Elep

# 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].

## 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 going to be set to this
    value.
  * `actual` {any} The `actual` property on the error instance is going to
    contain this value. Internally used for the `actual` error input in case
    e.g., [`assert.strictEqual()`] is used.
  * `expected` {any} The `expected` property on the error instance is going to
    contain this value. Internally used for the `expected` error input in case
    e.g., [`assert.strictEqual()`] is used.
  * `operator` {string} The `operator` property on the error instance is going
    to contain this value. Internally used to indicate what operation was used
    for comparison (or what assertion function triggered the error).
  * `stackStartFn` {Function} If provided, the generated stack trace is going to
    remove all frames up to the provided 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 value in case e.g.,
  [`assert.strictEqual()`] is used.
* `expected` {any} Set to the expected value in case e.g.,
  [`assert.strictEqual()`] is used.
* `generatedMessage` {boolean} Indicates if the message was auto-generated
  (`true`) or not.
* `code` {string} This is always set to the string `ERR_ASSERTION` to indicate
  that the error is actually 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 [ERR_ASSERTION]');
  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

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:

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

assert.deepEqual([[[1, 2, 3]], 4, 5], [[[1, 2, '3']], 4, 5]);
// AssertionError: Input A expected to strictly deep-equal 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 "strict"
in its name, such as [`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. 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.
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`][]. `Map`s and `Set`s 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

// 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: Input A expected to strictly deep-equal input B:
// + expected - actual
//   {
// -   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: Input A expected to strictly deep-equal input B:
// + expected - actual
// - {}
// + Date {}

// Different type tags:
assert.deepStrictEqual(date, fakeDate);
// AssertionError: Input A expected to strictly deep-equal input B:
// + expected - actual
// - 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: Input A expected to strictly deep-equal input B:
// + expected - actual
// - [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: Input A expected to strictly deep-equal input B:
// + expected - actual
// - 0
// + -0

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: Input A expected to strictly deep-equal input B:
// + expected - actual
//   WeakMap {
// -   [items unknown]
// +   [items unknown],
// +   unequal: true
//   }
```

## 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 } }
```

## 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]: Identical input passed to 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` {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(() => {
  // ...
});
```

Note that `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]: Input A expected to strictly equal input B:
// + expected - actual
// - 1
// + 2

assert.strictEqual(1, 1);
// OK

assert.strictEqual(1, '1');
// AssertionError [ERR_ASSERTION]: Input A expected to strictly equal input B:
// + expected - actual
// - 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. 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'
    }
    // Note that 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');
  },
  function(err) {
    if ((err instanceof Error) && /value/.test(err)) {
      return true;
    }
  },
  'unexpected error'
);
```

Note that `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 thrown an `ERR_AMBIGUOUS_ARGUMENT` error.
assert.throws(throwingSecond, 'Second');
// Throws an error:
// 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:
assert.throws(throwingSecond, /Second$/);
// Does not throw because the error messages match.
assert.throws(throwingFirst, /Second$/);
// Throws an 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.

[`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
[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