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<h2>Table of Contents</h2>
<ul>
<li><span class="stability_2"><a href="#assert_assert">Assert</a></span><ul>
<li><a href="#assert_class_assert_assertionerror">Class: assert.AssertionError</a><ul>
<li><a href="#assert_new_assert_assertionerror_options">new assert.AssertionError(options)</a></li>
</ul>
</li>
<li><a href="#assert_strict_mode">Strict mode</a></li>
<li><span class="stability_0"><a href="#assert_legacy_mode">Legacy mode</a></span></li>
<li><a href="#assert_assert_value_message">assert(value[, message])</a></li>
<li><a href="#assert_assert_deepequal_actual_expected_message">assert.deepEqual(actual, expected[, message])</a></li>
<li><a href="#assert_assert_deepstrictequal_actual_expected_message">assert.deepStrictEqual(actual, expected[, message])</a><ul>
<li><a href="#assert_comparison_details">Comparison details</a></li>
</ul>
</li>
<li><a href="#assert_assert_doesnotreject_asyncfn_error_message">assert.doesNotReject(asyncFn[, error][, message])</a></li>
<li><a href="#assert_assert_doesnotthrow_fn_error_message">assert.doesNotThrow(fn[, error][, message])</a></li>
<li><a href="#assert_assert_equal_actual_expected_message">assert.equal(actual, expected[, message])</a></li>
<li><a href="#assert_assert_fail_message">assert.fail([message])</a></li>
<li><a href="#assert_assert_fail_actual_expected_message_operator_stackstartfn">assert.fail(actual, expected[, message[, operator[, stackStartFn]]])</a></li>
<li><a href="#assert_assert_iferror_value">assert.ifError(value)</a></li>
<li><a href="#assert_assert_notdeepequal_actual_expected_message">assert.notDeepEqual(actual, expected[, message])</a></li>
<li><a href="#assert_assert_notdeepstrictequal_actual_expected_message">assert.notDeepStrictEqual(actual, expected[, message])</a></li>
<li><a href="#assert_assert_notequal_actual_expected_message">assert.notEqual(actual, expected[, message])</a></li>
<li><a href="#assert_assert_notstrictequal_actual_expected_message">assert.notStrictEqual(actual, expected[, message])</a></li>
<li><a href="#assert_assert_ok_value_message">assert.ok(value[, message])</a></li>
<li><a href="#assert_assert_rejects_asyncfn_error_message">assert.rejects(asyncFn[, error][, message])</a></li>
<li><a href="#assert_assert_strictequal_actual_expected_message">assert.strictEqual(actual, expected[, message])</a></li>
<li><a href="#assert_assert_throws_fn_error_message">assert.throws(fn[, error][, message])</a></li>
</ul>
</li>
</ul>
</div>
<div id="apicontent">
<h1>Assert<span><a class="mark" href="#assert_assert" id="assert_assert">#</a></span></h1>
<!--introduced_in=v0.1.21-->
<div class="api_stability api_stability_2"><a href="documentation.html#documentation_stability_index">Stability: 2</a> - Stable</div><p>The <code>assert</code> module provides a simple set of assertion tests that can be used to
test invariants.</p>
<p>A <code>strict</code> and a <code>legacy</code> mode exist, while it is recommended to only use
<a href="#assert_strict_mode"><code>strict mode</code></a>.</p>
<p>For more information about the used equality comparisons see
<a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Equality_comparisons_and_sameness">MDN's guide on equality comparisons and sameness</a>.</p>
<h2>Class: assert.AssertionError<span><a class="mark" href="#assert_class_assert_assertionerror" id="assert_class_assert_assertionerror">#</a></span></h2>
<p>A subclass of <code>Error</code> that indicates the failure of an assertion. All errors
thrown by the <code>assert</code> module will be instances of the <code>AssertionError</code> class.</p>
<h3>new assert.AssertionError(options)<span><a class="mark" href="#assert_new_assert_assertionerror_options" id="assert_new_assert_assertionerror_options">#</a></span></h3>
<div class="api_metadata">
<span>Added in: v0.1.21</span>
</div><ul>
<li><code>options</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a><ul>
<li><code>message</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> If provided, the error message is going to be set to this
value.</li>
<li><code>actual</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a> The <code>actual</code> property on the error instance is going to
contain this value. Internally used for the <code>actual</code> error input in case
e.g., <a href="#assert_assert_strictequal_actual_expected_message"><code>assert.strictEqual()</code></a> is used.</li>
<li><code>expected</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a> The <code>expected</code> property on the error instance is going to
contain this value. Internally used for the <code>expected</code> error input in case
e.g., <a href="#assert_assert_strictequal_actual_expected_message"><code>assert.strictEqual()</code></a> is used.</li>
<li><code>operator</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> The <code>operator</code> 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).</li>
<li><code>stackStartFn</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a> If provided, the generated stack trace is going to
remove all frames up to the provided function.</li>
</ul>
</li>
</ul>
<p>A subclass of <code>Error</code> that indicates the failure of an assertion.</p>
<p>All instances contain the built-in <code>Error</code> properties (<code>message</code> and <code>name</code>)
and:</p>
<ul>
<li><code>actual</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a> Set to the actual value in case e.g.,
<a href="#assert_assert_strictequal_actual_expected_message"><code>assert.strictEqual()</code></a> is used.</li>
<li><code>expected</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a> Set to the expected value in case e.g.,
<a href="#assert_assert_strictequal_actual_expected_message"><code>assert.strictEqual()</code></a> is used.</li>
<li><code>generatedMessage</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Boolean_type" class="type"><boolean></a> Indicates if the message was auto-generated
(<code>true</code>) or not.</li>
<li><code>code</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> This is always set to the string <code>ERR_ASSERTION</code> to indicate
that the error is actually an assertion error.</li>
<li><code>operator</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> Set to the passed in operator value.</li>
</ul>
<pre><code class="language-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);
}</code></pre>
<h2>Strict mode<span><a class="mark" href="#assert_strict_mode" id="assert_strict_mode">#</a></span></h2>
<div class="api_metadata">
<details class="changelog"><summary>History</summary>
<table>
<tr><th>Version</th><th>Changes</th></tr>
<tr><td>v9.9.0</td>
<td><p>Added error diffs to the strict mode</p>
</td></tr>
<tr><td>v9.9.0</td>
<td><p>Added strict mode to the assert module.</p>
</td></tr>
<tr><td>v9.9.0</td>
<td><p><span>Added in: v9.9.0</span></p>
</td></tr>
</table>
</details>
</div><p>When using the <code>strict mode</code>, any <code>assert</code> function will use the equality used
in the strict function mode. So <a href="#assert_assert_deepequal_actual_expected_message"><code>assert.deepEqual()</code></a> will, for example,
work the same as <a href="#assert_assert_deepstrictequal_actual_expected_message"><code>assert.deepStrictEqual()</code></a>.</p>
<p>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.</p>
<p>It can be accessed using:</p>
<pre><code class="language-js">const assert = require('assert').strict;</code></pre>
<p>Example error diff:</p>
<pre><code class="language-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
// ]</code></pre>
<p>To deactivate the colors, use the <code>NODE_DISABLE_COLORS</code> environment variable.
