# 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][mdn-equality-guide]. ## Strict mode 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: ```js const assert = require('assert').strict; ``` Example error diff (the `expected`, `actual`, and `Lines skipped` will be on a single row): ```js const assert = require('assert').strict; assert.deepEqual([[[1, 2, 3]], 4, 5], [[[1, 2, '3']], 4, 5]); ``` ```diff AssertionError [ERR_ASSERTION]: Input A expected to deepStrictEqual 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 a "strict" in its name (e.g. [`assert.deepEqual()`][]). It can be accessed using: ```js const assert = require('assert'); ``` It is recommended to use the [`strict mode`][] instead as the [Abstract Equality Comparison][] can often have surprising results. Especially in case of [`assert.deepEqual()`][] as the used comparison rules there are very lax. E.g. ```js // WARNING: This does not throw an AssertionError! assert.deepEqual(/a/gi, new Date()); ``` ## assert(value[, message]) * `value` {any} * `message` {any} An alias of [`assert.ok()`][]. ## assert.deepEqual(actual, expected[, message]) * `actual` {any} * `expected` {any} * `message` {any} **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]]`][prototype-spec] 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: ```js // WARNING: This does not throw an AssertionError! assert.deepEqual(/a/gi, new Date()); ``` An exception is made for [`Map`][] and [`Set`][]. Maps and Sets have their contained items compared too, as expected. "Deep" equality means that the enumerable "own" properties of child objects are evaluated also: ```js const assert = require('assert'); const obj1 = { a: { b: 1 } }; const obj2 = { a: { b: 2 } }; const obj3 = { a: { b: 1 } }; const obj4 = Object.create(obj1); assert.deepEqual(obj1, obj1); // OK, object is equal to itself assert.deepEqual(obj1, obj2); // AssertionError: { a: { b: 1 } } deepEqual { a: { b: 2 } } // values of b are different assert.deepEqual(obj1, obj3); // OK, objects are equal assert.deepEqual(obj1, obj4); // AssertionError: { a: { b: 1 } } deepEqual {} // Prototypes are ignored ``` 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]) * `actual` {any} * `expected` {any} * `message` {any} Tests for deep equality between the `actual` and `expected` parameters. "Deep" equality means that the enumerable "own" properties of child objects are recursively evaluated also by the following rules. ### Comparison details * Primitive values are compared using the [SameValue Comparison][], used by [`Object.is()`][]. * [Type tags][Object.prototype.toString()] of objects should be the same. * [`[[Prototype]]`][prototype-spec] of objects are compared using the [Strict Equality Comparison][]. * Only [enumerable "own" properties][] are considered. * [`Error`][] names and messages are always compared, even if these are not enumerable properties. * Enumerable own [`Symbol`][] properties are compared as well. * [Object wrappers][] are compared both as objects and unwrapped values. * Object properties are compared unordered. * Map keys and Set items are compared unordered. * Recursion stops when both sides differ or both sides encounter a circular reference. * [`WeakMap`][] and [`WeakSet`][] comparison does not rely on their values. See below for further details. ```js const assert = require('assert').strict; assert.deepStrictEqual({ a: 1 }, { a: '1' }); // AssertionError: { a: 1 } deepStrictEqual { a: '1' } // because 1 !== '1' using SameValue comparison // The following objects don't have own properties const date = new Date(); const object = {}; const fakeDate = {}; Object.setPrototypeOf(fakeDate, Date.prototype); assert.deepStrictEqual(object, fakeDate); // AssertionError: {} deepStrictEqual Date {} // Different [[Prototype]] assert.deepStrictEqual(date, fakeDate); // AssertionError: 2017-03-11T14:25:31.849Z deepStrictEqual Date {} // Different type tags assert.deepStrictEqual(NaN, NaN); // OK, because of the SameValue comparison assert.deepStrictEqual(new Number(1), new Number(2)); // Fails because the wrapped number is unwrapped and compared as well. assert.deepStrictEqual(new String('foo'), Object('foo')); // OK because the object and the string are identical when unwrapped. assert.deepStrictEqual(-0, -0); // OK assert.deepStrictEqual(0, -0); // AssertionError: 0 deepStrictEqual -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 }); // Fails because symbol1 !