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|
# Assert
> Stability: 2 - Stable
The `assert` module provides a simple set of assertion tests that can be used to
test invariants.
## assert(value[, message])
<!-- YAML
added: v0.5.9
-->
* `value` {any}
* `message` {any}
An alias of [`assert.ok()`][].
## assert.deepEqual(actual, expected[, message])
<!-- YAML
added: v0.1.21
changes:
- version: v8.0.0
pr-url: https://github.com/nodejs/node/pull/12142
description: Set and Map content is also compared
- version: v6.4.0, v4.7.1
pr-url: https://github.com/nodejs/node/pull/8002
description: Typed array slices are handled correctly now.
- version: v6.1.0, v4.5.0
pr-url: https://github.com/nodejs/node/pull/6432
description: Objects with circular references can be used as inputs now.
- version: v5.10.1, v4.4.3
pr-url: https://github.com/nodejs/node/pull/5910
description: Handle non-`Uint8Array` typed arrays correctly.
-->
* `actual` {any}
* `expected` {any}
* `message` {any}
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, attached symbols, or
non-enumerable properties — for such checks, consider using
[`assert.deepStrictEqual()`][] instead. This can lead to some
potentially surprising results. For example, the following example does not
throw an `AssertionError` because the properties on the [`Error`][] object are
not enumerable:
```js
// WARNING: This does not throw an AssertionError!
assert.deepEqual(Error('a'), Error('b'));
```
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.
## assert.deepStrictEqual(actual, expected[, message])
<!-- YAML
added: v1.2.0
changes:
- version: v8.0.0
pr-url: https://github.com/nodejs/node/pull/12142
description: Set and Map content is also compared
- version: v6.4.0, v4.7.1
pr-url: https://github.com/nodejs/node/pull/8002
description: Typed array slices are handled correctly now.
- version: v6.1.0
pr-url: https://github.com/nodejs/node/pull/6432
description: Objects with circular references can be used as inputs now.
- version: v5.10.1, v4.4.3
pr-url: https://github.com/nodejs/node/pull/5910
description: Handle non-`Uint8Array` typed arrays correctly.
-->
* `actual` {any}
* `expected` {any}
* `message` {any}
Generally identical to `assert.deepEqual()` with three exceptions:
1. Primitive values are compared using the [Strict Equality Comparison][]
( `===` ). Set values and Map keys are compared using the [SameValueZero][]
comparison. (Which means they are free of the [caveats][]).
2. [`[[Prototype]]`][prototype-spec] of objects are compared using
the [Strict Equality Comparison][] too.
3. [Type tags][Object.prototype.toString()] of objects should be the same.
```js
const assert = require('assert');
assert.deepEqual({a: 1}, {a: '1'});
// OK, because 1 == '1'
assert.deepStrictEqual({a: 1}, {a: '1'});
// AssertionError: { a: 1 } deepStrictEqual { a: '1' }
// because 1 !== '1' using strict equality
// The following objects don't have own properties
const date = new Date();
const object = {};
const fakeDate = {};
Object.setPrototypeOf(fakeDate, Date.prototype);
assert.deepEqual(object, fakeDate);
// OK, doesn't check [[Prototype]]
assert.deepStrictEqual(object, fakeDate);
// AssertionError: {} deepStrictEqual Date {}
// Different [[Prototype]]
assert.deepEqual(date, fakeDate);
// OK, doesn't check type tags
assert.deepStrictEqual(date, fakeDate);
// AssertionError: 2017-03-11T14:25:31.849Z deepStrictEqual Date {}
// Different type tags
```
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.
## assert.doesNotThrow(block[, error][, message])
<!-- YAML
added: v0.1.21
changes:
- version: v5.11.0, v4.4.5
pr-url: https://github.com/nodejs/node/pull/2407
description: The `message` parameter is respected now.
- version: v4.2.0
pr-url: https://github.com/nodejs/node/pull/3276
description: The `error` parameter can now be an arrow function.
-->
* `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])
<!-- YAML
added: v0.1.21
-->
* `actual` {any}
* `expected` {any}
* `message` {any}
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.
## assert.fail(message)
## assert.fail(actual, expected, message, operator)
<!-- YAML
added: v0.1.21
-->
* `actual` {any}
* `expected` {any}
* `message` {any}
* `operator` {string} (default: '!=')
Throws an `AssertionError`. If `message` is falsy, the error message is set as
the values of `actual` and `expected` separated by the provided `operator`.
Otherwise, the error message is the value of `message`.
