Node.js v26.2.0 documentation

Assert#

Strict assertion mode#

Added strict assertion mode to the assert module.

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

Message parameter semantics#

For assertion methods that accept an optional message parameter, the message may be provided in one of the following forms:

• string: Used as-is. If additional arguments are supplied after the message string, they are treated as printf-like substitutions (see util.format()).
• Error: If an Error instance is provided as message, that error is thrown directly instead of an AssertionError.
• function: A function of the form (actual, expected) => string. It is called only when the assertion fails and should return a string to be used as the error message. Non-string return values are ignored and the default message is used instead.

If additional arguments are passed along with an Error or a function as message, the call is rejected with ERR_AMBIGUOUS_ARGUMENT.

If the first item is neither a string, Error, nor function, ERR_INVALID_ARG_TYPE is thrown.

To use strict assertion mode:

import { strict as assert } from 'node:assert';
const assert = require('node:assert').strict;
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import assert from 'node:assert/strict';
const assert = require('node:assert/strict');
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Example error diff:

import { strict as assert } from 'node: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('node: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
//   ]
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Legacy assertion mode#

Legacy assertion mode uses the == operator in:

• assert.deepEqual()
• assert.equal()
• assert.notDeepEqual()
• assert.notEqual()

To use legacy assertion mode:

import assert from 'node:assert';
const assert = require('node:assert');
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// WARNING: This does not throw an AssertionError in legacy assertion mode!
assert.deepEqual(/a/gi, new Date());
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Class: assert.AssertionError#

• Extends: <errors.Error>

new assert.AssertionError(options)#

• 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.
  • diff <string> If set to 'full', shows the full diff in assertion errors. Defaults to 'simple'. Accepted values: 'simple', 'full'.

A subclass of <Error> that indicates the failure of an assertion.

• 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 'node: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('node: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);
}
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Class: assert.Assert#

new assert.Assert([options])#

Added skipPrototype option.

• options <Object>
  • diff <string> If set to 'full', shows the full diff in assertion errors. Defaults to 'simple'. Accepted values: 'simple', 'full'.
  • strict <boolean> If set to true, non-strict methods behave like their corresponding strict methods. Defaults to true.
  • skipPrototype <boolean> If set to true, skips prototype and constructor comparison in deep equality checks. Defaults to false.

Creates a new assertion instance. The diff option controls the verbosity of diffs in assertion error messages.

const { Assert } = require('node:assert');
const assertInstance = new Assert({ diff: 'full' });
assertInstance.deepStrictEqual({ a: 1 }, { a: 2 });
// Shows a full diff in the error message.
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Important: When destructuring assertion methods from an Assert instance, the methods lose their connection to the instance's configuration options (such as diff, strict, and skipPrototype settings). The destructured methods will fall back to default behavior instead.

const myAssert = new Assert({ diff: 'full' });

// This works as expected - uses 'full' diff
myAssert.strictEqual({ a: 1 }, { b: { c: 1 } });

// This loses the 'full' diff setting - falls back to default 'simple' diff
const { strictEqual } = myAssert;
strictEqual({ a: 1 }, { b: { c: 1 } });
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The skipPrototype option affects all deep equality methods:

class Foo {
  constructor(a) {
    this.a = a;
  }
}

class Bar {
  constructor(a) {
    this.a = a;
  }
}

const foo = new Foo(1);
const bar = new Bar(1);

// Default behavior - fails due to different constructors
const assert1 = new Assert();
assert1.deepStrictEqual(foo, bar); // AssertionError

// Skip prototype comparison - passes if properties are equal
const assert2 = new Assert({ skipPrototype: true });
assert2.deepStrictEqual(foo, bar); // OK
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assert(value[, message])#

Message may now be a printf-like format string or function.

• value <any> The input that is checked for being truthy.
• message <string> | <Error> | <Function>

assert.deepEqual(actual, expected[, message])#

Handle non-Uint8Array typed arrays correctly.

• actual <any>
• expected <any>
• message <string> | <Error> | <Function>

Strict assertion mode

An alias of assert.deepStrictEqual().