Please note that this will also deactivate the colors in the REPL.</p>
<h2>Legacy mode<span><a class="mark" href="#assert_legacy_mode" id="assert_legacy_mode">#</a></span></h2>
<div class="api_stability api_stability_0"><a href="documentation.html#documentation_stability_index">Stability: 0</a> - Deprecated: Use strict mode instead.</div><p>When accessing <code>assert</code> directly instead of using the <code>strict</code> property, the
<a href="https://tc39.github.io/ecma262/#sec-abstract-equality-comparison">Abstract Equality Comparison</a> will be used for any function without "strict"
in its name, such as <a href="#assert_assert_deepequal_actual_expected_message"><code>assert.deepEqual()</code></a>.</p>
<p>It can be accessed using:</p>
<pre><code class="language-js">const assert = require('assert');</code></pre>
<p>It is recommended to use the <a href="#assert_strict_mode"><code>strict mode</code></a> instead as the
<a href="https://tc39.github.io/ecma262/#sec-abstract-equality-comparison">Abstract Equality Comparison</a> can often have surprising results. This is
especially true for <a href="#assert_assert_deepequal_actual_expected_message"><code>assert.deepEqual()</code></a>, where the comparison rules are
lax:</p>
<pre><code class="language-js">// WARNING: This does not throw an AssertionError!
assert.deepEqual(/a/gi, new Date());</code></pre>
<h2>assert(value[, message])<span><a class="mark" href="#assert_assert_value_message" id="assert_assert_value_message">#</a></span></h2>
<div class="api_metadata">
<span>Added in: v0.5.9</span>
</div><ul>
<li><code>value</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a> The input that is checked for being truthy.</li>
<li><code>message</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Error" class="type"><Error></a></li>
</ul>
<p>An alias of <a href="#assert_assert_ok_value_message"><code>assert.ok()</code></a>.</p>
<h2>assert.deepEqual(actual, expected[, message])<span><a class="mark" href="#assert_assert_deepequal_actual_expected_message" id="assert_assert_deepequal_actual_expected_message">#</a></span></h2>
<div class="api_metadata">
<details class="changelog"><summary>History</summary>
<table>
<tr><th>Version</th><th>Changes</th></tr>
<tr><td>v9.0.0</td>
<td><p>The <code>Error</code> names and messages are now properly compared</p>
</td></tr>
<tr><td>v8.0.0</td>
<td><p>The <code>Set</code> and <code>Map</code> content is also compared</p>
</td></tr>
<tr><td>v6.4.0, v4.7.1</td>
<td><p>Typed array slices are handled correctly now.</p>
</td></tr>
<tr><td>v6.1.0, v4.5.0</td>
<td><p>Objects with circular references can be used as inputs now.</p>
</td></tr>
<tr><td>v5.10.1, v4.4.3</td>
<td><p>Handle non-<code>Uint8Array</code> typed arrays correctly.</p>
</td></tr>
<tr><td>v0.1.21</td>
<td><p><span>Added in: v0.1.21</span></p>
</td></tr>
</table>
</details>
</div><ul>
<li><code>actual</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a></li>
<li><code>expected</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a></li>
<li><code>message</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Error" class="type"><Error></a></li>
</ul>
<p><strong>Strict mode</strong></p>
<p>An alias of <a href="#assert_assert_deepstrictequal_actual_expected_message"><code>assert.deepStrictEqual()</code></a>.</p>
<p><strong>Legacy mode</strong></p>
<div class="api_stability api_stability_0"><a href="documentation.html#documentation_stability_index">Stability: 0</a> - Deprecated: Use [`assert.deepStrictEqual()`][] instead.</div><p>Tests for deep equality between the <code>actual</code> and <code>expected</code> parameters.
