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<h2>Table of Contents</h2>
<ul>
<li><span class="stability_2"><a href="#assert_assert">Assert</a></span><ul>
<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></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>
</ul>
</li>
</ul>
</div>
<div id="apicontent">
<h1>Assert<span><a class="mark" href="#assert_assert"id="assert_assert">#</a></span></h1>
<!--introduced_in=v0.10.0-->
<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>
<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"><any></span></li>
<li><code>message</code> <span class="type"><any></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">
<span>Added in: v0.1.21</span>
</div><ul>
<li><code>actual</code> <span class="type"><any></span></li>
<li><code>expected</code> <span class="type"><any></span></li>
<li><code>message</code> <span class="type"><any></span></li>
</ul>
<p>Tests for deep equality between the <code>actual</code> and <code>expected</code> parameters.
Primitive values are compared with the equal comparison operator ( <code>==</code> ).</p>
<p>Only enumerable "own" properties are considered. The <code>deepEqual()</code>
implementation does not test object prototypes, attached symbols, or
non-enumerable properties. This can lead to some potentially surprising
results. For example, the following example does not throw an <code>AssertionError</code>
because the properties on the <a href="errors.html#errors_class_error"><code>Error</code></a> object are non-enumerable:</p>
<pre><code class="lang-js">// WARNING: This does not throw an AssertionError!
assert.deepEqual(Error('a'), Error('b'));
</code></pre>
<p>"Deep" equality means that the enumerable "own" properties of child objects
are evaluated also:</p>
<pre><code class="lang-js">const assert = require('assert');
const obj1 = {
a: {
b: 1
}
};
const obj2 = {
a: {
b: 2
}
};
const obj3 = {
a: {
b: 1
}
};
const obj4 = Object.create(obj1);
assert.deepEqual(obj1, obj1);
// OK, object is equal to itself
assert.deepEqual(obj1, obj2);
// AssertionError: { a: { b: 1 } } deepEqual { a: { b: 2 } }
// values of b are different
assert.deepEqual(obj1, obj3);
// OK, objects are equal
assert.deepEqual(obj1, obj4);
// AssertionError: { a: { b: 1 } } deepEqual {}
// Prototypes are ignored
</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.</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">
<span>Added in: v1.2.0</span>
</div><ul>
<li><code>actual</code> <span class="type"><any></span></li>
<li><code>expected</code> <span class="type"><any></span></li>
<li><code>message</code> <span class="type"><any></span></li>
</ul>
<p>Generally identical to <code>assert.deepEqual()</code> with two exceptions. First,
primitive values are compared using the strict equality operator ( <code>===</code> ).
Second, object comparisons include a strict equality check of their prototypes.</p>
<pre><code class="lang-js">const assert = require('assert');
assert.deepEqual({ a: 1 }, { a: '1' });
// OK, because 1 == '1'
assert.deepStrictEqual({ a: 1 }, { a: '1' });
// AssertionError: { a: 1 } deepStrictEqual { a: '1' }
// because 1 !== '1' using strict equality
</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.</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">
<span>Added in: v0.1.21</span>
</div><ul>
<li><code>block</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> <span class="type"><any></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>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>
<pre><code class="lang-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 (TypeError)..':</p>
<pre><code class="lang-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>
<pre><code class="lang-js">assert.doesNotThrow(
() => {
throw new TypeError('Wrong value');
},
TypeError,
'Whoops'
);
// 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"><any></span></li>
<li><code>expected</code> <span class="type"><any></span></li>
<li><code>message</code> <span class="type"><any></span></li>
</ul>
<p>Tests shallow, coercive equality between the <code>actual</code> and <code>expected</code> parameters
using the equal comparison operator ( <code>==</code> ).</p>
<pre><code class="lang-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.</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"><any></span></li>
<li><code>expected</code> <span class="type"><any></span></li>
<li><code>message</code> <span class="type"><any></span></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> '!='</li>
<li><code>stackStartFunction</code> <span class="type"><function></span> <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>.
Otherwise, the error message is the value of <code>message</code>.
