# Assert > Stability: 2 - Stable The `assert` module provides a set of assertion functions for verifying invariants. The module provides a recommended [`strict` mode][] and a more lenient legacy mode. ## Class: assert.AssertionError 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) * `options` {Object} * `message` {string} If provided, the error message is set to this value. * `actual` {any} The `actual` property on the error instance. * `expected` {any} The `expected` property on the error instance. * `operator` {string} The `operator` property on the error instance. * `stackStartFn` {Function} If provided, the generated stack trace omits frames before this function. A subclass of `Error` that indicates the failure of an assertion. All instances contain the built-in `Error` properties (`message` and `name`) and: * `actual` {any} Set to the `actual` argument for methods such as [`assert.strictEqual()`]. * `expected` {any} Set to the `expected` value for methods such as [`assert.strictEqual()`]. * `generatedMessage` {boolean} Indicates if the message was auto-generated (`true`) or not. * `code` {string} Value is always `ERR_ASSERTION` to show that the error is an assertion error. * `operator` {string} Set to the passed in operator value. ```js const assert = require('assert'); // Generate an AssertionError to compare the error message later: const { message } = new assert.AssertionError({ actual: 1, expected: 2, operator: 'strictEqual' }); // Verify error output: try { assert.strictEqual(1, 2); } catch (err) { assert(err instanceof assert.AssertionError); assert.strictEqual(err.message, message); assert.strictEqual(err.name, 'AssertionError'); assert.strictEqual(err.actual, 1); assert.strictEqual(err.expected, 2); assert.strictEqual(err.code, 'ERR_ASSERTION'); assert.strictEqual(err.operator, 'strictEqual'); assert.strictEqual(err.generatedMessage, true); } ``` ## Strict mode In `strict` mode, `assert` functions use the comparison in the corresponding strict functions. For example, [`assert.deepEqual()`][] will behave like [`assert.deepStrictEqual()`][]. In `strict` mode, error messages for objects display a diff. In legacy mode, error messages for objects display the objects, often truncated. To use `strict` mode: ```js const assert = require('assert').strict; ``` Example error diff: ```js const assert = require('assert').strict; assert.deepEqual([[[1, 2, 3]], 4, 5], [[[1, 2, '3']], 4, 5]); // AssertionError: Expected inputs to be strictly deep-equal: // + actual - expected ... Lines skipped // // [ // [ // ... // 2, // + 3 // - '3' // ], // ... // 5 // ] ``` To deactivate the colors, use the `NODE_DISABLE_COLORS` environment variable. This will also deactivate the colors in the REPL. ## Legacy mode > Stability: 0 - Deprecated: Use strict mode instead. Legacy mode uses the [Abstract Equality Comparison][] in: * [`assert.deepEqual()`][] * [`assert.equal()`][] * [`assert.notDeepEqual()`][] * [`assert.notEqual()`][] To use legacy mode: ```js const assert = require('assert'); ``` Whenever possible, use the [`strict` mode][] instead. Otherwise, the [Abstract Equality Comparison][] may cause surprising results. This is especially true for [`assert.deepEqual()`][], where the comparison rules are lax: ```js // WARNING: This does not throw an AssertionError! assert.deepEqual(/a/gi, new Date()); ``` ## assert(value[, message]) * `value` {any} The input that is checked for being truthy. * `message` {string|Error} An alias of [`assert.ok()`][]. ## assert.deepEqual(actual, expected[, message]) * `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. Consider using [`assert.deepStrictEqual()`][] instead. [`assert.deepEqual()`][] can have potentially surprising results. "Deep" equality means that the enumerable "own" properties of child objects are also recursively evaluated by the following rules. ### Comparison details * Primitive values are compared with the [Abstract Equality Comparison][] ( `==` ). * [Type tags][Object.prototype.toString()] of objects should be the same. * Only [enumerable "own" properties][] are considered. * [`Error`][] names and messages are always compared, even if these are not enumerable properties. * [Object wrappers][] are compared both as objects and unwrapped values. * `Object` properties are compared unordered. * [`Map`][] keys and [`Set`][] items are compared unordered. * Recursion stops when both sides differ or both sides encounter a circular reference. * Implementation does not test the [`[[Prototype]]`][prototype-spec] of objects. * [`Symbol`][] properties are not compared. * [`WeakMap`][] and [`WeakSet`][] comparison does not rely on their values. The following example does not throw an `AssertionError` because the primitives are considered equal by the [Abstract Equality Comparison][] ( `==` ). ```js // WARNING: This does not throw an AssertionError! assert.deepEqual('+00000000', false); ``` "Deep" equality means that the enumerable "own" properties of child objects are evaluated also: ```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 // 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]) * `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][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; // This fails because 1 !