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        <ul>
<li><span class="stability_2"><a href="#assert_assert">Assert</a></span><ul>
<li><span class="stability_undefined"><a href="#assert_strict_mode">Strict mode</a></span></li>
<li><span class="stability_0"><a href="#assert_legacy_mode">Legacy mode</a></span></li>
<li><span class="stability_undefined"><a href="#assert_assert_value_message">assert(value[, message])</a></span></li>
<li><span class="stability_undefined"><a href="#assert_assert_deepequal_actual_expected_message">assert.deepEqual(actual, expected[, message])</a></span></li>
<li><span class="stability_undefined"><a href="#assert_assert_deepstrictequal_actual_expected_message">assert.deepStrictEqual(actual, expected[, message])</a></span><ul>
<li><span class="stability_undefined"><a href="#assert_comparison_details">Comparison details</a></span></li>
</ul>
</li>
<li><span class="stability_undefined"><a href="#assert_assert_doesnotthrow_block_error_message">assert.doesNotThrow(block[, error][, message])</a></span></li>
<li><span class="stability_undefined"><a href="#assert_assert_equal_actual_expected_message">assert.equal(actual, expected[, message])</a></span></li>
<li><span class="stability_undefined"><a href="#assert_assert_fail_message">assert.fail([message])</a></span></li>
<li><span class="stability_undefined"><a href="#assert_assert_fail_actual_expected_message_operator_stackstartfunction">assert.fail(actual, expected[, message[, operator[, stackStartFunction]]])</a></span></li>
<li><span class="stability_undefined"><a href="#assert_assert_iferror_value">assert.ifError(value)</a></span></li>
<li><span class="stability_undefined"><a href="#assert_assert_notdeepequal_actual_expected_message">assert.notDeepEqual(actual, expected[, message])</a></span></li>
<li><span class="stability_undefined"><a href="#assert_assert_notdeepstrictequal_actual_expected_message">assert.notDeepStrictEqual(actual, expected[, message])</a></span></li>
<li><span class="stability_undefined"><a href="#assert_assert_notequal_actual_expected_message">assert.notEqual(actual, expected[, message])</a></span></li>
<li><span class="stability_undefined"><a href="#assert_assert_notstrictequal_actual_expected_message">assert.notStrictEqual(actual, expected[, message])</a></span></li>
<li><span class="stability_undefined"><a href="#assert_assert_ok_value_message">assert.ok(value[, message])</a></span></li>
<li><span class="stability_undefined"><a href="#assert_assert_strictequal_actual_expected_message">assert.strictEqual(actual, expected[, message])</a></span></li>
<li><span class="stability_undefined"><a href="#assert_assert_throws_block_error_message">assert.throws(block[, error][, message])</a></span></li>
<li><span class="stability_undefined"><a href="#assert_caveats">Caveats</a></span></li>
</ul>
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      <div id="apicontent">
        <h1>Assert<span><a class="mark" href="#assert_assert" id="assert_assert">#</a></span></h1>
<!--introduced_in=v0.10.0-->
<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&#39;s guide on equality comparisons and sameness</a>.</p>
<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="lang-js">const assert = require(&#39;assert&#39;).strict;
</code></pre>
<p>Example error diff (the <code>expected</code>, <code>actual</code>, and <code>Lines skipped</code> will be on a
single row):</p>
<pre><code class="lang-js">const assert = require(&#39;assert&#39;).strict;

assert.deepEqual([[[1, 2, 3]], 4, 5], [[[1, 2, &#39;3&#39;]], 4, 5]);
</code></pre>
<pre><code class="lang-diff">AssertionError [ERR_ASSERTION]: Input A expected to deepStrictEqual input B:
+ expected
- actual
... Lines skipped

  [
    [
...
      2,
-     3
+     &#39;3&#39;
    ],
...
    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 &quot;strict&quot;
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="lang-js">const assert = require(&#39;assert&#39;);
</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="lang-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> <span class="type">&lt;any&gt;</span></li>
<li><code>message</code> <span class="type">&lt;any&gt;</span></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>Error names and messages are now properly compared</p>
</td></tr>
<tr><td>v8.0.0</td>
<td><p>Set and Map 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> <span class="type">&lt;any&gt;</span></li>
<li><code>expected</code> <span class="type">&lt;any&gt;</span></li>
<li><code>message</code> <span class="type">&lt;any&gt;</span></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 <a href="#assert_assert_deepstrictequal_actual_expected_message"><code>assert.deepStrictEqual()</code></a> 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 &quot;own&quot; 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="lang-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>. Maps and Sets have their
contained items compared too, as expected.</p>
<p>&quot;Deep&quot; equality means that the enumerable &quot;own&quot; properties of child objects
are evaluated also:</p>
<pre><code class="lang-js">const assert = require(&#39;assert&#39;);