Primitive values are compared with the <a href="https://tc39.github.io/ecma262/#sec-abstract-equality-comparison">Abstract Equality Comparison</a>
( <code>==</code> ).</p>
<p>Only <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Enumerability_and_ownership_of_properties">enumerable "own" properties</a> are considered. The
<a href="#assert_assert_deepequal_actual_expected_message"><code>assert.deepEqual()</code></a> implementation does not test the
<a href="https://tc39.github.io/ecma262/#sec-ordinary-object-internal-methods-and-internal-slots"><code>[[Prototype]]</code></a> of objects or enumerable own <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Symbol"><code>Symbol</code></a>
properties. For such checks, consider using <a href="#assert_assert_deepstrictequal_actual_expected_message"><code>assert.deepStrictEqual()</code></a>
instead. <a href="#assert_assert_deepequal_actual_expected_message"><code>assert.deepEqual()</code></a> can have potentially surprising results. The
following example does not throw an <code>AssertionError</code> because the properties on
the <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Regular_Expressions"><code>RegExp</code></a> object are not enumerable:</p>
<pre><code class="language-js">// WARNING: This does not throw an AssertionError!
assert.deepEqual(/a/gi, new Date());</code></pre>
<p>An exception is made for <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Map"><code>Map</code></a> and <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Set"><code>Set</code></a>. <code>Map</code>s and <code>Set</code>s have their
contained items compared too, as expected.</p>
<p>"Deep" equality means that the enumerable "own" properties of child objects
are evaluated also:</p>
<pre><code class="language-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 {}</code></pre>
<p>If the values are not equal, an <code>AssertionError</code> is thrown with a <code>message</code>
property set equal to the value of the <code>message</code> parameter. If the <code>message</code>
parameter is undefined, a default error message is assigned. If the <code>message</code>
parameter is an instance of an <a href="errors.html#errors_class_error"><code>Error</code></a> then it will be thrown instead of the
<code>AssertionError</code>.</p>
<h2>assert.deepStrictEqual(actual, expected[, message])<span><a class="mark" href="#assert_assert_deepstrictequal_actual_expected_message" id="assert_assert_deepstrictequal_actual_expected_message">#</a></span></h2>
<div class="api_metadata">
<details class="changelog"><summary>History</summary>
<table>
<tr><th>Version</th><th>Changes</th></tr>
<tr><td>v9.0.0</td>
<td><p>Enumerable symbol properties are now compared.</p>
</td></tr>
<tr><td>v9.0.0</td>
<td><p>The <code>NaN</code> is now compared using the <a href="https://tc39.github.io/ecma262/#sec-samevaluezero">SameValueZero</a> comparison.</p>
</td></tr>
<tr><td>v8.5.0</td>
<td><p>The <code>Error</code> names and messages are now properly compared</p>
</td></tr>
<tr><td>v8.0.0</td>
<td><p>The <code>Set</code> and <code>Map</code> content is also compared</p>
</td></tr>
<tr><td>v6.4.0, v4.7.1</td>
<td><p>Typed array slices are handled correctly now.</p>
</td></tr>
<tr><td>v6.1.0</td>
<td><p>Objects with circular references can be used as inputs now.</p>
</td></tr>
<tr><td>v5.10.1, v4.4.3</td>
<td><p>Handle non-<code>Uint8Array</code> typed arrays correctly.</p>
</td></tr>
<tr><td>v1.2.0</td>
<td><p><span>Added in: v1.2.0</span></p>
</td></tr>
</table>
</details>
</div><ul>
<li><code>actual</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a></li>
<li><code>expected</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a></li>
<li><code>message</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Error" class="type"><Error></a></li>
</ul>
<p>Tests for deep equality between the <code>actual</code> and <code>expected</code> parameters.
"Deep" equality means that the enumerable "own" properties of child objects
are recursively evaluated also by the following rules.</p>
<h3>Comparison details<span><a class="mark" href="#assert_comparison_details" id="assert_comparison_details">#</a></span></h3>
<ul>
<li>Primitive values are compared using the <a href="https://tc39.github.io/ecma262/#sec-samevalue">SameValue Comparison</a>, used by
<a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/is"><code>Object.is()</code></a>.</li>
<li><a href="https://tc39.github.io/ecma262/#sec-object.prototype.tostring">Type tags</a> of objects should be the same.</li>
<li><a href="https://tc39.github.io/ecma262/#sec-ordinary-object-internal-methods-and-internal-slots"><code>[[Prototype]]</code></a> of objects are compared using
the <a href="https://tc39.github.io/ecma262/#sec-strict-equality-comparison">Strict Equality Comparison</a>.</li>
<li>Only <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Enumerability_and_ownership_of_properties">enumerable "own" properties</a> are considered.</li>
<li><a href="errors.html#errors_class_error"><code>Error</code></a> names and messages are always compared, even if these are not
enumerable properties.</li>
<li>Enumerable own <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Symbol"><code>Symbol</code></a> properties are compared as well.</li>
<li><a href="https://developer.mozilla.org/en-US/docs/Glossary/Primitive#Primitive_wrapper_objects_in_JavaScript">Object wrappers</a> are compared both as objects and unwrapped values.</li>
<li><code>Object</code> properties are compared unordered.</li>
<li><code>Map</code> keys and <code>Set</code> items are compared unordered.</li>
<li>Recursion stops when both sides differ or both sides encounter a circular
reference.</li>
<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/WeakMap"><code>WeakMap</code></a> and <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/WeakSet"><code>WeakSet</code></a> comparison does not rely on their values. See
below for further details.</li>
</ul>
<pre><code class="language-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 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]: Input objects not identical:
// {
// [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: Input A expected to strictly deep-equal input B:
// + expected - actual
// WeakMap {
// - [items unknown]
// + [items unknown],
// + unequal: true
// }</code></pre>
<p>If the values are not equal, an <code>AssertionError</code> is thrown with a <code>message</code>
property set equal to the value of the <code>message</code> parameter. If the <code>message</code>
parameter is undefined, a default error message is assigned. If the <code>message</code>
parameter is an instance of an <a href="errors.html#errors_class_error"><code>Error</code></a> then it will be thrown instead of the
<code>AssertionError</code>.</p>
<h2>assert.doesNotReject(asyncFn[, error][, message])<span><a class="mark" href="#assert_assert_doesnotreject_asyncfn_error_message" id="assert_assert_doesnotreject_asyncfn_error_message">#</a></span></h2>
<div class="api_metadata">
<span>Added in: v10.0.0</span>
</div><ul>
<li><code>asyncFn</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise" class="type"><Promise></a></li>
<li><code>error</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp" class="type"><RegExp></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a></li>
<li><code>message</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a></li>
</ul>
<p>Awaits the <code>asyncFn</code> promise or, if <code>asyncFn</code> is a function, immediately
calls the function and awaits the returned promise to complete. It will then
check that the promise is not rejected.</p>
<p>If <code>asyncFn</code> is a function and it throws an error synchronously,
<code>assert.doesNotReject()</code> will return a rejected <code>Promise</code> with that error. If
the function does not return a promise, <code>assert.doesNotReject()</code> will return a
rejected <code>Promise</code> with an <a href="errors.html#errors_err_invalid_return_value"><code>ERR_INVALID_RETURN_VALUE</code></a> error. In both cases
the error handler is skipped.</p>
<p>Using <code>assert.doesNotReject()</code> 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.</p>
<p>If specified, <code>error</code> can be a <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Classes"><code>Class</code></a>, <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Regular_Expressions"><code>RegExp</code></a> or a validation
function. See <a href="#assert_assert_throws_fn_error_message"><code>assert.throws()</code></a> for more details.</p>
<p>Besides the async nature to await the completion behaves identically to
<a href="#assert_assert_doesnotthrow_fn_error_message"><code>assert.doesNotThrow()</code></a>.</p>
<pre><code class="language-js">(async () => {
await assert.doesNotReject(
async () => {
throw new TypeError('Wrong value');
},
SyntaxError
);
})();</code></pre>
<pre><code class="language-js">assert.doesNotReject(Promise.reject(new TypeError('Wrong value')))
.then(() => {
// ...
});</code></pre>
<h2>assert.doesNotThrow(fn[, error][, message])<span><a class="mark" href="#assert_assert_doesnotthrow_fn_error_message" id="assert_assert_doesnotthrow_fn_error_message">#</a></span></h2>
<div class="api_metadata">
<details class="changelog"><summary>History</summary>
<table>
<tr><th>Version</th><th>Changes</th></tr>
<tr><td>v5.11.0, v4.4.5</td>
<td><p>The <code>message</code> parameter is respected now.</p>
</td></tr>
<tr><td>v4.2.0</td>
<td><p>The <code>error</code> parameter can now be an arrow function.</p>
</td></tr>
<tr><td>v0.1.21</td>
<td><p><span>Added in: v0.1.21</span></p>
</td></tr>
</table>
</details>
</div><ul>
<li><code>fn</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a></li>
<li><code>error</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp" class="type"><RegExp></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a></li>
<li><code>message</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a></li>
</ul>
<p>Asserts that the function <code>fn</code> does not throw an error.</p>
<p>Using <code>assert.doesNotThrow()</code> 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.</p>
<p>When <code>assert.doesNotThrow()</code> is called, it will immediately call the <code>fn</code>
function.</p>
<p>If an error is thrown and it is the same type as that specified by the <code>error</code>
parameter, then an <code>AssertionError</code> is thrown. If the error is of a different
type, or if the <code>error</code> parameter is undefined, the error is propagated back
to the caller.</p>
<p>If specified, <code>error</code> can be a <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Classes"><code>Class</code></a>, <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Regular_Expressions"><code>RegExp</code></a> or a validation
function. See <a href="#assert_assert_throws_fn_error_message"><code>assert.throws()</code></a> for more details.</p>
<p>The following, for instance, will throw the <a href="errors.html#errors_class_typeerror"><code>TypeError</code></a> because there is no
matching error type in the assertion:</p>
<!-- eslint-disable no-restricted-syntax -->
<pre><code class="language-js">assert.doesNotThrow(
() => {
throw new TypeError('Wrong value');
},
SyntaxError
);</code></pre>
<p>However, the following will result in an <code>AssertionError</code> with the message
'Got unwanted exception...':</p>
<!-- eslint-disable no-restricted-syntax -->
<pre><code class="language-js">assert.doesNotThrow(
() => {
throw new TypeError('Wrong value');
},
TypeError
);</code></pre>
<p>If an <code>AssertionError</code> is thrown and a value is provided for the <code>message</code>
parameter, the value of <code>message</code> will be appended to the <code>AssertionError</code>
message:</p>
<!-- eslint-disable no-restricted-syntax -->
<pre><code class="language-js">assert.doesNotThrow(
() => {
throw new TypeError('Wrong value');
},
/Wrong value/,
'Whoops'
);
// Throws: AssertionError: Got unwanted exception: Whoops</code></pre>