```js
const assert = require('assert');
assert.fail(1, 2, undefined, '>');
// AssertionError: 1 > 2
assert.fail(1, 2, 'whoops', '>');
// AssertionError: whoops
assert.fail('boom');
// AssertionError: boom
assert.fail('a', 'b');
// AssertionError: 'a' != 'b'
```
## assert.ifError(value)
<!-- YAML
added: v0.1.97
-->
* `value` {any}
Throws `value` if `value` is truthy. This is useful when testing the `error`
argument in callbacks.
```js
const assert = require('assert');
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])
<!-- YAML
added: v0.1.21
-->
* `actual` {any}
* `expected` {any}
* `message` {any}
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 obj2 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.
## assert.notDeepStrictEqual(actual, expected[, message])
<!-- YAML
added: v1.2.0
-->
* `actual` {any}
* `expected` {any}
* `message` {any}
Tests for deep strict inequality. Opposite of [`assert.deepStrictEqual()`][].
```js
const assert = require('assert');
assert.notDeepEqual({a: 1}, {a: '1'});
// AssertionError: { a: 1 } notDeepEqual { a: '1' }
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.
## assert.notEqual(actual, expected[, message])
<!-- YAML
added: v0.1.21
-->
* `actual` {any}
* `expected` {any}
* `message` {any}
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.
## assert.notStrictEqual(actual, expected[, message])
<!-- YAML
added: v0.1.21
-->
* `actual` {any}
* `expected` {any}
* `message` {any}
Tests strict inequality as determined by the [Strict Equality Comparison][]
( `!==` ).
```js
const assert = require('assert');
assert.notStrictEqual(1, 2);
// OK
assert.notStrictEqual(1, 1);
// AssertionError: 1 !== 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.
## assert.ok(value[, message])
<!-- YAML
added: v0.1.21
-->
* `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.
```js
const assert = require('assert');
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])
<!-- YAML
added: v0.1.21
-->
* `actual` {any}
* `expected` {any}
* `message` {any}
Tests strict equality as determined by the [Strict Equality Comparison][]
( `===` ).
```js
const assert = require('assert');
assert.strictEqual(1, 2);
// AssertionError: 1 === 2
assert.strictEqual(1, 1);
// OK
assert.strictEqual(1, '1');
// AssertionError: 1 === '1'
```
If the values are not strictly equal, an `AssertionError` is thrown with a
`message` property set equal to the value of the `message` parameter. If the
`message` parameter is undefined, a default error message is assigned.
## assert.throws(block[, error][, message])
<!-- YAML
added: v0.1.21
changes:
- version: v4.2.0
pr-url: https://github.com/nodejs/node/pull/3276
description: The `error` parameter can now be an arrow function.
-->
* `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`][]:
```js
assert.throws(
() => {
throw new Error('Wrong value');
},
/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:
<!-- eslint-disable assert-throws-arguments -->
```js
// THIS IS A MISTAKE! DO NOT DO THIS!
assert.throws(myFunction, 'missing foo', 'did not throw with expected message');
// Do this instead.
assert.throws(myFunction, /missing foo/, 'did not throw with expected message');
```
## Caveats
For the following cases, consider using ES2015 [`Object.is()`][],
which uses the [SameValueZero][] comparison.
```js
const a = 0;
const b = -a;
assert.notStrictEqual(a, b);
// AssertionError: 0 !== -0
// Strict Equality Comparison doesn't distinguish between -0 and +0...
assert(!Object.is(a, b));
// but Object.is() does!
const str1 = 'foo';
const str2 = 'foo';
assert.strictEqual(str1 / 1, str2 / 1);
// AssertionError: NaN === NaN
// Strict Equality Comparison can't be used to check NaN...
assert(Object.is(str1 / 1, str2 / 1));
// but Object.is() can!
```
For more information, see
[MDN's guide on equality comparisons and sameness][mdn-equality-guide].
[`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
[`TypeError`]: errors.html#errors_class_typeerror
[`assert.deepEqual()`]: #assert_assert_deepequal_actual_expected_message
[`assert.deepStrictEqual()`]: #assert_assert_deepstrictequal_actual_expected_message
[`assert.ok()`]: #assert_assert_ok_value_message
[`assert.throws()`]: #assert_assert_throws_block_error_message
[Abstract Equality Comparison]: https://tc39.github.io/ecma262/#sec-abstract-equality-comparison
[Object.prototype.toString()]: https://tc39.github.io/ecma262/#sec-object.prototype.tostring
[SameValueZero]: https://tc39.github.io/ecma262/#sec-samevaluezero
[Strict Equality Comparison]: https://tc39.github.io/ecma262/#sec-strict-equality-comparison
[caveats]: #assert_caveats
[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
|