

**Strict assertion mode**

An alias of [`assert.notDeepStrictEqual()`][].

**Legacy assertion mode**

> Stability: 3 - Legacy: Use [`assert.notDeepStrictEqual()`][] instead.

Tests for any deep inequality. Opposite of [`assert.deepEqual()`][].

```js
const assert = require('assert');

const obj1 = {
  a: {
    b: 1
  }
};
const obj2 = {
  a: {
    b: 2
  }
};
const obj3 = {
  a: {
    b: 1
  }
};
const obj4 = Object.create(obj1);

assert.notDeepEqual(obj1, obj1);
// AssertionError: { a: { b: 1 } } notDeepEqual { a: { b: 1 } }

assert.notDeepEqual(obj1, obj2);
// OK

assert.notDeepEqual(obj1, obj3);
// AssertionError: { a: { b: 1 } } notDeepEqual { a: { b: 1 } }

assert.notDeepEqual(obj1, obj4);
// OK
```

If the values are deeply equal, an [`AssertionError`][] is thrown with a
`message` property set equal to the value of the `message` parameter. If the
`message` parameter is undefined, a default error message is assigned. If the
`message` parameter is an instance of an [`Error`][] then it will be thrown
instead of the `AssertionError`.

## `assert.notDeepStrictEqual(actual, expected[, message])`
<!-- YAML
added: v1.2.0
changes:
  - version: v9.0.0
    pr-url: https://github.com/nodejs/node/pull/15398
    description: The `-0` and `+0` are not considered equal anymore.
  - version: v9.0.0
    pr-url: https://github.com/nodejs/node/pull/15036
    description: The `NaN` is now compared using the
              [SameValueZero](https://tc39.github.io/ecma262/#sec-samevaluezero)
              comparison.
  - version: v9.0.0
    pr-url: https://github.com/nodejs/node/pull/15001
    description: The `Error` names and messages are now properly compared.
  - version: v8.0.0
    pr-url: https://github.com/nodejs/node/pull/12142
    description: The `Set` and `Map` content is also compared.
  - version:
    - v6.4.0
    - v4.7.1
    pr-url: https://github.com/nodejs/node/pull/8002
    description: Typed array slices are handled correctly now.
  - version: v6.1.0
    pr-url: https://github.com/nodejs/node/pull/6432
    description: Objects with circular references can be used as inputs now.
  - version:
    - v5.10.1
    - v4.4.3
    pr-url: https://github.com/nodejs/node/pull/5910
    description: Handle non-`Uint8Array` typed arrays correctly.
-->

* `actual` {any}
* `expected` {any}
* `message` {string|Error}

Tests for deep strict inequality. Opposite of [`assert.deepStrictEqual()`][].

```js
const assert = require('assert').strict;

assert.notDeepStrictEqual({ a: 1 }, { a: '1' });
// OK
```

If the values are deeply and strictly equal, an [`AssertionError`][] is thrown
with a `message` property set equal to the value of the `message` parameter. If
the `message` parameter is undefined, a default error message is assigned. If
the `message` parameter is an instance of an [`Error`][] then it will be thrown
instead of the [`AssertionError`][].

## `assert.notEqual(actual, expected[, message])`
<!-- YAML
added: v0.1.21
changes:
  - version: v14.18.0
    pr-url: https://github.com/nodejs/node/pull/38113
    description: In Legacy assertion mode, changed status from Deprecated to
                 Legacy.
  - version: v14.0.0
    pr-url: https://github.com/nodejs/node/pull/30766
    description: NaN is now treated as being identical in case both sides are
                 NaN.
-->

* `actual` {any}
* `expected` {any}
* `message` {string|Error}

**Strict assertion mode**

An alias of [`assert.notStrictEqual()`][].

**Legacy assertion mode**

> Stability: 3 - Legacy: Use [`assert.notStrictEqual()`][] instead.

Tests shallow, coercive inequality with the [Abstract Equality Comparison][]
(`!=` ). `NaN` is special handled and treated as being identical in case both
sides are `NaN`.

```js
const assert = require('assert');

assert.notEqual(1, 2);
// OK

assert.notEqual(1, 1);
// AssertionError: 1 != 1

assert.notEqual(1, '1');
// AssertionError: 1 != '1'
```

If the values are equal, an [`AssertionError`][] is thrown with a `message`
property set equal to the value of the `message` parameter. If the `message`
parameter is undefined, a default error message is assigned. If the `message`
parameter is an instance of an [`Error`][] then it will be thrown instead of the
`AssertionError`.