const obj1 = {
  a: {
    b: 1
  }
};
const obj2 = {
  a: {
    b: 2
  }
};
const obj3 = {
  a: {
    b: 1
  }
};
const obj4 = Object.create(obj1);

assert.deepEqual(obj1, obj1);
// OK, object is equal to itself

assert.deepEqual(obj1, obj2);
// AssertionError: { a: { b: 1 } } deepEqual { a: { b: 2 } }
// values of b are different

assert.deepEqual(obj1, obj3);
// OK, objects are equal

assert.deepEqual(obj1, obj4);
// AssertionError: { a: { b: 1 } } deepEqual {}
// Prototypes are ignored
</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 [<code>Error</code>][] 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>NaN 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>Error names and messages are now properly compared</p>
</td></tr>
<tr><td>v8.0.0</td>
<td><p>Set and Map 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> <span class="type">&lt;any&gt;</span></li>
<li><code>expected</code> <span class="type">&lt;any&gt;</span></li>
<li><code>message</code> <span class="type">&lt;any&gt;</span></li>
</ul>
<p>Tests for deep equality between the <code>actual</code> and <code>expected</code> parameters.
&quot;Deep&quot; equality means that the enumerable &quot;own&quot; 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>
<div class="signature"><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 &quot;own&quot; properties</a> are considered.</li>
<li>[<code>Error</code>][] 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>Object properties are compared unordered.</li>
<li>Map keys and Set 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/docs/Web/JavaScript/Reference/Global_Objects/WeakMap"><code>WeakMap</code></a> and <a href="https://developer.mozilla.org/en/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>
</div><pre><code class="lang-js">const assert = require(&#39;assert&#39;).strict;

assert.deepStrictEqual({ a: 1 }, { a: &#39;1&#39; });
// AssertionError: { a: 1 } deepStrictEqual { a: &#39;1&#39; }
// because 1 !== &#39;1&#39; using SameValue comparison

// The following objects don&#39;t have own properties
const date = new Date();
const object = {};
const fakeDate = {};
Object.setPrototypeOf(fakeDate, Date.prototype);

assert.deepStrictEqual(object, fakeDate);
// AssertionError: {} deepStrictEqual Date {}
// Different [[Prototype]]

assert.deepStrictEqual(date, fakeDate);
// AssertionError: 2017-03-11T14:25:31.849Z deepStrictEqual Date {}
// Different type tags

assert.deepStrictEqual(NaN, NaN);
// OK, because of the SameValue comparison

assert.deepStrictEqual(new Number(1), new Number(2));
// Fails because the wrapped number is unwrapped and compared as well.
assert.deepStrictEqual(new String(&#39;foo&#39;), Object(&#39;foo&#39;));
// OK because the object and the string are identical when unwrapped.

assert.deepStrictEqual(-0, -0);
// OK
assert.deepStrictEqual(0, -0);
// AssertionError: 0 deepStrictEqual -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 });
// Fails because symbol1 !== symbol2!

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
assert.deepStrictEqual(weakMap1, weakMap3);
// Fails because weakMap3 has a property that weakMap1 does not contain!
</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 [<code>Error</code>][] then it will be thrown instead of the
<code>AssertionError</code>.</p>
<h2>assert.doesNotThrow(block[, error][, message])<span><a class="mark" href="#assert_assert_doesnotthrow_block_error_message" id="assert_assert_doesnotthrow_block_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>block</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type">&lt;Function&gt;</a></li>
<li><code>error</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp" class="type">&lt;RegExp&gt;</a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type">&lt;Function&gt;</a></li>
<li><code>message</code> <span class="type">&lt;any&gt;</span></li>
</ul>
<p>Asserts that the function <code>block</code> does not throw an error. See
<a href="#assert_assert_throws_block_error_message"><code>assert.throws()</code></a> for more details.</p>
<p>Please note: Using <code>assert.doesNotThrow()</code> is actually not useful because there
is no benefit by 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>block</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>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="lang-js">assert.doesNotThrow(
  () =&gt; {
    throw new TypeError(&#39;Wrong value&#39;);
  },
  SyntaxError
);
</code></pre>
<p>However, the following will result in an <code>AssertionError</code> with the message
&#39;Got unwanted exception (TypeError)..&#39;:</p>
<!-- eslint-disable no-restricted-syntax -->
<pre><code class="lang-js">assert.doesNotThrow(
  () =&gt; {
    throw new TypeError(&#39;Wrong value&#39;);
  },
  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="lang-js">assert.doesNotThrow(
  () =&gt; {
    throw new TypeError(&#39;Wrong value&#39;);
  },
  TypeError,
  &#39;Whoops&#39;
);
// Throws: AssertionError: Got unwanted exception (TypeError). 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> <span class="type">&lt;any&gt;</span></li>
<li><code>expected</code> <span class="type">&lt;any&gt;</span></li>
<li><code>message</code> <span class="type">&lt;any&gt;</span></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 <a href="#assert_assert_strictequal_actual_expected_message"><code>assert.strictEqual()</code></a> 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="lang-js">const assert = require(&#39;assert&#39;);