== symbol2! 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 assert.deepStrictEqual(weakMap1, weakMap3); // Fails because weakMap3 has a property that weakMap1 does not contain! ``` 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.doesNotThrow(block[, error][, message]) * `block` {Function} * `error` {RegExp|Function} * `message` {any} Asserts that the function `block` does not throw an error. See [`assert.throws()`][] for more details. When `assert.doesNotThrow()` is called, it will immediately call the `block` 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. The following, for instance, will throw the [`TypeError`][] because there is no matching error type in the assertion: ```js assert.doesNotThrow( () => { throw new TypeError('Wrong value'); }, SyntaxError ); ``` However, the following will result in an `AssertionError` with the message 'Got unwanted exception (TypeError)..': ```js assert.doesNotThrow( () => { throw new TypeError('Wrong value'); }, TypeError ); ``` If an `AssertionError` is thrown and a value is provided for the `message` parameter, the value of `message` will be appended to the `AssertionError` message: ```js assert.doesNotThrow( () => { throw new TypeError('Wrong value'); }, TypeError, 'Whoops' ); // Throws: AssertionError: Got unwanted exception (TypeError). Whoops ``` ## assert.equal(actual, expected[, message]) * `actual` {any} * `expected` {any} * `message` {any} **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][] ( `==` ). ```js 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]) * `message` {any} **Default:** `'Failed'` Throws an `AssertionError` with the provided error message or a default error message. If the `message` parameter is an instance of an [`Error`][] then it will be thrown instead of the `AssertionError`. ```js const assert = require('assert').strict; assert.fail(); // AssertionError [ERR_ASSERTION]: Failed assert.fail('boom'); // AssertionError [ERR_ASSERTION]: boom assert.fail(new TypeError('need array')); // TypeError: need array ``` Using `assert.fail()` with more than two arguments is possible but deprecated. See below for further details. ## assert.fail(actual, expected[, message[, operator[, stackStartFunction]]]) * `actual` {any} * `expected` {any} * `message` {any} * `operator` {string} **Default:** '!=' * `stackStartFunction` {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 `stackStartFunction` is provided, all stack frames above that function will be removed from stacktrace (see [`Error.captureStackTrace`]). If no arguments are given, the default message `Failed` will be used. ```js const assert = require('assert').strict; assert.fail('a', 'b'); // AssertionError [ERR_ASSERTION]: 'a' != 'b' assert.fail(1, 2, undefined, '>'); // AssertionError [ERR_ASSERTION]: 1 > 2 assert.fail(1, 2, 'fail'); // AssertionError [ERR_ASSERTION]: fail assert.fail(1, 2, 'whoops', '>'); // AssertionError [ERR_ASSERTION]: whoops assert.fail(1, 2, new TypeError('need array')); // TypeError: need array ``` In the last three cases `actual`, `expected`, and `operator` have no influence on the error message. Example use of `stackStartFunction` for truncating the exception's stacktrace: ```js function suppressFrame() { assert.fail('a', 'b', undefined, '!==', suppressFrame); } suppressFrame(); // AssertionError [ERR_ASSERTION]: 'a' !== 'b' // at repl:1:1 // at ContextifyScript.Script.runInThisContext (vm.js:44:33) // ... ``` ## assert.ifError(value) * `value` {any} Throws `value` if `value` is not `undefined` or `null`. This is useful when testing the `error` argument in callbacks. ```js const assert = require('assert').strict; assert.ifError(null); // OK assert.ifError(0); // AssertionError [ERR_ASSERTION]: ifError got unwanted exception: 0 assert.ifError('error'); // AssertionError [ERR_ASSERTION]: ifError got unwanted exception: 'error' assert.ifError(new Error()); // AssertionError [ERR_ASSERTION]: ifError got unwanted exception: Error ``` ## assert.notDeepEqual(actual, expected[, message]) * `actual` {any} * `expected` {any} * `message` {any} **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()`][]. ```js const assert = require('assert'); const obj1 = { a: { b: 1 } }; const obj2 = { a: { b: 2 } }; const