## `assert.notStrictEqual(actual, expected[, message])`
<!-- YAML
added: v0.1.21
changes:
  - version: v10.0.0
    pr-url: https://github.com/nodejs/node/pull/17003
    description: Used comparison changed from Strict Equality to `Object.is()`.
-->

* `actual` {any}
* `expected` {any}
* `message` {string|Error}

Tests strict inequality between the `actual` and `expected` parameters as
determined by the [SameValue Comparison][].

```js
const assert = require('assert').strict;

assert.notStrictEqual(1, 2);
// OK

assert.notStrictEqual(1, 1);
// AssertionError [ERR_ASSERTION]: Expected "actual" to be strictly unequal to:
//
// 1

assert.notStrictEqual(1, '1');
// OK
```

If the values are strictly equal, an [`AssertionError`][] is thrown with a
`message` property set equal to the value of the `message` parameter. If the
`message` parameter is undefined, a default error message is assigned. If the
`message` parameter is an instance of an [`Error`][] then it will be thrown
instead of the `AssertionError`.

## `assert.ok(value[, message])`
<!-- YAML
added: v0.1.21
changes:
  - version: v10.0.0
    pr-url: https://github.com/nodejs/node/pull/18319
    description: The `assert.ok()` (no arguments) will now use a predefined
                 error message.
-->

* `value` {any}
* `message` {string|Error}

Tests if `value` is truthy. It is equivalent to
`assert.equal(!!value, true, message)`.

If `value` is not truthy, an [`AssertionError`][] is thrown with a `message`
property set equal to the value of the `message` parameter. If the `message`
parameter is `undefined`, a default error message is assigned. If the `message`
parameter is an instance of an [`Error`][] then it will be thrown instead of the
`AssertionError`.
If no arguments are passed in at all `message` will be set to the string:
``'No value argument passed to `assert.ok()`'``.

Be aware that in the `repl` the error message will be different to the one
thrown in a file! See below for further details.

```js
const assert = require('assert').strict;

assert.ok(true);
// OK
assert.ok(1);
// OK

assert.ok();
// AssertionError: No value argument passed to `assert.ok()`

assert.ok(false, 'it\'s false');
// AssertionError: it's false

// In the repl:
assert.ok(typeof 123 === 'string');
// AssertionError: false == true

// In a file (e.g. test.js):
assert.ok(typeof 123 === 'string');
// AssertionError: The expression evaluated to a falsy value:
//
//   assert.ok(typeof 123 === 'string')

assert.ok(false);
// AssertionError: The expression evaluated to a falsy value:
//
//   assert.ok(false)

assert.ok(0);
// AssertionError: The expression evaluated to a falsy value:
//
//   assert.ok(0)

// Using `assert()` works the same:
assert(0);
// AssertionError: The expression evaluated to a falsy value:
//
//   assert(0)
```

## `assert.rejects(asyncFn[, error][, message])`
<!-- YAML
added: v10.0.0
-->

* `asyncFn` {Function|Promise}
* `error` {RegExp|Function|Object|Error}
* `message` {string}

Awaits the `asyncFn` promise or, if `asyncFn` is a function, immediately
calls the function and awaits the returned promise to complete. It will then
check that the promise is rejected.

If `asyncFn` is a function and it throws an error synchronously,
`assert.rejects()` will return a rejected `Promise` with that error. If the
function does not return a promise, `assert.rejects()` will return a rejected
`Promise` with an [`ERR_INVALID_RETURN_VALUE`][] error. In both cases the error
handler is skipped.

Besides the async nature to await the completion behaves identically to
[`assert.throws()`][].

If specified, `error` can be a [`Class`][], [`RegExp`][], a validation function,
an object where each property will be tested for, or an instance of error where
each property will be tested for including the non-enumerable `message` and
`name` properties.

If specified, `message` will be the message provided by the [`AssertionError`][]
if the `asyncFn` fails to reject.

```js
(async () => {
  await assert.rejects(
    async () => {
      throw new TypeError('Wrong value');
    },
    {
      name: 'TypeError',
      message: 'Wrong value'
    }
  );
})();
```

```js
(async () => {
  await assert.rejects(
    async () => {
      throw new TypeError('Wrong value');
    },
    (err) => {
      assert.strictEqual(err.name, 'TypeError');
      assert.strictEqual(err.message, 'Wrong value');
      return true;
    }
  );
})();
```

```js
assert.rejects(
  Promise.reject(new Error('Wrong value')),
  Error
).then(() => {
  // ...
});
```

`error` cannot be a string. If a string is provided as the second
argument, then `error` is assumed to be omitted and the string will be used for
`message` instead. This can lead to easy-to-miss mistakes. Please read the
example in [`assert.throws()`][] carefully if using a string as the second
argument gets considered.