Legacy assertion mode

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

Comparison details#

• Primitive values are compared with the == operator, 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.
• Object constructors are compared when available.
• <Error> names, messages, causes, and errors 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 either side encounters a circular reference.
• Implementation does not test the [[Prototype]] of objects.
• <Symbol> properties are not compared.
• <WeakMap>, <WeakSet> and <Promise> instances are not compared structurally. They are only equal if they reference the same object. Any comparison between different WeakMap, WeakSet, or Promise instances will result in inequality, even if they contain the same content.
• <RegExp> lastIndex, flags, and source are always compared, even if these are not enumerable properties.

The following example does not throw an AssertionError because the primitives are compared using the == operator.

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

assert.deepEqual('+00000000', false);
const assert = require('node:assert');
// WARNING: This does not throw an AssertionError!

assert.deepEqual('+00000000', false);
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"Deep" equality means that the enumerable "own" properties of child objects are evaluated also:

import assert from 'node:assert';

const obj1 = {
  a: {
    b: 1,
  },
};
const obj2 = {
  a: {
    b: 2,
  },
};
const obj3 = {
  a: {
    b: 1,
  },
};
const obj4 = { __proto__: 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('node:assert');

const obj1 = {
  a: {
    b: 1,
  },
};
const obj2 = {
  a: {
    b: 2,
  },
};
const obj3 = {
  a: {
    b: 1,
  },
};
const obj4 = { __proto__: 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 {}
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assert.deepStrictEqual(actual, expected[, message])#

Handle non-Uint8Array typed arrays correctly.

• actual <any>
• expected <any>
• message <string> | <Error> | <Function>

Comparison details#

• Primitive values are compared using Object.is().
• Type tags of objects should be the same.
• [[Prototype]] of objects are compared using the === operator.
• Only enumerable "own" properties are considered.
• <Error> names, messages, causes, and errors are always compared, even if these are not enumerable properties. errors is also compared.
• 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 either side encounters a circular reference.
• <WeakMap>, <WeakSet> and <Promise> instances are not compared structurally. They are only equal if they reference the same object. Any comparison between different WeakMap, WeakSet, or Promise instances will result in inequality, even if they contain the same content.
• <RegExp> lastIndex, flags, and source are always compared, even if these are not enumerable properties.

import assert from 'node: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 Object.is(NaN, NaN) is true.

// 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:
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 obj = {};

weakMap1.set(obj, 'value');
weakMap2.set(obj, 'value');

// Comparing different instances fails, even with same contents
assert.deepStrictEqual(weakMap1, weakMap2);
// AssertionError: Values have same structure but are not reference-equal:
//
// WeakMap {
//   <items unknown>
// }

// Comparing the same instance to itself succeeds
assert.deepStrictEqual(weakMap1, weakMap1);
// OK

const weakSet1 = new WeakSet();
const weakSet2 = new WeakSet();
weakSet1.add(obj);
weakSet2.add(obj);

// Comparing different instances fails, even with same contents
assert.deepStrictEqual(weakSet1, weakSet2);
// AssertionError: Values have same structure but are not reference-equal:
// + actual - expected
//
// WeakSet {
//   <items unknown>
// }

// Comparing the same instance to itself succeeds
assert.deepStrictEqual(weakSet1, weakSet1);
// OK
const assert = require('node: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 Object.is(NaN, NaN) is true.

// 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:
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 obj = {};

weakMap1.set(obj, 'value');
weakMap2.set(obj, 'value');

// Comparing different instances fails, even with same contents
assert.deepStrictEqual(weakMap1, weakMap2);
// AssertionError: Values have same structure but are not reference-equal:
//
// WeakMap {
//   <items unknown>
// }

// Comparing the same instance to itself succeeds
assert.deepStrictEqual(weakMap1, weakMap1);
// OK

const weakSet1 = new WeakSet();
const weakSet2 = new WeakSet();
weakSet1.add(obj);
weakSet2.add(obj);

// Comparing different instances fails, even with same contents
assert.deepStrictEqual(weakSet1, weakSet2);
// AssertionError: Values have same structure but are not reference-equal:
// + actual - expected
//
// WeakSet {
//   <items unknown>
// }

// Comparing the same instance to itself succeeds
assert.deepStrictEqual(weakSet1, weakSet1);
// OK
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assert.doesNotMatch(string, regexp[, message])#

This API is no longer experimental.