<h2>assert.equal(actual, expected[, message])<span><a class="mark" href="#assert_assert_equal_actual_expected_message" id="assert_assert_equal_actual_expected_message">#</a></span></h2>
<div class="api_metadata">
<span>Added in: v0.1.21</span>
</div><ul>
<li><code>actual</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a></li>
<li><code>expected</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a></li>
<li><code>message</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Error" class="type"><Error></a></li>
</ul>
<p><strong>Strict mode</strong></p>
<p>An alias of <a href="#assert_assert_strictequal_actual_expected_message"><code>assert.strictEqual()</code></a>.</p>
<p><strong>Legacy mode</strong></p>
<div class="api_stability api_stability_0"><a href="documentation.html#documentation_stability_index">Stability: 0</a> - Deprecated: Use [`assert.strictEqual()`][] instead.</div><p>Tests shallow, coercive equality between the <code>actual</code> and <code>expected</code> parameters
using the <a href="https://tc39.github.io/ecma262/#sec-abstract-equality-comparison">Abstract Equality Comparison</a> ( <code>==</code> ).</p>
<pre><code class="language-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 } }</code></pre>
<p>If the values are not equal, an <code>AssertionError</code> is thrown with a <code>message</code>
property set equal to the value of the <code>message</code> parameter. If the <code>message</code>
parameter is undefined, a default error message is assigned. If the <code>message</code>
parameter is an instance of an <a href="errors.html#errors_class_error"><code>Error</code></a> then it will be thrown instead of the
<code>AssertionError</code>.</p>
<h2>assert.fail([message])<span><a class="mark" href="#assert_assert_fail_message" id="assert_assert_fail_message">#</a></span></h2>
<div class="api_metadata">
<span>Added in: v0.1.21</span>
</div><ul>
<li><code>message</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Error" class="type"><Error></a> <strong>Default:</strong> <code>'Failed'</code></li>
</ul>
<p>Throws an <code>AssertionError</code> with the provided error message or a default error
message. If the <code>message</code> parameter is an instance of an <a href="errors.html#errors_class_error"><code>Error</code></a> then it
will be thrown instead of the <code>AssertionError</code>.</p>
<pre><code class="language-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</code></pre>
<p>Using <code>assert.fail()</code> with more than two arguments is possible but deprecated.
See below for further details.</p>
<h2>assert.fail(actual, expected[, message[, operator[, stackStartFn]]])<span><a class="mark" href="#assert_assert_fail_actual_expected_message_operator_stackstartfn" id="assert_assert_fail_actual_expected_message_operator_stackstartfn">#</a></span></h2>
<div class="api_metadata">
<details class="changelog"><summary>History</summary>
<table>
<tr><th>Version</th><th>Changes</th></tr>
<tr><td>v10.0.0</td>
<td><p>Calling <code>assert.fail()</code> with more than one argument is deprecated and emits a warning.</p>
</td></tr>
<tr><td>v0.1.21</td>
<td><p><span>Added in: v0.1.21</span></p>
</td></tr>
</table>
</details>
</div><ul>
<li><code>actual</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a></li>
<li><code>expected</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a></li>
<li><code>message</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Error" class="type"><Error></a></li>
<li><code>operator</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> <strong>Default:</strong> <code>'!='</code></li>
<li><code>stackStartFn</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a> <strong>Default:</strong> <code>assert.fail</code></li>
</ul>
<div class="api_stability api_stability_0"><a href="documentation.html#documentation_stability_index">Stability: 0</a> - Deprecated: Use `assert.fail([message])` or other assert functions instead.</div><p>If <code>message</code> is falsy, the error message is set as the values of <code>actual</code> and
<code>expected</code> separated by the provided <code>operator</code>. If just the two <code>actual</code> and
<code>expected</code> arguments are provided, <code>operator</code> will default to <code>'!='</code>. If
<code>message</code> 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
<code>stackStartFn</code> is provided, all stack frames above that function will be
removed from stacktrace (see <a href="errors.html#errors_error_capturestacktrace_targetobject_constructoropt"><code>Error.captureStackTrace</code></a>). If no arguments are
given, the default message <code>Failed</code> will be used.</p>
<pre><code class="language-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</code></pre>
<p>In the last three cases <code>actual</code>, <code>expected</code>, and <code>operator</code> have no
influence on the error message.</p>
<p>Example use of <code>stackStartFn</code> for truncating the exception's stacktrace:</p>
<pre><code class="language-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)
// ...</code></pre>
<h2>assert.ifError(value)<span><a class="mark" href="#assert_assert_iferror_value" id="assert_assert_iferror_value">#</a></span></h2>
<div class="api_metadata">
<details class="changelog"><summary>History</summary>
<table>
<tr><th>Version</th><th>Changes</th></tr>
<tr><td>v10.0.0</td>
<td><p>Instead of throwing the original error it is now wrapped into an <code>AssertionError</code> that contains the full stack trace.</p>
</td></tr>
<tr><td>v10.0.0</td>
<td><p>Value may now only be <code>undefined</code> or <code>null</code>. Before all falsy values were handled the same as <code>null</code> and did not throw.</p>
</td></tr>
<tr><td>v0.1.97</td>
<td><p><span>Added in: v0.1.97</span></p>
</td></tr>
</table>
</details>
</div><ul>
<li><code>value</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a></li>
</ul>
<p>Throws <code>value</code> if <code>value</code> is not <code>undefined</code> or <code>null</code>. This is useful when
testing the <code>error</code> argument in callbacks. The stack trace contains all frames