== '1'. assert.deepStrictEqual({ a: 1 }, { a: '1' }); // AssertionError: Expected inputs to be strictly deep-equal: // + actual - expected // // { // + a: 1 // - a: '1' // } // The following objects don't have own properties const date = new Date(); const object = {}; const fakeDate = {}; Object.setPrototypeOf(fakeDate, Date.prototype); // Different [[Prototype]]: assert.deepStrictEqual(object, fakeDate); // AssertionError: Expected inputs to be strictly deep-equal: // + actual - expected // // + {} // - Date {} // Different type tags: assert.deepStrictEqual(date, fakeDate); // AssertionError: Expected inputs to be strictly deep-equal: // + actual - expected // // + 2018-04-26T00:49:08.604Z // - Date {} assert.deepStrictEqual(NaN, NaN); // OK, because of the SameValue comparison // Different unwrapped numbers: assert.deepStrictEqual(new Number(1), new Number(2)); // AssertionError: Expected inputs to be strictly deep-equal: // + actual - expected // // + [Number: 1] // - [Number: 2] assert.deepStrictEqual(new String('foo'), Object('foo')); // OK because the object and the string are identical when unwrapped. assert.deepStrictEqual(-0, -0); // OK // Different zeros using the SameValue Comparison: assert.deepStrictEqual(0, -0); // AssertionError: Expected inputs to be strictly deep-equal: // + actual - expected // // + 0 // - -0 const symbol1 = Symbol(); const symbol2 = Symbol(); assert.deepStrictEqual({ [symbol1]: 1 }, { [symbol1]: 1 }); // OK, because it is the same symbol on both objects. assert.deepStrictEqual({ [symbol1]: 1 }, { [symbol2]: 1 }); // AssertionError [ERR_ASSERTION]: Inputs identical but not reference equal: // // { // [Symbol()]: 1 // } const weakMap1 = new WeakMap(); const weakMap2 = new WeakMap([[{}, {}]]); const weakMap3 = new WeakMap(); weakMap3.unequal = true; assert.deepStrictEqual(weakMap1, weakMap2); // OK, because it is impossible to compare the entries // Fails because weakMap3 has a property that weakMap1 does not contain: assert.deepStrictEqual(weakMap1, weakMap3); // AssertionError: Expected inputs to be strictly deep-equal: // + actual - expected // // WeakMap { // + [items unknown] // - [items unknown], // - unequal: true // } ``` If the values are not equal, an `AssertionError` is thrown with a `message` property set equal to the value of the `message` parameter. If the `message` parameter is undefined, a default error message is assigned. If the `message` parameter is an instance of an [`Error`][] then it will be thrown instead of the `AssertionError`. ## assert.doesNotReject(asyncFn[, error][, message]) * `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()`][]. ```js (async () => { await assert.doesNotReject( async () => { throw new TypeError('Wrong value'); }, SyntaxError ); })(); ``` ```js assert.doesNotReject(Promise.reject(new TypeError('Wrong value'))) .then(() => { // ... }); ``` ## assert.doesNotThrow(fn[, error][, message]) * `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: ```js assert.doesNotThrow( () => { throw new TypeError('Wrong value'); }, SyntaxError ); ``` However, the following will result in an `AssertionError` with the message 'Got unwanted exception...': ```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'); }, /Wrong value/, 'Whoops' ); // Throws: AssertionError: Got unwanted exception: Whoops ``` ## assert.equal(actual, expected[, message]) * `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][] ( `==` ). ```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` {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`. ```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[, stackStartFn]]]) * `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. ```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 `stackStartFn` 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. The stack trace contains all frames from the error passed to `ifError()` including the potential new frames for `ifError()` itself. ```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 // 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]) * `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()`][]. ```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 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]) * `actual` {any} * `expected` {any} * `message` {string|Error} 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` {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][] ( `!