• string <string>
• regexp <RegExp>
• message <string> | <Error> | <Function>

Expects the string input not to match the regular expression.

import assert from 'node: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
const assert = require('node: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
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assert.doesNotReject(asyncFn[, error][, message])#

• asyncFn <Function> | <Promise>
• error <RegExp> | <Function>
• message <string>
• Returns: <Promise>

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.

import assert from 'node:assert/strict';

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

(async () => {
  await assert.doesNotReject(
    async () => {
      throw new TypeError('Wrong value');
    },
    SyntaxError,
  );
})();
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import assert from 'node:assert/strict';

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

assert.doesNotReject(Promise.reject(new TypeError('Wrong value')))
  .then(() => {
    // ...
  });
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assert.doesNotThrow(fn[, error][, message])#

The error parameter can now be an arrow function.

• fn <Function>
• error <RegExp> | <Function>
• message <string>

Asserts that the function fn does not throw an error.

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

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

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

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

The following, for instance, will throw the <TypeError> because there is no matching error type in the assertion:

import assert from 'node:assert/strict';

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

assert.doesNotThrow(
  () => {
    throw new TypeError('Wrong value');
  },
  SyntaxError,
);
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However, the following will result in an AssertionError with the message 'Got unwanted exception...':

import assert from 'node:assert/strict';

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

assert.doesNotThrow(
  () => {
    throw new TypeError('Wrong value');
  },
  TypeError,
);
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import assert from 'node:assert/strict';

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

assert.doesNotThrow(
  () => {
    throw new TypeError('Wrong value');
  },
  /Wrong value/,
  'Whoops',
);
// Throws: AssertionError: Got unwanted exception: Whoops
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assert.equal(actual, expected[, message])#

NaN is now treated as being identical if both sides are NaN.

• actual <any>
• expected <any>
• message <string> | <Error> | <Function>

Strict assertion mode

An alias of assert.strictEqual().

Legacy assertion mode

Tests shallow, coercive equality between the actual and expected parameters using the == operator. NaN is specially handled and treated as being identical if both sides are NaN.

import assert from 'node: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('node: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 } }
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assert.fail([message])#

• message <string> | <Error> Default: 'Failed'

import assert from 'node:assert/strict';

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

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

assert.fail(new TypeError('need array'));
// TypeError: need array
const assert = require('node:assert/strict');

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

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

assert.fail(new TypeError('need array'));
// TypeError: need array
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assert.ifError(value)#

Value may now only be undefined or null. Before all falsy values were handled the same as null and did not throw.

• value <any>

import assert from 'node: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
const assert = require('node: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
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assert.match(string, regexp[, message])#

This API is no longer experimental.

• string <string>
• regexp <RegExp>
• message <string> | <Error> | <Function>

Expects the string input to match the regular expression.

import assert from 'node: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
const assert = require('node: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
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assert.notDeepEqual(actual, expected[, message])#

Handle non-Uint8Array typed arrays correctly.

• actual <any>
• expected <any>
• message <string> | <Error> | <Function>

Strict assertion mode

An alias of assert.notDeepStrictEqual().

Legacy assertion mode

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

import assert from 'node:assert';

const obj1 = {
  a: {
    b: 1,
  },
};
const obj2 = {
  a: {
    b: 2,
  },
};
const obj3 = {
  a: {
    b: 1,
  },
};
const obj4 = { __proto__: 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
const assert = require('node:assert');

const obj1 = {
  a: {
    b: 1,
  },
};
const obj2 = {
  a: {
    b: 2,
  },
};
const obj3 = {
  a: {
    b: 1,
  },
};
const obj4 = { __proto__: 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
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assert.notDeepStrictEqual(actual, expected[, message])#

Handle non-Uint8Array typed arrays correctly.

