# 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()`][]. It can be accessed using: ```js const assert = require('assert').strict; ``` ## 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. ```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! ``` 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]) ## assert.fail(actual, expected[, message[, operator[, stackStartFunction]]]) * `actual` {any} * `expected` {any} * `message` {any} **Default:** `'Failed'` * `operator` {string} **Default:** '!=' * `stackStartFunction` {function} **Default:** `assert.fail` Throws an `AssertionError`. If `message` is falsy, the error message is set as the values of `actual` and `expected` separated by the provided `operator`. If the `message` parameter is an instance of an [`Error`][] then it will be thrown instead of the `AssertionError`. If just the two `actual` and `expected` arguments are provided, `operator` will default to `'!='`. If `message` is provided only it will be used as the error message, 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(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 ``` *Note*: In the last two cases `actual`, `expected`, and `operator` have no influence on the error message. ```js assert.fail(); // AssertionError [ERR_ASSERTION]: Failed assert.fail('boom'); // AssertionError [ERR_ASSERTION]: boom assert.fail('a', 'b'); // AssertionError [ERR_ASSERTION]: 'a' != 'b' ``` 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 truthy. This is useful when testing the `error` argument in callbacks. ```js const assert = require('assert').strict; assert.ifError(0); // OK assert.ifError(1); // Throws 1 assert.ifError('error'); // Throws 'error' assert.ifError(new Error()); // Throws 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`. ```js const assert = require('assert').strict; assert.ok(true); // OK assert.ok(1); // OK assert.ok(false); // throws "AssertionError: false == true" assert.ok(0); // throws "AssertionError: 0 == true" assert.ok(false, 'it\'s false'); // throws "AssertionError: it's false" ``` ## 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} * `message` {any} Expects the function `block` to throw an error. If specified, `error` can be a constructor, [`RegExp`][], or validation function. 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$/ ); ``` Custom error validation: ```js assert.throws( () => { throw new Error('Wrong value'); }, function(err) { if ((err instanceof Error) && /value/.test(err)) { return true; } }, 'unexpected error' ); ``` Note that `error` can not be a string. If a string is provided as the second argument, then `error` is assumed to be omitted and the string will be used for `message` instead. This can lead to easy-to-miss mistakes. Please read the example below carefully if using a string as the second argument gets considered: ```js function throwingFirst() { throw new Error('First'); } function throwingSecond() { throw new Error('Second'); } function notThrowing() {} // The second argument is a string and the input function threw an Error. // In that case both cases do not throw as neither is going to try to // match for the error message thrown by the input function! assert.throws(throwingFirst, 'Second'); assert.throws(throwingSecond, 'Second'); // The string is only used (as message) in case the function does not throw: assert.throws(notThrowing, 'Second'); // AssertionError [ERR_ASSERTION]: Missing expected exception: Second // If it was intended to match for the error message do this instead: assert.throws(throwingSecond, /Second$/); // 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 [`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 [`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