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Table of contents

- Assert
  - Strict assertion mode
  - Legacy assertion mode
  - Class: assert.AssertionError
    - `new assert.AssertionError(options)`
  - Class: `assert.CallTracker`
    - `new assert.CallTracker()`
    - `tracker.calls([fn][, exact])`
    - `tracker.report()`
    - `tracker.verify()`
  - `assert(value[, message])`
  - `assert.deepEqual(actual, expected[, message])`
    - Comparison details
  - `assert.deepStrictEqual(actual, expected[, message])`
    - Comparison details
  - `assert.doesNotMatch(string, regexp[, message])`
  - `assert.doesNotReject(asyncFn[, error][, message])`
  - `assert.doesNotThrow(fn[, error][, message])`
  - `assert.equal(actual, expected[, message])`
  - `assert.fail([message])`
  - `assert.fail(actual, expected[, message[, operator[, stackStartFn]]])`
  - `assert.ifError(value)`
  - `assert.match(string, regexp[, message])`
  - `assert.notDeepEqual(actual, expected[, message])`
  - `assert.notDeepStrictEqual(actual, expected[, message])`
  - `assert.notEqual(actual, expected[, message])`
  - `assert.notStrictEqual(actual, expected[, message])`
  - `assert.ok(value[, message])`
  - `assert.rejects(asyncFn[, error][, message])`
  - `assert.strictEqual(actual, expected[, message])`
  - `assert.throws(fn[, error][, message])`

## Assert#

Stability: 2 - Stable

Source Code: lib/assert.js

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

### Strict assertion mode#

History

Version | Changes
--- | ---
v15.0.0 | Exposed as `require('assert/strict')`.
v13.9.0, v12.16.2 | 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".
v9.9.0 | Added error diffs to the strict assertion mode.
v9.9.0 | Added strict assertion mode to the assert module.
v9.9.0 | Added in: v9.9.0

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:

    import { strict as assert } from 'assert';const assert = require('assert').strict;

    import assert from 'assert/strict';const assert = require('assert/strict');

Example error diff:

    import { strict as assert } from 'assert';

    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
    //   ]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()` 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:

    import assert from 'assert';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:

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

### Class: assert.AssertionError[src]#

- 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)`#

Added in: 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.

    import assert from '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);
    }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`#

Added in: v14.2.0, v12.19.0

Stability: 1 - Experimental

This feature is currently experimental and behavior might still change.

#### `new assert.CallTracker()`#

Added in: v14.2.0, v12.19.0

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.

    import assert from 'assert';
    import process from 'process';

    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();
    });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])`#

Added in: v14.2.0, v12.19.0

- `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.

    import assert from '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);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()`#

Added in: v14.2.0, v12.19.0

- 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.

    import assert from '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
    //  }
    // ]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()`#

Added in: v14.2.0, v12.19.0

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.

    import assert from '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();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])`#

Added in: 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])`#

History

Version | Changes
--- | ---
v16.0.0 | In Legacy assertion mode, changed status from Deprecated to Legacy.
v14.0.0 | NaN is now treated as being identical in case both sides are NaN.
v12.0.0 | The type tags are now properly compared and there are a couple minor comparison adjustments to make the check less surprising.
v9.0.0 | The `Error` names and messages are now properly compared.
v8.0.0 | The `Set` and `Map` content is also compared.
v6.4.0, v4.7.1 | Typed array slices are handled correctly now.
v6.1.0, v4.5.0 | Objects with circular references can be used as inputs now.
v5.10.1, v4.4.3 | Handle non-`Uint8Array` typed arrays correctly.
v0.1.21 | Added in: v0.1.21

- `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 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]]` 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 ( `==` ).

    import assert from 'assert';
    // WARNING: This does not throw an AssertionError!

    assert.deepEqual('+00000000', false);const assert = require('assert');
    // 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:

    import assert from '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 {}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])`#