assert.equal(1, 1);
// OK, 1 == 1
assert.equal(1, &#39;1&#39;);
// OK, 1 == &#39;1&#39;

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 [<code>Error</code>][] 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>
<h2>assert.fail(actual, expected[, message[, operator[, stackStartFunction]]])<span><a class="mark" href="#assert_assert_fail_actual_expected_message_operator_stackstartfunction" id="assert_assert_fail_actual_expected_message_operator_stackstartfunction">#</a></span></h2>
<div class="api_metadata">
<span>Added in: v0.1.21</span>
</div><ul>
<li><code>actual</code> <span class="type">&lt;any&gt;</span></li>
<li><code>expected</code> <span class="type">&lt;any&gt;</span></li>
<li><code>message</code> <span class="type">&lt;any&gt;</span> <strong>Default:</strong> <code>&#39;Failed&#39;</code></li>
<li><code>operator</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type">&lt;string&gt;</a> <strong>Default:</strong> &#39;!=&#39;</li>
<li><code>stackStartFunction</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type">&lt;Function&gt;</a> <strong>Default:</strong> <code>assert.fail</code></li>
</ul>
<p>Throws an <code>AssertionError</code>. 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
the <code>message</code> parameter is an instance of an [<code>Error</code>][] then it will be thrown
instead of the <code>AssertionError</code>. If just the two <code>actual</code> and <code>expected</code>
arguments are provided, <code>operator</code> will default to <code>&#39;!=&#39;</code>. If <code>message</code> is
provided only it will be used as the error message, the other arguments will be
stored as properties on the thrown object. If <code>stackStartFunction</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="lang-js">const assert = require(&#39;assert&#39;).strict;

assert.fail(1, 2, undefined, &#39;&gt;&#39;);
// AssertionError [ERR_ASSERTION]: 1 &gt; 2

assert.fail(1, 2, &#39;fail&#39;);
// AssertionError [ERR_ASSERTION]: fail

assert.fail(1, 2, &#39;whoops&#39;, &#39;&gt;&#39;);
// AssertionError [ERR_ASSERTION]: whoops

assert.fail(1, 2, new TypeError(&#39;need array&#39;));
// TypeError: need array
</code></pre>
<p><em>Note</em>: In the last two cases <code>actual</code>, <code>expected</code>, and <code>operator</code> have no
influence on the error message.</p>
<pre><code class="lang-js">assert.fail();
// AssertionError [ERR_ASSERTION]: Failed

assert.fail(&#39;boom&#39;);
// AssertionError [ERR_ASSERTION]: boom

assert.fail(&#39;a&#39;, &#39;b&#39;);
// AssertionError [ERR_ASSERTION]: &#39;a&#39; != &#39;b&#39;
</code></pre>
<p>Example use of <code>stackStartFunction</code> for truncating the exception&#39;s stacktrace:</p>
<pre><code class="lang-js">function suppressFrame() {
  assert.fail(&#39;a&#39;, &#39;b&#39;, undefined, &#39;!==&#39;, suppressFrame);
}
suppressFrame();
// AssertionError [ERR_ASSERTION]: &#39;a&#39; !== &#39;b&#39;
//     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">
<span>Added in: v0.1.97</span>
</div><ul>
<li><code>value</code> <span class="type">&lt;any&gt;</span></li>
</ul>
<p>Throws <code>value</code> if <code>value</code> is truthy. This is useful when testing the <code>error</code>
argument in callbacks.</p>
<pre><code class="lang-js">const assert = require(&#39;assert&#39;).strict;