from the error passed to <code>ifError()</code> including the potential new frames for
<code>ifError()</code> itself.</p>
<pre><code class="language-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</code></pre>
<h2>assert.notDeepEqual(actual, expected[, message])<span><a class="mark" href="#assert_assert_notdeepequal_actual_expected_message" id="assert_assert_notdeepequal_actual_expected_message">#</a></span></h2>
<div class="api_metadata">
<details class="changelog"><summary>History</summary>
<table>
<tr><th>Version</th><th>Changes</th></tr>
<tr><td>v9.0.0</td>
<td><p>The <code>Error</code> names and messages are now properly compared</p>
</td></tr>
<tr><td>v8.0.0</td>
<td><p>The <code>Set</code> and <code>Map</code> content is also compared</p>
</td></tr>
<tr><td>v6.4.0, v4.7.1</td>
<td><p>Typed array slices are handled correctly now.</p>
</td></tr>
<tr><td>v6.1.0, v4.5.0</td>
<td><p>Objects with circular references can be used as inputs now.</p>
</td></tr>
<tr><td>v5.10.1, v4.4.3</td>
<td><p>Handle non-<code>Uint8Array</code> typed arrays correctly.</p>
</td></tr>
<tr><td>v0.1.21</td>
<td><p><span>Added in: v0.1.21</span></p>
</td></tr>
</table>
</details>
</div><ul>
<li><code>actual</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a></li>
<li><code>expected</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a></li>
<li><code>message</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Error" class="type"><Error></a></li>
</ul>
<p><strong>Strict mode</strong></p>
<p>An alias of <a href="#assert_assert_notdeepstrictequal_actual_expected_message"><code>assert.notDeepStrictEqual()</code></a>.</p>
<p><strong>Legacy mode</strong></p>
<div class="api_stability api_stability_0"><a href="documentation.html#documentation_stability_index">Stability: 0</a> - Deprecated: Use [`assert.notDeepStrictEqual()`][] instead.</div><p>Tests for any deep inequality. Opposite of <a href="#assert_assert_deepequal_actual_expected_message"><code>assert.deepEqual()</code></a>.</p>
<pre><code class="language-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</code></pre>
<p>If the values are deeply equal, an <code>AssertionError</code> is thrown with a <code>message</code>
property set equal to the value of the <code>message</code> parameter. If the <code>message</code>
parameter is undefined, a default error message is assigned. If the <code>message</code>
parameter is an instance of an <a href="errors.html#errors_class_error"><code>Error</code></a> then it will be thrown instead of the
<code>AssertionError</code>.</p>
<h2>assert.notDeepStrictEqual(actual, expected[, message])<span><a class="mark" href="#assert_assert_notdeepstrictequal_actual_expected_message" id="assert_assert_notdeepstrictequal_actual_expected_message">#</a></span></h2>
<div class="api_metadata">
<details class="changelog"><summary>History</summary>
<table>
<tr><th>Version</th><th>Changes</th></tr>
<tr><td>v9.0.0</td>
<td><p>The <code>-0</code> and <code>+0</code> are not considered equal anymore.</p>
</td></tr>
<tr><td>v9.0.0</td>
<td><p>The <code>NaN</code> is now compared using the <a href="https://tc39.github.io/ecma262/#sec-samevaluezero">SameValueZero</a> comparison.</p>
</td></tr>
<tr><td>v9.0.0</td>
<td><p>The <code>Error</code> names and messages are now properly compared</p>
</td></tr>
<tr><td>v8.0.0</td>
<td><p>The <code>Set</code> and <code>Map</code> content is also compared</p>
</td></tr>
<tr><td>v6.4.0, v4.7.1</td>
<td><p>Typed array slices are handled correctly now.</p>
</td></tr>
<tr><td>v6.1.0</td>
<td><p>Objects with circular references can be used as inputs now.</p>
</td></tr>
<tr><td>v5.10.1, v4.4.3</td>
<td><p>Handle non-<code>Uint8Array</code> typed arrays correctly.</p>
</td></tr>
<tr><td>v1.2.0</td>
<td><p><span>Added in: v1.2.0</span></p>
</td></tr>
</table>
</details>
</div><ul>
<li><code>actual</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a></li>
<li><code>expected</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a></li>
<li><code>message</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Error" class="type"><Error></a></li>
</ul>
<p>Tests for deep strict inequality. Opposite of <a href="#assert_assert_deepstrictequal_actual_expected_message"><code>assert.deepStrictEqual()</code></a>.</p>
<pre><code class="language-js">const assert = require('assert').strict;
assert.notDeepStrictEqual({ a: 1 }, { a: '1' });
// OK</code></pre>
<p>If the values are deeply and strictly equal, an <code>AssertionError</code> is thrown with
a <code>message</code> property set equal to the value of the <code>message</code> parameter. If the
<code>message</code> parameter is undefined, a default error message is assigned. If the
<code>message</code> parameter is an instance of an <a href="errors.html#errors_class_error"><code>Error</code></a> then it will be thrown
instead of the <code>AssertionError</code>.</p>
<h2>assert.notEqual(actual, expected[, message])<span><a class="mark" href="#assert_assert_notequal_actual_expected_message" id="assert_assert_notequal_actual_expected_message">#</a></span></h2>
<div class="api_metadata">
<span>Added in: v0.1.21</span>
</div><ul>
<li><code>actual</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a></li>
<li><code>expected</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a></li>
<li><code>message</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Error" class="type"><Error></a></li>
</ul>
<p><strong>Strict mode</strong></p>
<p>An alias of <a href="#assert_assert_notstrictequal_actual_expected_message"><code>assert.notStrictEqual()</code></a>.</p>
<p><strong>Legacy mode</strong></p>
<div class="api_stability api_stability_0"><a href="documentation.html#documentation_stability_index">Stability: 0</a> - Deprecated: Use [`assert.notStrictEqual()`][] instead.</div><p>Tests shallow, coercive inequality with the <a href="https://tc39.github.io/ecma262/#sec-abstract-equality-comparison">Abstract Equality Comparison</a>
( <code>!=</code> ).</p>
<pre><code class="language-js">const assert = require('assert');
assert.notEqual(1, 2);
// OK
assert.notEqual(1, 1);