History

Version | Changes
--- | ---
v9.0.0 | Enumerable symbol properties are now compared.
v9.0.0 | The `NaN` is now compared using the SameValueZero comparison.
v8.5.0 | The `Error` names and messages are now properly compared.
v8.0.0 | The `Set` and `Map` content is also compared.
v6.1.0 | Objects with circular references can be used as inputs now.
v6.4.0, v4.7.1 | Typed array slices are handled correctly now.
v5.10.1, v4.4.3 | Handle non-`Uint8Array` typed arrays correctly.
v1.2.0 | Added in: v1.2.0

- `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 of objects should be the same.
- `[[Prototype]]` 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.

    import assert from '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
    //   }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])`#

History

Version | Changes
--- | ---
v16.0.0 | This API is no longer experimental.
v13.6.0, v12.16.0 | Added in: v13.6.0, v12.16.0

- `string` <string>
- `regexp` <RegExp>
- `message` <string> | <Error>

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

    import assert from '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/);
    // OKconst 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])`#

Added in: 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()`.

    import assert from 'assert/strict';

    await assert.doesNotReject(
      async () => {
        throw new TypeError('Wrong value');
      },
      SyntaxError
    );const assert = require('assert/strict');

    (async () => {
      await assert.doesNotReject(
        async () => {
          throw new TypeError('Wrong value');
        },
        SyntaxError
      );
    })();

    import assert from 'assert/strict';

    assert.doesNotReject(Promise.reject(new TypeError('Wrong value')))
      .then(() => {
        // ...
      });const assert = require('assert/strict');

    assert.doesNotReject(Promise.reject(new TypeError('Wrong value')))
      .then(() => {
        // ...
      });

### `assert.doesNotThrow(fn[, error][, message])`#

History

Version | Changes
--- | ---
v5.11.0, v4.4.5 | The `message` parameter is respected now.
v4.2.0 | The `error` parameter can now be an arrow function.
v0.1.21 | Added in: v0.1.21

- `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:

    import assert from 'assert/strict';

    assert.doesNotThrow(
      () => {
        throw new TypeError('Wrong value');
      },
      SyntaxError
    );const assert = require('assert/strict');

    assert.doesNotThrow(
      () => {
        throw new TypeError('Wrong value');
      },
      SyntaxError
    );

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

    import assert from 'assert/strict';

    assert.doesNotThrow(
      () => {
        throw new TypeError('Wrong value');
      },
      TypeError
    );const assert = require('assert/strict');

    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:

    import assert from 'assert/strict';

    assert.doesNotThrow(
      () => {
        throw new TypeError('Wrong value');
      },
      /Wrong value/,
      'Whoops'
    );
    // Throws: AssertionError: Got unwanted exception: Whoopsconst assert = require('assert/strict');

    assert.doesNotThrow(
      () => {
        throw new TypeError('Wrong value');
      },
      /Wrong value/,
      'Whoops'
    );
    // Throws: AssertionError: Got unwanted exception: Whoops

### `assert.equal(actual, expected[, message])`#

History

Version | Changes
--- | ---
v16.0.0 | In Legacy assertion mode, changed status from Deprecated to Legacy.
v14.0.0 | NaN is now treated as being identical in case both sides are NaN.
v0.1.21 | Added in: v0.1.21

- `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`.

    import assert from '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 } }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])`#

Added in: 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`.

    import assert from 'assert/strict';

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

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

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

History

Version | Changes
--- | ---
v10.0.0 | Calling `assert.fail()` with more than one argument is deprecated and emits a warning.
v0.1.21 | Added in: v0.1.21

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.

    import assert from '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 arrayconst 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:

    import assert from 'assert/strict';

    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)
    //     ...const assert = require('assert/strict');

    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)`#

History

Version | Changes
--- | ---
v10.0.0 | Instead of throwing the original error it is now wrapped into an [`AssertionError`][] that contains the full stack trace.
v10.0.0 | Value may now only be `undefined` or `null`. Before all falsy values were handled the same as `null` and did not throw.
v0.1.97 | Added in: v0.1.97

- `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.

    import assert from '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 errorFrameconst 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])`#

History

Version | Changes
--- | ---
v16.0.0 | This API is no longer experimental.
v13.6.0, v12.16.0 | Added in: v13.6.0, v12.16.0