assert.ifError(null);
// OK
assert.ifError(0);
// OK
assert.ifError(1);
// Throws 1
assert.ifError(&#39;error&#39;);
// Throws &#39;error&#39;
assert.ifError(new Error());
// Throws Error
</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>Error names and messages are now properly compared</p>
</td></tr>
<tr><td>v8.0.0</td>
<td><p>Set and Map 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> <span class="type">&lt;any&gt;</span></li>
<li><code>expected</code> <span class="type">&lt;any&gt;</span></li>
<li><code>message</code> <span class="type">&lt;any&gt;</span></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 <a href="#assert_assert_notdeepstrictequal_actual_expected_message"><code>assert.notDeepStrictEqual()</code></a> 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="lang-js">const assert = require(&#39;assert&#39;);

const obj1 = {
  a: {
    b: 1
  }
};
const obj2 = {
  a: {
    b: 2
  }
};
const obj3 = {
  a: {
    b: 1
  }
};
const obj4 = Object.create(obj1);

assert.notDeepEqual(obj1, obj1);
// AssertionError: { a: { b: 1 } } notDeepEqual { a: { b: 1 } }

assert.notDeepEqual(obj1, obj2);
// OK: obj1 and obj2 are not deeply equal

assert.notDeepEqual(obj1, obj3);
// AssertionError: { a: { b: 1 } } notDeepEqual { a: { b: 1 } }

assert.notDeepEqual(obj1, obj4);
// OK: obj1 and obj4 are not deeply equal
</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 [<code>Error</code>][] 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>-0 and +0 are not considered equal anymore.</p>
</td></tr>
<tr><td>v9.0.0</td>
<td><p>NaN 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>Error names and messages are now properly compared</p>
</td></tr>
<tr><td>v8.0.0</td>
<td><p>Set and Map 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> <span class="type">&lt;any&gt;</span></li>
<li><code>expected</code> <span class="type">&lt;any&gt;</span></li>
<li><code>message</code> <span class="type">&lt;any&gt;</span></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="lang-js">const assert = require(&#39;assert&#39;).strict;

assert.notDeepStrictEqual({ a: 1 }, { a: &#39;1&#39; });
// 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 [<code>Error</code>][] 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> <span class="type">&lt;any&gt;</span></li>
<li><code>expected</code> <span class="type">&lt;any&gt;</span></li>
<li><code>message</code> <span class="type">&lt;any&gt;</span></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 <a href="#assert_assert_notstrictequal_actual_expected_message"><code>assert.notStrictEqual()</code></a> 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="lang-js">const assert = require(&#39;assert&#39;);

assert.notEqual(1, 2);
// OK

assert.notEqual(1, 1);
// AssertionError: 1 != 1

assert.notEqual(1, &#39;1&#39;);
// AssertionError: 1 != &#39;1&#39;
</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 [<code>Error</code>][] 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">
<span>Added in: v0.1.21</span>
</div><ul>
<li><code>actual</code> <span class="type">&lt;any&gt;</span></li>
<li><code>expected</code> <span class="type">&lt;any&gt;</span></li>
<li><code>message</code> <span class="type">&lt;any&gt;</span></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="lang-js">const assert = require(&#39;assert&#39;).strict;

assert.notStrictEqual(1, 2);
// OK

assert.notStrictEqual(1, 1);
// AssertionError: 1 !== 1

assert.notStrictEqual(1, &#39;1&#39;);
// 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 [<code>Error</code>][] 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">
<span>Added in: v0.1.21</span>
</div><ul>
<li><code>value</code> <span class="type">&lt;any&gt;</span></li>
<li><code>message</code> <span class="type">&lt;any&gt;</span></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 [<code>Error</code>][] then it will be thrown instead of the
<code>AssertionError</code>.</p>
<pre><code class="lang-js">const assert = require(&#39;assert&#39;).strict;

assert.ok(true);
// OK
assert.ok(1);
// OK
assert.ok(false);
// throws &quot;AssertionError: false == true&quot;
assert.ok(0);
// throws &quot;AssertionError: 0 == true&quot;
assert.ok(false, &#39;it\&#39;s false&#39;);
// throws &quot;AssertionError: it&#39;s false&quot;
</code></pre>
<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">
<span>Added in: v0.1.21</span>
</div><ul>
<li><code>actual</code> <span class="type">&lt;any&gt;</span></li>
<li><code>expected</code> <span class="type">&lt;any&gt;</span></li>
<li><code>message</code> <span class="type">&lt;any&gt;</span></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="lang-js">const assert = require(&#39;assert&#39;).strict;