• actual <any>
• expected <any>
• message <string> | <Error> | <Function>

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

import assert from 'node:assert/strict';

assert.notDeepStrictEqual({ a: 1 }, { a: '1' });
// OK
const assert = require('node:assert/strict');

assert.notDeepStrictEqual({ a: 1 }, { a: '1' });
// OK
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assert.notEqual(actual, expected[, message])#

NaN is now treated as being identical if both sides are NaN.

• actual <any>
• expected <any>
• message <string> | <Error> | <Function>

Strict assertion mode

An alias of assert.notStrictEqual().

Legacy assertion mode

Tests shallow, coercive inequality with the != operator. NaN is specially handled and treated as being identical if both sides are NaN.

import assert from 'node:assert';

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

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

assert.notEqual(1, '1');
// AssertionError: 1 != '1'
const assert = require('node:assert');

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

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

assert.notEqual(1, '1');
// AssertionError: 1 != '1'
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assert.notStrictEqual(actual, expected[, message])#

Used comparison changed from Strict Equality to Object.is().

• actual <any>
• expected <any>
• message <string> | <Error> | <Function>

Tests strict inequality between the actual and expected parameters as determined by Object.is().

import assert from 'node: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
const assert = require('node: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
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assert.ok(value[, message])#

The assert.ok() (no arguments) will now use a predefined error message.

• value <any>
• message <string> | <Error> | <Function>

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 <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()`'.

import assert from 'node: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('node: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)
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import assert from 'node:assert/strict';

// Using `assert()` works the same:
assert(2 + 2 > 5);
// AssertionError: The expression evaluated to a falsy value:
//
//   assert(2 + 2 > 5)
const assert = require('node:assert');

// Using `assert()` works the same:
assert(2 + 2 > 5);
// AssertionError: The expression evaluated to a falsy value:
//
//   assert(2 + 2 > 5)
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assert.rejects(asyncFn[, error][, message])#

• asyncFn <Function> | <Promise>
• error <RegExp> | <Function> | <Object> | <Error>
• message <string>
• Returns: <Promise>

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 'node:assert/strict';

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

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

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

assert.rejects(
  Promise.reject(new Error('Wrong value')),
  Error,
).then(() => {
  // ...
});
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assert.strictEqual(actual, expected[, message])#

Used comparison changed from Strict Equality to Object.is().

• actual <any>
• expected <any>
• message <string> | <Error> | <Function> Postfix printf-like arguments in case it's used as format string. If message is a function, it is called in case of a comparison failure. The function receives the actual and expected arguments and has to return a string that is going to be used as error message. printf-like format strings and functions are beneficial for performance reasons in case arguments are passed through. In addition, it allows nice formatting with ease.

Tests strict equality between the actual and expected parameters as determined by Object.is().

import assert from 'node: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(apples, oranges, 'apples %s !== oranges %s', apples, oranges);
// AssertionError [ERR_ASSERTION]: apples 1 !== oranges 2

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

assert.strictEqual(apples, oranges, (actual, expected) => {
  // Do 'heavy' computations
  return `I expected ${expected} but I got ${actual}`;
});
// AssertionError [ERR_ASSERTION]: I expected oranges but I got apples
const assert = require('node: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(apples, oranges, 'apples %s !== oranges %s', apples, oranges);
// AssertionError [ERR_ASSERTION]: apples 1 !== oranges 2

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

assert.strictEqual(apples, oranges, (actual, expected) => {
  // Do 'heavy' computations
  return `I expected ${expected} but I got ${actual}`;
});
// AssertionError [ERR_ASSERTION]: I expected oranges but I got apples
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assert.throws(fn[, error][, message])#

The error parameter can now be an arrow function.

• fn <Function>
• error <RegExp> | <Function> | <Object> | <Error>
• message <string>

Expects the function fn to throw an error.