// AssertionError: 1 != 1
assert.notEqual(1, '1');
// AssertionError: 1 != '1'</code></pre>
<p>If the values are equal, an <code>AssertionError</code> is thrown with a <code>message</code> property
set equal to the value of the <code>message</code> parameter. If the <code>message</code> parameter is
undefined, a default error message is assigned. If the <code>message</code> parameter is an
instance of an <a href="errors.html#errors_class_error"><code>Error</code></a> then it will be thrown instead of the
<code>AssertionError</code>.</p>
<h2>assert.notStrictEqual(actual, expected[, message])<span><a class="mark" href="#assert_assert_notstrictequal_actual_expected_message" id="assert_assert_notstrictequal_actual_expected_message">#</a></span></h2>
<div class="api_metadata">
<details class="changelog"><summary>History</summary>
<table>
<tr><th>Version</th><th>Changes</th></tr>
<tr><td>v10.0.0</td>
<td><p>Used comparison changed from Strict Equality to <code>Object.is()</code></p>
</td></tr>
<tr><td>v0.1.21</td>
<td><p><span>Added in: v0.1.21</span></p>
</td></tr>
</table>
</details>
</div><ul>
<li><code>actual</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a></li>
<li><code>expected</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a></li>
<li><code>message</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Error" class="type"><Error></a></li>
</ul>
<p>Tests strict inequality between the <code>actual</code> and <code>expected</code> parameters as
determined by the <a href="https://tc39.github.io/ecma262/#sec-samevalue">SameValue Comparison</a>.</p>
<pre><code class="language-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</code></pre>
<p>If the values are strictly equal, an <code>AssertionError</code> is thrown with a <code>message</code>
property set equal to the value of the <code>message</code> parameter. If the <code>message</code>
parameter is undefined, a default error message is assigned. If the <code>message</code>
parameter is an instance of an <a href="errors.html#errors_class_error"><code>Error</code></a> then it will be thrown instead of the
<code>AssertionError</code>.</p>
<h2>assert.ok(value[, message])<span><a class="mark" href="#assert_assert_ok_value_message" id="assert_assert_ok_value_message">#</a></span></h2>
<div class="api_metadata">
<details class="changelog"><summary>History</summary>
<table>
<tr><th>Version</th><th>Changes</th></tr>
<tr><td>v10.0.0</td>
<td><p>The <code>assert.ok()</code> (no arguments) will now use a predefined error message.</p>
</td></tr>
<tr><td>v0.1.21</td>
<td><p><span>Added in: v0.1.21</span></p>
</td></tr>
</table>
</details>
</div><ul>
<li><code>value</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a></li>
<li><code>message</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Error" class="type"><Error></a></li>
</ul>
<p>Tests if <code>value</code> is truthy. It is equivalent to
<code>assert.equal(!!value, true, message)</code>.</p>
<p>If <code>value</code> is not truthy, an <code>AssertionError</code> is thrown with a <code>message</code>
property set equal to the value of the <code>message</code> parameter. If the <code>message</code>
parameter is <code>undefined</code>, a default error message is assigned. If the <code>message</code>
parameter is an instance of an <a href="errors.html#errors_class_error"><code>Error</code></a> then it will be thrown instead of the
<code>AssertionError</code>.
If no arguments are passed in at all <code>message</code> will be set to the string:
<code>'No value argument passed to `assert.ok()`'</code>.</p>
<p>Be aware that in the <code>repl</code> the error message will be different to the one
thrown in a file! See below for further details.</p>
<pre><code class="language-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)</code></pre>
<h2>assert.rejects(asyncFn[, error][, message])<span><a class="mark" href="#assert_assert_rejects_asyncfn_error_message" id="assert_assert_rejects_asyncfn_error_message">#</a></span></h2>
<div class="api_metadata">
<span>Added in: v10.0.0</span>
</div><ul>
<li><code>asyncFn</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise" class="type"><Promise></a></li>
<li><code>error</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp" class="type"><RegExp></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Error" class="type"><Error></a></li>
<li><code>message</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a></li>
</ul>
<p>Awaits the <code>asyncFn</code> promise or, if <code>asyncFn</code> is a function, immediately
calls the function and awaits the returned promise to complete. It will then
check that the promise is rejected.</p>
<p>If <code>asyncFn</code> is a function and it throws an error synchronously,
<code>assert.rejects()</code> will return a rejected <code>Promise</code> with that error. If the
function does not return a promise, <code>assert.rejects()</code> will return a rejected
<code>Promise</code> with an <a href="errors.html#errors_err_invalid_return_value"><code>ERR_INVALID_RETURN_VALUE</code></a> error. In both cases the error
handler is skipped.</p>
<p>Besides the async nature to await the completion behaves identically to
<a href="#assert_assert_throws_fn_error_message"><code>assert.throws()</code></a>.</p>
<p>If specified, <code>error</code> can be a <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Classes"><code>Class</code></a>, <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Regular_Expressions"><code>RegExp</code></a>, 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 <code>message</code> and
<code>name</code> properties.</p>
<p>If specified, <code>message</code> will be the message provided by the <code>AssertionError</code> if
the <code>asyncFn</code> fails to reject.</p>
<pre><code class="language-js">(async () => {
await assert.rejects(
async () => {
throw new TypeError('Wrong value');
},
{
name: 'TypeError',
message: 'Wrong value'
}
);
})();</code></pre>
<pre><code class="language-js">assert.rejects(
Promise.reject(new Error('Wrong value')),
Error
).then(() => {
// ...