- `string` <string>
- `regexp` <RegExp>
- `message` <string> | <Error>

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

    import assert from '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/);
    // OKconst 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])`#

History

Version | Changes
--- | ---
v16.0.0 | In Legacy assertion mode, changed status from Deprecated to Legacy.
v14.0.0 | NaN is now treated as being identical in case both sides are NaN.
v9.0.0 | The `Error` names and messages are now properly compared.
v8.0.0 | The `Set` and `Map` content is also compared.
v6.4.0, v4.7.1 | Typed array slices are handled correctly now.
v6.1.0, v4.5.0 | Objects with circular references can be used as inputs now.
v5.10.1, v4.4.3 | Handle non-`Uint8Array` typed arrays correctly.
v0.1.21 | Added in: v0.1.21

- `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()`.

    import assert from '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);
    // OKconst 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])`#

History

Version | Changes
--- | ---
v9.0.0 | The `-0` and `+0` are not considered equal anymore.
v9.0.0 | The `NaN` is now compared using the SameValueZero comparison.
v9.0.0 | The `Error` names and messages are now properly compared.
v8.0.0 | The `Set` and `Map` content is also compared.
v6.1.0 | Objects with circular references can be used as inputs now.
v6.4.0, v4.7.1 | Typed array slices are handled correctly now.
v5.10.1, v4.4.3 | Handle non-`Uint8Array` typed arrays correctly.
v1.2.0 | Added in: v1.2.0

- `actual` <any>
- `expected` <any>
- `message` <string> | <Error>

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

    import assert from 'assert/strict';

    assert.notDeepStrictEqual({ a: 1 }, { a: '1' });
    // OKconst 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])`#

History

Version | Changes
--- | ---
v16.0.0 | In Legacy assertion mode, changed status from Deprecated to Legacy.
v14.0.0 | NaN is now treated as being identical in case both sides are NaN.
v0.1.21 | Added in: v0.1.21

- `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`.

    import assert from 'assert';

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

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

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

History

Version | Changes
--- | ---
v10.0.0 | Used comparison changed from Strict Equality to `Object.is()`.
v0.1.21 | Added in: v0.1.21

- `actual` <any>
- `expected` <any>
- `message` <string> | <Error>

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

    import assert from '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');
    // OKconst 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])`#

History

Version | Changes
--- | ---
v10.0.0 | The `assert.ok()` (no arguments) will now use a predefined error message.
v0.1.21 | Added in: v0.1.21

- `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.

    import assert from '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)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)

    import assert from 'assert/strict';

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

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

### `assert.rejects(asyncFn[, error][, message])`#

Added in: 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.

    import assert from 'assert/strict';

    await assert.rejects(
      async () => {
        throw new TypeError('Wrong value');
      },
      {
        name: 'TypeError',
        message: 'Wrong value'
      }
    );const assert = require('assert/strict');

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

    import assert from 'assert/strict';

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

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

    import assert from 'assert/strict';

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

    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])`#

History

Version | Changes
--- | ---
v10.0.0 | Used comparison changed from Strict Equality to `Object.is()`.
v0.1.21 | Added in: v0.1.21

- `actual` <any>
- `expected` <any>
- `message` <string> | <Error>

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

    import assert from '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 identicalconst 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])`#

History

Version | Changes
--- | ---
v10.2.0 | The `error` parameter can be an object containing regular expressions now.
v9.9.0 | The `error` parameter can now be an object as well.
v4.2.0 | The `error` parameter can now be an arrow function.
v0.1.21 | Added in: v0.1.21

- `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:

    import assert from 'assert/strict';

    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:
    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:
    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
    );const assert = require('assert/strict');

    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:
    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:
    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:

    import assert from 'assert/strict';

    assert.throws(
      () => {
        throw new Error('Wrong value');
      },
      Error
    );const assert = require('assert/strict');

    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.

    import assert from 'assert/strict';

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

    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`.

    import assert from 'assert/strict';

    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'
    );const assert = require('assert/strict');

    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:

    import assert from 'assert/strict';

    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]const assert = require('assert/strict');

    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.