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

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

Custom validation object/error instance:

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

// Fails due to the different `message` and `name` properties:
assert.throws(
  () => {
    const otherErr = new Error('Not found');
    // Copy all enumerable properties from `err` to `otherErr`.
    for (const [key, value] of Object.entries(err)) {
      otherErr[key] = value;
    }
    throw otherErr;
  },
  // The error's `message` and `name` properties will also be checked when using
  // an error as validation object.
  err,
);
const assert = require('node: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:
assert.throws(
  () => {
    throw err;
  },
  {
    // The `name` and `message` properties are strings and using regular
    // expressions on those will match against the string. If they fail, an
    // error is thrown.
    name: /^TypeError$/,
    message: /Wrong/,
    foo: 'bar',
    info: {
      nested: true,
      // It is not possible to use regular expressions for nested properties!
      baz: 'text',
    },
    // The `reg` property contains a regular expression and only if the
    // validation object contains an identical regular expression, it is going
    // to pass.
    reg: /abc/i,
  },
);

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

import assert from 'node:assert/strict';

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

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

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

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

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

import assert from 'node: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('node: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',
);
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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 'node: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('node: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]
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assert.partialDeepStrictEqual(actual, expected[, message])#

partialDeepStrictEqual is now Stable. Previously, it had been Experimental.

• actual <any>
• expected <any>
• message <string> | <Error> | <Function>

Tests for partial 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. "Partial" equality means that only properties that exist on the expected parameter are going to be compared.

This method always passes the same test cases as assert.deepStrictEqual(), behaving as a super set of it.

import assert from 'node:assert';

assert.partialDeepStrictEqual(
  { a: { b: { c: 1 } } },
  { a: { b: { c: 1 } } },
);
// OK

assert.partialDeepStrictEqual(
  { a: 1, b: 2, c: 3 },
  { b: 2 },
);
// OK

assert.partialDeepStrictEqual(
  [1, 2, 3, 4, 5, 6, 7, 8, 9],
  [4, 5, 8],
);
// OK

assert.partialDeepStrictEqual(
  new Set([{ a: 1 }, { b: 1 }]),
  new Set([{ a: 1 }]),
);
// OK

assert.partialDeepStrictEqual(
  new Map([['key1', 'value1'], ['key2', 'value2']]),
  new Map([['key2', 'value2']]),
);
// OK

assert.partialDeepStrictEqual(123n, 123n);
// OK

assert.partialDeepStrictEqual(
  [1, 2, 3, 4, 5, 6, 7, 8, 9],
  [5, 4, 8],
);
// AssertionError

assert.partialDeepStrictEqual(
  { a: 1 },
  { a: 1, b: 2 },
);
// AssertionError

assert.partialDeepStrictEqual(
  { a: { b: 2 } },
  { a: { b: '2' } },
);
// AssertionError
const assert = require('node:assert');

assert.partialDeepStrictEqual(
  { a: { b: { c: 1 } } },
  { a: { b: { c: 1 } } },
);
// OK

assert.partialDeepStrictEqual(
  { a: 1, b: 2, c: 3 },
  { b: 2 },
);
// OK

assert.partialDeepStrictEqual(
  [1, 2, 3, 4, 5, 6, 7, 8, 9],
  [4, 5, 8],
);
// OK

assert.partialDeepStrictEqual(
  new Set([{ a: 1 }, { b: 1 }]),
  new Set([{ a: 1 }]),
);
// OK

assert.partialDeepStrictEqual(
  new Map([['key1', 'value1'], ['key2', 'value2']]),
  new Map([['key2', 'value2']]),
);
// OK

assert.partialDeepStrictEqual(123n, 123n);
// OK

assert.partialDeepStrictEqual(
  [1, 2, 3, 4, 5, 6, 7, 8, 9],
  [5, 4, 8],
);
// AssertionError

assert.partialDeepStrictEqual(
  { a: 1 },
  { a: 1, b: 2 },
);
// AssertionError

assert.partialDeepStrictEqual(
  { a: { b: 2 } },
  { a: { b: '2' } },
);
// AssertionError
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