});</code></pre>
<p>Note that <code>error</code> cannot be a string. If a string is provided as the second
argument, then <code>error</code> is assumed to be omitted and the string will be used for
<code>message</code> instead. This can lead to easy-to-miss mistakes. Please read the
example in <a href="#assert_assert_throws_fn_error_message"><code>assert.throws()</code></a> carefully if using a string as the second
argument gets considered.</p>
<h2>assert.strictEqual(actual, expected[, message])<span><a class="mark" href="#assert_assert_strictequal_actual_expected_message" id="assert_assert_strictequal_actual_expected_message">#</a></span></h2>
<div class="api_metadata">
<details class="changelog"><summary>History</summary>
<table>
<tr><th>Version</th><th>Changes</th></tr>
<tr><td>v10.0.0</td>
<td><p>Used comparison changed from Strict Equality to <code>Object.is()</code></p>
</td></tr>
<tr><td>v0.1.21</td>
<td><p><span>Added in: v0.1.21</span></p>
</td></tr>
</table>
</details>
</div><ul>
<li><code>actual</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a></li>
<li><code>expected</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a></li>
<li><code>message</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Error" class="type"><Error></a></li>
</ul>
<p>Tests strict equality between the <code>actual</code> and <code>expected</code> parameters as
determined by the <a href="https://tc39.github.io/ecma262/#sec-samevalue">SameValue Comparison</a>.</p>
<pre><code class="language-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'</code></pre>
<p>If the values are not strictly equal, an <code>AssertionError</code> is thrown with a
<code>message</code> property set equal to the value of the <code>message</code> parameter. If the
<code>message</code> parameter is undefined, a default error message is assigned. If the
<code>message</code> parameter is an instance of an <a href="errors.html#errors_class_error"><code>Error</code></a> then it will be thrown
instead of the <code>AssertionError</code>.</p>
<h2>assert.throws(fn[, error][, message])<span><a class="mark" href="#assert_assert_throws_fn_error_message" id="assert_assert_throws_fn_error_message">#</a></span></h2>
<div class="api_metadata">
<details class="changelog"><summary>History</summary>
<table>
<tr><th>Version</th><th>Changes</th></tr>
<tr><td>v10.2.0</td>
<td><p>The <code>error</code> parameter can be an object containing regular expressions now.</p>
</td></tr>
<tr><td>v9.9.0</td>
<td><p>The <code>error</code> parameter can now be an object as well.</p>
</td></tr>
<tr><td>v4.2.0</td>
<td><p>The <code>error</code> parameter can now be an arrow function.</p>
</td></tr>
<tr><td>v0.1.21</td>
<td><p><span>Added in: v0.1.21</span></p>
</td></tr>
</table>
</details>
</div><ul>
<li><code>fn</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a></li>
<li><code>error</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp" class="type"><RegExp></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Error" class="type"><Error></a></li>
<li><code>message</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a></li>
</ul>
<p>Expects the function <code>fn</code> to throw an error.</p>
<p>If specified, <code>error</code> can be a <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Classes"><code>Class</code></a>, <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Regular_Expressions"><code>RegExp</code></a>, 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 <code>message</code> and <code>name</code> properties. When
using an object, it is also possible to use a regular expression, when
validating against a string property. See below for examples.</p>
<p>If specified, <code>message</code> will be appended to the message provided by the
<code>AssertionError</code> if the <code>fn</code> call fails to throw or in case the error validation
fails.</p>
<p>Custom validation object/error instance:</p>
<pre><code class="language-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$/,
message: /Wrong/,
foo: 'bar',
info: {
nested: true,
// It is not possible to use regular expressions for nested properties!
baz: 'text'
},
// The `reg` property contains a regular expression and only if the
// validation object contains an identical regular expression, it is going
// to pass.
reg: /abc/i
}
);
// Fails due to the different `message` and `name` properties:
assert.throws(
() => {
const otherErr = new Error('Not found');
otherErr.code = 404;
throw otherErr;
},
err // This tests for `message`, `name` and `code`.
);</code></pre>
<p>Validate instanceof using constructor:</p>
<pre><code class="language-js">assert.throws(
() => {
throw new Error('Wrong value');
},
Error
);</code></pre>
<p>Validate error message using <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Regular_Expressions"><code>RegExp</code></a>:</p>
<p>Using a regular expression runs <code>.toString</code> on the error object, and will
therefore also include the error name.</p>
<pre><code class="language-js">assert.throws(
() => {
throw new Error('Wrong value');
},
/^Error: Wrong value$/
);</code></pre>
<p>Custom error validation:</p>
<pre><code class="language-js">assert.throws(
() => {
throw new Error('Wrong value');
},
function(err) {
if ((err instanceof Error) && /value/.test(err)) {
return true;
}
},
'unexpected error'
);</code></pre>
<p>Note that <code>error</code> cannot be a string. If a string is provided as the second
argument, then <code>error</code> is assumed to be omitted and the string will be used for
<code>message</code> instead. This can lead to easy-to-miss mistakes. Using the same
message as the thrown error message is going to result in an
<code>ERR_AMBIGUOUS_ARGUMENT</code> error. Please read the example below carefully if using
a string as the second argument gets considered:</p>
<!-- eslint-disable no-restricted-syntax -->
<pre><code class="language-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)</code></pre>
<p>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.</p>
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