obj3 = { a: { b: 1 } }; const obj4 = Object.create(obj1); assert.notDeepEqual(obj1, obj1); // AssertionError: { a: { b: 1 } } notDeepEqual { a: { b: 1 } } assert.notDeepEqual(obj1, obj2); // OK: obj1 and obj2 are not deeply equal assert.notDeepEqual(obj1, obj3); // AssertionError: { a: { b: 1 } } notDeepEqual { a: { b: 1 } } assert.notDeepEqual(obj1, obj4); // OK: obj1 and obj4 are not deeply equal ``` 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]) * `actual` {any} * `expected` {any} * `message` {any} Tests for deep strict inequality. Opposite of [`assert.deepStrictEqual()`][]. ```js const assert = require('assert').strict; assert.notDeepStrictEqual({ a: 1 }, { a: '1' }); // OK ``` If the values are deeply and strictly equal, an `AssertionError` is thrown with a `message` property set equal to the value of the `message` parameter. If the `message` parameter is undefined, a default error message is assigned. If the `message` parameter is an instance of an [`Error`][] then it will be thrown instead of the `AssertionError`. ## assert.notEqual(actual, expected[, message]) * `actual` {any} * `expected` {any} * `message` {any} **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][] ( `!=` ). ```js const assert = require('assert'); assert.notEqual(1, 2); // OK assert.notEqual(1, 1); // AssertionError: 1 != 1 assert.notEqual(1, '1'); // AssertionError: 1 != '1' ``` If the values are equal, an `AssertionError` is thrown with a `message` property set equal to the value of the `message` parameter. If the `message` parameter is undefined, a default error message is assigned. If the `message` parameter is an instance of an [`Error`][] then it will be thrown instead of the `AssertionError`. ## assert.notStrictEqual(actual, expected[, message]) * `actual` {any} * `expected` {any} * `message` {any} Tests strict inequality between the `actual` and `expected` parameters as determined by the [SameValue Comparison][]. ```js const assert = require('assert').strict; assert.notStrictEqual(1, 2); // OK assert.notStrictEqual(1, 1); // AssertionError: 1 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]) * `value` {any} * `message` {any} Tests if `value` is truthy. It is equivalent to `assert.equal(!!value, true, message)`. If `value` is not truthy, an `AssertionError` is thrown with a `message` property set equal to the value of the `message` parameter. If the `message` parameter is `undefined`, a default error message is assigned. If the `message` parameter is an instance of an [`Error`][] then it will be thrown instead of the `AssertionError`. If no arguments are passed in at all `message` will be set to the string: "No value argument passed to assert.ok". Be aware that in the `repl` the error message will be different to the one thrown in a file! See below for further details. ```js const assert = require('assert').strict; assert.ok(true); // OK assert.ok(1); // OK assert.ok(); // throws: // "AssertionError: No value argument passed to `assert.ok`. assert.ok(false, 'it\'s false'); // throws "AssertionError: it's false" // In the repl: assert.ok(typeof 123 === 'string'); // throws: // "AssertionError: false == true // In a file (e.g. test.js): assert.ok(typeof 123 === 'string'); // throws: // "AssertionError: The expression evaluated to a falsy value: // // assert.ok(typeof 123 === 'string') assert.ok(false); // throws: // "AssertionError: The expression evaluated to a falsy value: // // assert.ok(false) assert.ok(0); // throws: // "AssertionError: The expression evaluated to a falsy value: // // assert.ok(0) // Using `assert()` works the same: assert(0); // throws: // "AssertionError: The expression evaluated to a falsy value: // // assert(0) ``` ## assert.strictEqual(actual, expected[, message]) * `actual` {any} * `expected` {any} * `message` {any} Tests strict equality between the `actual` and `expected` parameters as determined by the [SameValue Comparison][]. ```js const assert = require('assert').strict; assert.strictEqual(1, 2); // AssertionError: 1 strictEqual 2 assert.strictEqual(1, 1); // OK assert.strictEqual(1, '1'); // AssertionError: 1 strictEqual '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(block[, error][, message]) * `block` {Function} * `error` {RegExp|Function|Object} * `message` {any} Expects the function `block` to throw an error. If