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# Assert#

Stability: 2 - Stable

The `assert` module provides a simple set of assertion tests that can be used to test invariants.

A `strict` and a `legacy` mode exist, while it is recommended to only use `strict mode`.

For more information about the used equality comparisons see MDN's guide on equality comparisons and sameness.

## Class: assert.AssertionError[src]#

A subclass of `Error` that 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 going to be set to this value.
  - `actual` <any> The `actual` property on the error instance is going to contain this value. Internally used for the `actual` error input in case e.g., `assert.strictEqual()` is used.
  - `expected` <any> The `expected` property on the error instance is going to contain this value. Internally used for the `expected` error input in case e.g., `assert.strictEqual()` is used.
  - `operator` <string> The `operator` property on the error instance is going to contain this value. Internally used to indicate what operation was used for comparison (or what assertion function triggered the error).
  - `stackStartFn` <Function> If provided, the generated stack trace is going to remove all frames up to the provided 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 value in case e.g., `assert.strictEqual()` is used.
- `expected` <any> Set to the expected value in case e.g., `assert.strictEqual()` is used.
- `generatedMessage` <boolean> Indicates if the message was auto-generated (`true`) or not.
- `code` <string> This is always set to the string `ERR_ASSERTION` to indicate that the error is actually an assertion error.
- `operator` <string> Set to the passed in operator value.

    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 [ERR_ASSERTION]');
      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);
    }

## Strict mode#

History

Version | Changes
--- | ---
v9.9.0 | Added error diffs to the strict mode
v9.9.0 | Added strict mode to the assert module.
v9.9.0 | Added in: v9.9.0

When using the `strict mode`, any `assert` function will use the equality used in the strict function mode. So `assert.deepEqual()` will, for example, work the same as `assert.deepStrictEqual()`.

On top of that, error messages which involve objects produce an error diff instead of displaying both objects. That is not the case for the legacy mode.

It can be accessed using:

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

Example error diff:

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

    assert.deepEqual([[[1, 2, 3]], 4, 5], [[[1, 2, '3']], 4, 5]);
    // AssertionError: Input A expected to strictly deep-equal input B:
    // + expected - actual ... Lines skipped
    //
    //   [
    //     [
    // ...
    //       2,
    // -     3
    // +     '3'
    //     ],
    // ...
    //     5
    //   ]

To deactivate the colors, use the `NODE_DISABLE_COLORS` environment variable. Please note that this will also deactivate the colors in the REPL.

## Legacy mode#

Stability: 0 - Deprecated: Use strict mode instead.

When accessing `assert` directly instead of using the `strict` property, the Abstract Equality Comparison will be used for any function without "strict" in its name, such as `assert.deepEqual()`.

It can be accessed using:

    const assert = require('assert');

It is recommended to use the `strict mode` instead as the Abstract Equality Comparison can often have 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());

## assert(value[, message])[src]#

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])[src]#

History

Version | Changes
--- | ---
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 mode

An alias of `assert.deepStrictEqual()`.

Legacy mode

Stability: 0 - Deprecated: Use `assert.deepStrictEqual()` instead.

Tests for deep equality between the `actual` and `expected` parameters. Primitive values are compared with the Abstract Equality Comparison ( `==` ).

Only enumerable "own" properties are considered. The `assert.deepEqual()` implementation does not test the `[[Prototype]]` of objects or enumerable own `Symbol` properties. For such checks, consider using `assert.deepStrictEqual()` instead. `assert.deepEqual()` can have potentially surprising results. The following example does not throw an `AssertionError` because the properties on the `RegExp` object are not enumerable:

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

An exception is made for `Map` and `Set`. `Map`s and `Set`s have their contained items compared too, as expected.

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

    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])[src]#

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
v6.4.0, v4.7.1 | Typed array slices are handled correctly now.
v6.1.0 | Objects with circular references can be used as inputs 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.