=` ). ```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` {string|Error} 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 [ERR_ASSERTION]: Expected "actual" to be strictly unequal to: // // 1 assert.notStrictEqual(1, '1'); // OK ``` If the values are strictly equal, an `AssertionError` is thrown with a `message` property set equal to the value of the `message` parameter. If the `message` parameter is undefined, a default error message is assigned. If the `message` parameter is an instance of an [`Error`][] then it will be thrown instead of the `AssertionError`. ## assert.ok(value[, message]) * `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. ```js 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]) * `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. ```js (async () => { await assert.rejects( async () => { throw new TypeError('Wrong value'); }, { name: 'TypeError', message: 'Wrong value' } ); })(); ``` ```js assert.rejects( Promise.reject(new Error('Wrong value')), Error ).then(() => { // ... }); ``` `error` cannot be a string. If a string is provided as the second argument, then `error` is assumed to be omitted and the string will be used for `message` instead. This can lead to easy-to-miss mistakes. Please read the example in [`assert.throws()`][] carefully if using a string as the second argument gets considered. ## assert.strictEqual(actual, expected[, message]) * `actual` {any} * `expected` {any} * `message` {string|Error} 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 [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!' // ^ ``` 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]) * `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: ```js 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$/, message: /Wrong/, foo: 'bar', info: { nested: true, // It is not possible to use regular expressions for nested properties! baz: 'text' }, // The `reg` property contains a regular expression and only if the // validation object contains an identical regular expression, it is going // to pass. reg: /abc/i } ); // Fails due to the different `message` and `name` properties: assert.throws( () => { const otherErr = new Error('Not found'); otherErr.code = 404; throw otherErr; }, err // This tests for `message`, `name` and `code`. ); ``` Validate instanceof using constructor: ```js assert.throws( () => { throw new Error('Wrong value'); }, Error ); ``` Validate error message using [`RegExp`][]: Using a regular expression runs `.toString` on the error object, and will therefore also include the error name. ```js assert.throws( () => { throw new Error('Wrong value'); }, /^Error: Wrong value$/ ); ``` Custom error validation: ```js assert.throws( () => { throw new Error('Wrong value'); }, (err) => { assert(err instanceof Error); assert(/value/.test(err)); // Returning anything from validation functions besides `true` is not // recommended. Doing so results in the caught error being thrown again. // That is usually not the desired outcome. Throw an error about the // specific validation that failed instead (as done in this example). return true; }, 'unexpected error' ); ``` `error` cannot be a string. If a string is provided as the second argument, then `error` is assumed to be omitted and the string will be used for `message` instead. This can lead to easy-to-miss mistakes. Using the same message as the thrown error message is going to result in an `ERR_AMBIGUOUS_ARGUMENT` error. Please read the example below carefully if using a string as the second argument gets considered: ```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. // 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$/); // If the error message does not match, the error from within the function is // not caught. assert.throws(throwingFirst, /Second$/); // 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. [`Class`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Classes [`ERR_INVALID_RETURN_VALUE`]: errors.html#errors_err_invalid_return_value [`Error.captureStackTrace`]: errors.html#errors_error_capturestacktrace_targetobject_constructoropt [`Error`]: errors.html#errors_class_error [`Map`]: https://developer.mozilla.org/en-US/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-US/docs/Web/JavaScript/Reference/Global_Objects/Set [`Symbol`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Symbol [`TypeError`]: errors.html#errors_class_typeerror [`WeakMap`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/WeakMap [`WeakSet`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/WeakSet [`assert.deepEqual()`]: #assert_assert_deepequal_actual_expected_message [`assert.deepStrictEqual()`]: #assert_assert_deepstrictequal_actual_expected_message [`assert.doesNotThrow()`]: #assert_assert_doesnotthrow_fn_error_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_fn_error_message [`strict` mode]: #assert_strict_mode [Abstract Equality Comparison]: https://tc39.github.io/ecma262/#sec-abstract-equality-comparison [Object wrappers]: https://developer.mozilla.org/en-US/docs/Glossary/Primitive#Primitive_wrapper_objects_in_JavaScript [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 [prototype-spec]: https://tc39.github.io/ecma262/#sec-ordinary-object-internal-methods-and-internal-slots