# Assert

<!--introduced_in=v0.10.0-->

> Stability: 2 - Stable

The `assert` module provides a simple set of assertion tests that can be used to
test invariants.

## assert(value[, message])
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* `value` {any}
* `message` {any}

An alias of [`assert.ok()`][].

## assert.deepEqual(actual, expected[, message])
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* `actual` {any}
* `expected` {any}
* `message` {any}

Tests for deep equality between the `actual` and `expected` parameters.
Primitive values are compared with the equal comparison operator ( `==` ).

Only enumerable "own" properties are considered. The `deepEqual()`
implementation does not test object prototypes, attached symbols, or
non-enumerable properties. 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 non-enumerable:

```js
// WARNING: This does not throw an AssertionError!
assert.deepEqual(Error('a'), Error('b'));
```

"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])
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* `actual` {any}
* `expected` {any}
* `message` {any}

Generally identical to `assert.deepEqual()` with two exceptions. First,
primitive values are compared using the strict equality operator ( `===` ).
Second, object comparisons include a strict equality check of their prototypes.

```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
```

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])
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* `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])
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* `actual` {any}
* `expected` {any}
* `message` {any}

Tests shallow, coercive equality between the `actual` and `expected` parameters
using the equal comparison operator ( `==` ).

```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(actual, expected[, message[, operator[, stackStartFunction]]])
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* `actual` {any}
* `expected` {any}
* `message` {any}
* `operator` {string}
* `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`.
Otherwise, the error message is the value of `message`.
If `stackStartFunction` is provided, all stack frames above that function will
be removed from stacktrace (see [`Error.captureStackTrace`]).

```js
const assert = require('assert');

assert.fail(1, 2, undefined, '>');
// AssertionError: 1 > 2

assert.fail(1, 2, 'fail');
// AssertionError: fail

assert.fail(1, 2, 'whoops', '>');
// AssertionError: whoops
```

Example use of `stackStartFunction` for truncating the exception's stacktrace:
```js
function suppressFrame() {
  assert.fail('a', 'b', undefined, '!==', suppressFrame);
}
suppressFrame();
// AssertionError: 'a' !== 'b'
//     at repl:1:1
//     at ContextifyScript.Script.runInThisContext (vm.js:44:33)
//     ...
```

## assert.ifError(value)
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* `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])
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* `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])
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* `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])
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* `actual` {any}
* `expected` {any}
* `message` {any}

Tests shallow, coercive inequality with the not equal comparison operator
( `!=` ).

```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])
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* `actual` {any}
* `expected` {any}
* `message` {any}

Tests strict inequality as determined by the strict not equal operator
( `!==` ).

```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])
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* `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])
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* `actual` {any}
* `expected` {any}
* `message` {any}

Tests strict equality as determined by the strict equality operator ( `===` ).

```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])
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* `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 no-restricted-syntax -->
```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');
```

[`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
[`Error`]: errors.html#errors_class_error
[`Error.captureStackTrace`]: errors.html#errors_error_capturestacktrace_targetobject_constructoropt
[`RegExp`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Regular_Expressions
[`TypeError`]: errors.html#errors_class_typeerror