## `assert.strictEqual(actual, expected[, message])`
<!-- YAML
added: v0.1.21
changes:
  - version: v10.0.0
    pr-url: https://github.com/nodejs/node/pull/17003
    description: Used comparison changed from Strict Equality to `Object.is()`.
-->

* `actual` {any}
* `expected` {any}
* `message` {string|Error}

Tests strict equality between the `actual` and `expected` parameters as
determined by the [SameValue Comparison][].

```js
const assert = require('assert').strict;

assert.strictEqual(1, 2);
// AssertionError [ERR_ASSERTION]: Expected inputs to be strictly equal:
//
// 1 !== 2

assert.strictEqual(1, 1);
// OK

assert.strictEqual('Hello foobar', 'Hello World!');
// AssertionError [ERR_ASSERTION]: Expected inputs to be strictly equal:
// + actual - expected
//
// + 'Hello foobar'
// - 'Hello World!'
//          ^

const apples = 1;
const oranges = 2;
assert.strictEqual(apples, oranges, `apples ${apples} !== oranges ${oranges}`);
// AssertionError [ERR_ASSERTION]: apples 1 !== oranges 2

assert.strictEqual(1, '1', new TypeError('Inputs are not identical'));
// TypeError: Inputs are not identical
```

If the values are not strictly equal, an [`AssertionError`][] is thrown with a
`message` property set equal to the value of the `message` parameter. If the
`message` parameter is undefined, a default error message is assigned. If the
`message` parameter is an instance of an [`Error`][] then it will be thrown
instead of the [`AssertionError`][].

## `assert.throws(fn[, error][, message])`
<!-- YAML
added: v0.1.21
changes:
  - version: v10.2.0
    pr-url: https://github.com/nodejs/node/pull/20485
    description: The `error` parameter can be an object containing regular
                 expressions now.
  - version: v9.9.0
    pr-url: https://github.com/nodejs/node/pull/17584
    description: The `error` parameter can now be an object as well.
  - version: v4.2.0
    pr-url: https://github.com/nodejs/node/pull/3276
    description: The `error` parameter can now be an arrow function.
-->

* `fn` {Function}
* `error` {RegExp|Function|Object|Error}
* `message` {string}

Expects the function `fn` to throw an error.

If specified, `error` can be a [`Class`][], [`RegExp`][], a validation function,
a validation object where each property will be tested for strict deep equality,
or an instance of error where each property will be tested for strict deep
equality including the non-enumerable `message` and `name` properties. When
using an object, it is also possible to use a regular expression, when
validating against a string property. See below for examples.

If specified, `message` will be appended to the message provided by the
`AssertionError` if the `fn` call fails to throw or in case the error validation
fails.

Custom validation object/error instance:

```js
const err = new TypeError('Wrong value');
err.code = 404;
err.foo = 'bar';
err.info = {
  nested: true,
  baz: 'text'
};
err.reg = /abc/i;

assert.throws(
  () => {
    throw err;
  },
  {
    name: 'TypeError',
    message: 'Wrong value',
    info: {
      nested: true,
      baz: 'text'
    }
    // Only properties on the validation object will be tested for.
    // Using nested objects requires all properties to be present. Otherwise
    // the validation is going to fail.
  }
);

// Using regular expressions to validate error properties:
assert.throws(
  () => {
    throw err;
  },
  {
    // The `name` and `message` properties are strings and using regular
    // expressions on those will match against the string. If they fail, an
    // error is thrown.
    name: /^TypeError$/,
    message: /Wrong/,
    foo: 'bar',
    info: {
      nested: true,
      // It is not possible to use regular expressions for nested properties!
      baz: 'text'
    },
    // The `reg` property contains a regular expression and only if the
    // validation object contains an identical regular expression, it is going
    // to pass.
    reg: /abc/i
  }
);

// Fails due to the different `message` and `name` properties:
assert.throws(
  () => {
    const otherErr = new Error('Not found');
    // Copy all enumerable properties from `err` to `otherErr`.
    for (const [key, value] of Object.entries(err)) {
      otherErr[key] = value;
    }
    throw otherErr;
  },
  // The error's `message` and `name` properties will also be checked when using
  // an error as validation object.
  err
);
```

Validate instanceof using constructor:

```js
assert.throws(
  () => {
    throw new Error('Wrong value');
  },
  Error
);
```

Validate error message using [`RegExp`][]:

Using a regular expression runs `.toString` on the error object, and will
therefore also include the error name.

```js
assert.throws(
  () => {
    throw new Error('Wrong value');
  },
  /^Error: Wrong value$/
);
```

Custom error validation:

The function must return `true` to indicate all internal validations passed.
It will otherwise fail with an [`AssertionError`][].

```js
assert.throws(
  () => {
    throw new Error('Wrong value');
  },
  (err) => {
    assert(err instanceof Error);
    assert(/value/.test(err));
    // Avoid returning anything from validation functions besides `true`.
    // Otherwise, it's not clear what part of the validation failed. Instead,
    // throw an error about the specific validation that failed (as done in this
    // example) and add as much helpful debugging information to that error as
    // possible.
    return true;
  },
  'unexpected error'
);
```

`error` cannot be a string. If a string is provided as the second
argument, then `error` is assumed to be omitted and the string will be used for
`message` instead. This can lead to easy-to-miss mistakes. Using the same
message as the thrown error message is going to result in an
`ERR_AMBIGUOUS_ARGUMENT` error. Please read the example below carefully if using
a string as the second argument gets considered:

<!-- eslint-disable no-restricted-syntax -->
```js
function throwingFirst() {
  throw new Error('First');
}

function throwingSecond() {
  throw new Error('Second');
}

function notThrowing() {}

// The second argument is a string and the input function threw an Error.
// The first case will not throw as it does not match for the error message
// thrown by the input function!
assert.throws(throwingFirst, 'Second');
// In the next example the message has no benefit over the message from the
// error and since it is not clear if the user intended to actually match
// against the error message, Node.js throws an `ERR_AMBIGUOUS_ARGUMENT` error.
assert.throws(throwingSecond, 'Second');
// TypeError [ERR_AMBIGUOUS_ARGUMENT]

// The string is only used (as message) in case the function does not throw:
assert.throws(notThrowing, 'Second');
// AssertionError [ERR_ASSERTION]: Missing expected exception: Second

// If it was intended to match for the error message do this instead:
// It does not throw because the error messages match.
assert.throws(throwingSecond, /Second$/);

// If the error message does not match, an AssertionError is thrown.
assert.throws(throwingFirst, /Second$/);
// AssertionError [ERR_ASSERTION]
```

Due to the confusing error-prone notation, avoid a string as the second
argument.

[Abstract Equality Comparison]: https://tc39.github.io/ecma262/#sec-abstract-equality-comparison
[Object wrappers]: https://developer.mozilla.org/en-US/docs/Glossary/Primitive#Primitive_wrapper_objects_in_JavaScript
[Object.prototype.toString()]: https://tc39.github.io/ecma262/#sec-object.prototype.tostring
[SameValue Comparison]: https://tc39.github.io/ecma262/#sec-samevalue
[Strict Equality Comparison]: https://tc39.github.io/ecma262/#sec-strict-equality-comparison
[`AssertionError`]: #assert_class_assert_assertionerror
[`CallTracker`]: #assert_class_assert_calltracker
[`Class`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Classes
[`ERR_INVALID_RETURN_VALUE`]: errors.md#errors_err_invalid_return_value
[`Error.captureStackTrace`]: errors.md#errors_error_capturestacktrace_targetobject_constructoropt
[`Error`]: errors.md#errors_class_error
[`Map`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Map
[`Object.is()`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/is
[`RegExp`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Regular_Expressions
[`Set`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Set
[`Symbol`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Symbol
[`TypeError`]: errors.md#errors_class_typeerror
[`WeakMap`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/WeakMap
[`WeakSet`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/WeakSet
[`assert.deepEqual()`]: #assert_assert_deepequal_actual_expected_message
[`assert.deepStrictEqual()`]: #assert_assert_deepstrictequal_actual_expected_message
[`assert.doesNotThrow()`]: #assert_assert_doesnotthrow_fn_error_message
[`assert.equal()`]: #assert_assert_equal_actual_expected_message
[`assert.notDeepEqual()`]: #assert_assert_notdeepequal_actual_expected_message
[`assert.notDeepStrictEqual()`]: #assert_assert_notdeepstrictequal_actual_expected_message
[`assert.notEqual()`]: #assert_assert_notequal_actual_expected_message
[`assert.notStrictEqual()`]: #assert_assert_notstrictequal_actual_expected_message
[`assert.ok()`]: #assert_assert_ok_value_message
[`assert.strictEqual()`]: #assert_assert_strictequal_actual_expected_message
[`assert.throws()`]: #assert_assert_throws_fn_error_message
[`process.on('exit')`]: process.md#process_event_exit
[`tracker.calls()`]: #assert_tracker_calls_fn_exact
[`tracker.verify()`]: #assert_tracker_verify
[enumerable "own" properties]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Enumerability_and_ownership_of_properties
[prototype-spec]: https://tc39.github.io/ecma262/#sec-ordinary-object-internal-methods-and-internal-slots
[strict assertion mode]: #assert_strict_assertion_mode