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

    // This fails because 1 !== '1'.
    assert.deepStrictEqual({ a: 1 }, { a: '1' });
    // AssertionError: Input A expected to strictly deep-equal input B:
    // + expected - actual
    //   {
    // -   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: Input A expected to strictly deep-equal input B:
    // + expected - actual
    // - {}
    // + Date {}

    // Different type tags:
    assert.deepStrictEqual(date, fakeDate);
    // AssertionError: Input A expected to strictly deep-equal input B:
    // + expected - actual
    // - 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: Input A expected to strictly deep-equal input B:
    // + expected - actual
    // - [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: Input A expected to strictly deep-equal input B:
    // + expected - actual
    // - 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]: Input objects not identical:
    // {
    //   [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: Input A expected to strictly deep-equal input B:
    // + expected - actual
    //   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.doesNotReject(asyncFn[, error][, message])[src]#

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()`.

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

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

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

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:

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

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

    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:

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

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

Added in: v0.1.21

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

Strict mode

An alias of `assert.strictEqual()`.

Legacy mode

Stability: 0 - Deprecated: Use `assert.strictEqual()` instead.

Tests shallow, coercive equality between the `actual` and `expected` parameters using the Abstract Equality Comparison ( `==` ).

    const assert = require('assert');

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

    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])[src]#

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`.

    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]]])[src]#

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

- `actual` <any>
- `expected` <any>
- `message` <string> | <Error>
- `operator` <string> Default: `'!='`
- `stackStartFn` <Function> Default: `assert.fail`

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

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.

    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:

    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)[src]#

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.

    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.notDeepEqual(actual, expected[, message])[src]#

History

Version | Changes
--- | ---
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 mode

An alias of `assert.notDeepStrictEqual()`.

Legacy mode

Stability: 0 - Deprecated: Use `assert.notDeepStrictEqual()` instead.

Tests for any deep inequality. Opposite of `assert.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.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])[src]#

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
v6.4.0, v4.7.1 | Typed array slices are handled correctly now.
v6.1.0 | Objects with circular references can be used as inputs 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()`.

    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])[src]#

Added in: v0.1.21

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

Strict mode

An alias of `assert.notStrictEqual()`.

Legacy mode

Stability: 0 - Deprecated: Use `assert.notStrictEqual()` instead.

Tests shallow, coercive inequality with the Abstract Equality Comparison ( `!=` ).

    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])[src]#

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.

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

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

    assert.notStrictEqual(1, 1);
    // AssertionError [ERR_ASSERTION]: Identical input passed to notStrictEqual: 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])[src]#

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.

    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])[src]#

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.

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

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

Note that `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])[src]#

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.

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

    assert.strictEqual(1, 2);
    // AssertionError [ERR_ASSERTION]: Input A expected to strictly equal input B:
    // + expected - actual
    // - 1
    // + 2

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

    assert.strictEqual(1, '1');
    // AssertionError [ERR_ASSERTION]: Input A expected to strictly equal input B:
    // + expected - actual
    // - 1
    // + '1'

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])[src]#

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:

    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'
        }
        // Note that 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');
        otherErr.code = 404;
        throw otherErr;
      },
      err // This tests for `message`, `name` and `code`.
    );

Validate instanceof using constructor:

    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.

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

Custom error validation:

    assert.throws(
      () => {
        throw new Error('Wrong value');
      },
      function(err) {
        if ((err instanceof Error) && /value/.test(err)) {
          return true;
        }
      },
      'unexpected error'
    );

Note that `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:

    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 thrown an `ERR_AMBIGUOUS_ARGUMENT` error.
    assert.throws(throwingSecond, 'Second');
    // Throws an error:
    // 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:
    assert.throws(throwingSecond, /Second$/);
    // Does not throw because the error messages match.
    assert.throws(throwingFirst, /Second$/);
    // Throws an error:
    // Error: First
    //     at throwingFirst (repl:2:9)

Due to the confusing notation, it is recommended not to use a string as the second argument. This might lead to difficult-to-spot errors.