assert.strictEqual(1, 2);
// AssertionError: 1 === 2

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

assert.strictEqual(1, &#39;1&#39;);
// AssertionError: 1 === &#39;1&#39;
</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 [<code>Error</code>][] then it will be thrown
instead of the <code>AssertionError</code>.</p>
<h2>assert.throws(block[, error][, message])<span><a class="mark" href="#assert_assert_throws_block_error_message" id="assert_assert_throws_block_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>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>block</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type">&lt;Function&gt;</a></li>
<li><code>error</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp" class="type">&lt;RegExp&gt;</a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type">&lt;Function&gt;</a> | <span class="type">&lt;object&gt;</span></li>
<li><code>message</code> <span class="type">&lt;any&gt;</span></li>
</ul>
<p>Expects the function <code>block</code> to throw an error.</p>
<p>If specified, <code>error</code> can be a constructor, <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Regular_Expressions"><code>RegExp</code></a>, a validation
function, or an object where each property will be tested for.</p>
<p>If specified, <code>message</code> will be the message provided by the <code>AssertionError</code> if
the block fails to throw.</p>
<p>Validate instanceof using constructor:</p>
<pre><code class="lang-js">assert.throws(
  () =&gt; {
    throw new Error(&#39;Wrong value&#39;);
  },
  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="lang-js">assert.throws(
  () =&gt; {
    throw new Error(&#39;Wrong value&#39;);
  },
  /^Error: Wrong value$/
);
</code></pre>
<p>Custom error validation:</p>
<pre><code class="lang-js">assert.throws(
  () =&gt; {
    throw new Error(&#39;Wrong value&#39;);
  },
  function(err) {
    if ((err instanceof Error) &amp;&amp; /value/.test(err)) {
      return true;
    }
  },
  &#39;unexpected error&#39;
);
</code></pre>
<p>Custom error object / error instance:</p>
<pre><code class="lang-js">assert.throws(
  () =&gt; {
    const err = new TypeError(&#39;Wrong value&#39;);
    err.code = 404;
    throw err;
  },
  {
    name: &#39;TypeError&#39;,
    message: &#39;Wrong value&#39;
    // Note that only properties on the error object will be tested!
  }
);
</code></pre>
<p>Note that <code>error</code> can not 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 below carefully if using a string as the second argument gets
considered:</p>
<!-- eslint-disable no-restricted-syntax -->
<pre><code class="lang-js">function throwingFirst() {
  throw new Error(&#39;First&#39;);
}
function throwingSecond() {
  throw new Error(&#39;Second&#39;);
}
function notThrowing() {}

// The second argument is a string and the input function threw an Error.
// In that case both cases do not throw as neither is going to try to
// match for the error message thrown by the input function!
assert.throws(throwingFirst, &#39;Second&#39;);
assert.throws(throwingSecond, &#39;Second&#39;);

// The string is only used (as message) in case the function does not throw:
assert.throws(notThrowing, &#39;Second&#39;);
// 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 a 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>
<h2>Caveats<span><a class="mark" href="#assert_caveats" id="assert_caveats">#</a></span></h2>
<p>For the following cases, consider using ES2015 <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/is"><code>Object.is()</code></a>,
which uses the <a href="https://tc39.github.io/ecma262/#sec-samevaluezero">SameValueZero</a> comparison.</p>
<pre><code class="lang-js">const a = 0;
const b = -a;
assert.notStrictEqual(a, b);
// AssertionError: 0 !== -0
// Strict Equality Comparison doesn&#39;t distinguish between -0 and +0...
assert(!Object.is(a, b));
// but Object.is() does!

const str1 = &#39;foo&#39;;
const str2 = &#39;foo&#39;;
assert.strictEqual(str1 / 1, str2 / 1);
// AssertionError: NaN === NaN
// Strict Equality Comparison can&#39;t be used to check NaN...
assert(Object.is(str1 / 1, str2 / 1));
// but Object.is() can!
</code></pre>
<p>For more information, see
<a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Equality_comparisons_and_sameness">MDN&#39;s guide on equality comparisons and sameness</a>.</p>

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