specified, `error` can be a constructor, [`RegExp`][], a validation function, or an object where each property will be tested for. If specified, `message` will be the message provided by the `AssertionError` if the block fails to throw. Validate instanceof using constructor: ```js assert.throws( () => { throw new Error('Wrong value'); }, Error ); ``` Validate error message using [`RegExp`][]: Using a regular expression runs `.toString` on the error object, and will therefore also include the error name. ```js assert.throws( () => { throw new Error('Wrong value'); }, /^Error: Wrong value$/ ); ``` Custom error validation: ```js assert.throws( () => { throw new Error('Wrong value'); }, function(err) { if ((err instanceof Error) && /value/.test(err)) { return true; } }, 'unexpected error' ); ``` Custom error object / error instance: ```js assert.throws( () => { const err = new TypeError('Wrong value'); err.code = 404; throw err; }, { name: 'TypeError', message: 'Wrong value' // Note that only properties on the error object will be tested! } ); ``` Note that `error` can not be a string. If a string is provided as the second argument, then `error` is assumed to be omitted and the string will be used for `message` instead. This can lead to easy-to-miss mistakes. Please read the example below carefully if using a string as the second argument gets considered: ```js function throwingFirst() { throw new Error('First'); } function throwingSecond() { throw new Error('Second'); } function notThrowing() {} // The second argument is a string and the input function threw an Error. // In that case both cases do not throw as neither is going to try to // match for the error message thrown by the input function! assert.throws(throwingFirst, 'Second'); assert.throws(throwingSecond, 'Second'); // The string is only used (as message) in case the function does not throw: assert.throws(notThrowing, 'Second'); // AssertionError [ERR_ASSERTION]: Missing expected exception: Second // If it was intended to match for the error message do this instead: assert.throws(throwingSecond, /Second$/); // Does not throw because the error messages match. assert.throws(throwingFirst, /Second$/); // Throws a 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. [`Error.captureStackTrace`]: errors.html#errors_error_capturestacktrace_targetobject_constructoropt [`Error`]: errors.html#errors_class_error [`Map`]: https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Map [`Object.is()`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/is [`RegExp`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Regular_Expressions [`Set`]: https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Set [`Symbol`]: https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Symbol [`TypeError`]: errors.html#errors_class_typeerror [`WeakMap`]: https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/WeakMap [`WeakSet`]: https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/WeakSet [`assert.deepEqual()`]: #assert_assert_deepequal_actual_expected_message [`assert.deepStrictEqual()`]: #assert_assert_deepstrictequal_actual_expected_message [`assert.notDeepStrictEqual()`]: #assert_assert_notdeepstrictequal_actual_expected_message [`assert.notStrictEqual()`]: #assert_assert_notstrictequal_actual_expected_message [`assert.ok()`]: #assert_assert_ok_value_message [`assert.strictEqual()`]: #assert_assert_strictequal_actual_expected_message [`assert.throws()`]: #assert_assert_throws_block_error_message [`strict mode`]: #assert_strict_mode [Abstract Equality Comparison]: https://tc39.github.io/ecma262/#sec-abstract-equality-comparison [Object.prototype.toString()]: https://tc39.github.io/ecma262/#sec-object.prototype.tostring [SameValue Comparison]: https://tc39.github.io/ecma262/#sec-samevalue [Strict Equality Comparison]: https://tc39.github.io/ecma262/#sec-strict-equality-comparison [enumerable "own" properties]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Enumerability_and_ownership_of_properties [mdn-equality-guide]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Equality_comparisons_and_sameness [prototype-spec]: https://tc39.github.io/ecma262/#sec-ordinary-object-internal-methods-and-internal-slots [Object wrappers]: https://developer.mozilla.org/en-US/docs/Glossary/Primitive#Primitive_wrapper_objects_in_JavaScript