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>).</p>
<pre><code class="lang-js">const assert = require('assert');
assert.fail(1, 2, undefined, '>');
// AssertionError: 1 > 2
assert.fail(1, 2, 'fail');
// AssertionError: fail
assert.fail(1, 2, 'whoops', '>');
// AssertionError: whoops
assert.fail('boom');
// AssertionError: boom
assert.fail('a', 'b');
// AssertionError: 'a' != 'b'
</code></pre>
<p>Example use of <code>stackStartFunction</code> for truncating the exception's stacktrace:</p>
<pre><code class="lang-js">function suppressFrame() {
assert.fail('a', 'b', undefined, '!==', suppressFrame);
}
suppressFrame();
// AssertionError: 'a' !== 'b'
// at repl:1:1
// at ContextifyScript.Script.runInThisContext (vm.js:44:33)
// ...
</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"><any></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('assert');
assert.ifError(null);
// OK
assert.ifError(0);
// OK
assert.ifError(1);
// Throws 1
assert.ifError('error');
// Throws 'error'
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">
<span>Added in: v0.1.21</span>
</div><ul>
<li><code>actual</code> <span class="type"><any></span></li>
<li><code>expected</code> <span class="type"><any></span></li>
<li><code>message</code> <span class="type"><any></span></li>
</ul>
<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('assert');
const obj1 = {
a: {
b: 1
}
};
const obj2 = {
a: {
b: 2
}
};
const obj3 = {
a: {
b: 1
}
};
const obj4 = Object.create(obj1);
assert.notDeepEqual(obj1, obj1);
// AssertionError: { a: { b: 1 } } notDeepEqual { a: { b: 1 } }
assert.notDeepEqual(obj1, obj2);
// OK: obj1 and obj2 are not deeply equal
assert.notDeepEqual(obj1, obj3);
// AssertionError: { a: { b: 1 } } notDeepEqual { a: { b: 1 } }
assert.notDeepEqual(obj1, obj4);
// OK: obj1 and 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.</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">
<span>Added in: v1.2.0</span>
</div><ul>
<li><code>actual</code> <span class="type"><any></span></li>
<li><code>expected</code> <span class="type"><any></span></li>
<li><code>message</code> <span class="type"><any></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('assert');
assert.notDeepEqual({a: 1}, {a: '1'});
// AssertionError: { a: 1 } notDeepEqual { a: '1' }
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.</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"><any></span></li>
<li><code>expected</code> <span class="type"><any></span></li>
<li><code>message</code> <span class="type"><any></span></li>
</ul>
<p>Tests shallow, coercive inequality with the not equal comparison operator
( <code>!=</code> ).</p>
<pre><code class="lang-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.</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"><any></span></li>
<li><code>expected</code> <span class="type"><any></span></li>
<li><code>message</code> <span class="type"><any></span></li>
</ul>
<p>Tests strict inequality as determined by the strict not equal operator
( <code>!==</code> ).</p>
<pre><code class="lang-js">const assert = require('assert');
assert.notStrictEqual(1, 2);
// OK
assert.notStrictEqual(1, 1);
// AssertionError: 1 !== 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.</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"><any></span></li>
<li><code>message</code> <span class="type"><any></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.</p>
<pre><code class="lang-js">const assert = require('assert');
assert.ok(true);
// OK
assert.ok(1);
// OK
assert.ok(false);
// throws "AssertionError: false == true"
assert.ok(0);
// throws "AssertionError: 0 == true"
assert.ok(false, 'it\'s false');
// throws "AssertionError: it's false"
</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"><any></span></li>
<li><code>expected</code> <span class="type"><any></span></li>
<li><code>message</code> <span class="type"><any></span></li>
</ul>
<p>Tests strict equality as determined by the strict equality operator ( <code>===</code> ).</p>
<pre><code class="lang-js">const assert = require('assert');
assert.strictEqual(1, 2);
// AssertionError: 1 === 2
assert.strictEqual(1, 1);
// OK
assert.strictEqual(1, '1');
// AssertionError: 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.</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">
<span>Added in: v0.1.21</span>
</div><ul>
<li><code>block</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> <span class="type"><any></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>, or validation
function.</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(
() => {
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>
<pre><code class="lang-js">assert.throws(
() => {
throw new Error('Wrong value');
},
/value/
);
</code></pre>
<p>Custom error validation:</p>
<pre><code class="lang-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> 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:</p>
<!-- eslint-disable no-restricted-syntax -->
<pre><code class="lang-js">// THIS IS A MISTAKE! DO NOT DO THIS!
assert.throws(myFunction, 'missing foo', 'did not throw with expected message');
// Do this instead.
assert.throws(myFunction, /missing foo/, 'did not throw with expected message');
</code></pre>
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