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---
title: Iterator
slug: Web/JavaScript/Reference/Global_Objects/Iterator
tags:
- JavaScript
- Reference
translation_of: Archive/Web/Iterator
---
<div>{{jsSidebar("Objects")}}</div>
<div> </div>
<div class="blockIndicator warning">
<p><strong>Non-standard.</strong> The <code><strong>Iterator</strong></code> function is a SpiderMonkey-specific feature, and will be removed at some point. For future-facing usages, consider using <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/for...of" title="/en-US/docs/Web/JavaScript/Reference/Statements/for...of">for..of</a> loops and the <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/The_Iterator_protocol">iterator protocol</a>.</p>
</div>
<p>The <code><strong>Iterator</strong></code> function returns an object which implements legacy iterator protocol and iterates over enumerable properties of an object.</p>
<h2 id="Syntax">Syntax</h2>
<pre>Iterator(<var>object</var>, [keyOnly])</pre>
<h3 id="Parameters">Parameters</h3>
<dl>
<dt><code>object</code></dt>
<dd>Object to iterate over properties.</dd>
<dt><code>keyOnly</code></dt>
<dd>If <code>keyOnly</code> is truthy value, <code>Iterator.prototype.next</code> returns <code>property_name</code> only.</dd>
</dl>
<h2 id="Description">Description</h2>
<p>Returns <code>Iterator</code> instance that iterates over <code>object</code>. <code>Iterator</code> instance returns <code>[property_name, property_value]</code> array for each iteration if <code>keyOnly</code> is falsy, otherwise, if <code>keyOnly</code> is truthy, it returns <code>property_name</code> for each iteration. If <code>object</code> is the <code>Iterator</code> instance or Generator instance, it returns <code>object</code> itself.</p>
<h2 id="Properties">Properties</h2>
<dl>
<dt><code><strong>Iterator.prototype[@@iterator]</strong></code></dt>
<dd>Returns a function that returns iterator object.</dd>
</dl>
<h2 id="Methods">Methods</h2>
<dl>
<dt><code><strong>Iterator.prototype.next</strong></code></dt>
<dd>Returns next item in the <code>[property_name, property_value]</code> format or <code>property_name</code> only. It throws <code><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/StopIteration">StopIteration</a></code> if there are no more items.</dd>
</dl>
<h2 id="Examples">Examples</h2>
<h3 id="Iterating_over_properties_of_an_object">Iterating over properties of an object</h3>
<pre><code>var a = {
x: 10,
y: 20,
};
var iter = Iterator(a);
console.log(iter.next()); // ["x", 10]
console.log(iter.next()); // ["y", 20]
console.log(iter.next()); // throws StopIteration</code></pre>
<h3 id="Iterating_over_properties_of_an_object_with_legacy_destructuring_for-in_statement">Iterating over properties of an object with legacy destructuring <code>for-in</code> statement</h3>
<pre><code>var a = {
x: 10,
y: 20,
};
for (var [name, value] in Iterator(a)) {
console.log(name, value); // x 10
// y 20
}</code></pre>
<h3 id="Iterating_with_for-of">Iterating with <code>for-of</code></h3>
<pre><code>var a = {
x: 10,
y: 20,
};
for (var [name, value] of Iterator(a)) { // @@iterator is used
console.log(name, value); // x 10
// y 20
}</code></pre>
<h3 id="Iterates_over_property_name">Iterates over property name</h3>
<pre><code>var a = {
x: 10,
y: 20,
};
for (var name in Iterator(a, true)) {
console.log(name); // x
// y
}</code></pre>
<h3 id="Passing_Generator_instance">Passing Generator instance</h3>
<pre><code>function* f() {
yield 'a';
yield 'b';
}
var g = f();
console.log(g == Iterator(g)); // true
for (var v in Iterator(g)) {
console.log(v); // a
// b
}</code></pre>
<h3 id="Passing_Iterator_instance">Passing Iterator instance</h3>
<pre><code>var a = {
x: 10,
y: 20,
};
var i = Iterator(a);
console.log(i == Iterator(i)); // true</code></pre>
<h2 id="Specifications">Specifications</h2>
<p>Non-standard. Not part of any current standards document</p>
<h2 id="Browser_compatibility">Browser compatibility</h2>
<p>Not supported. Used to be in Firefox in versions prior to Firefox 57.</p>
<h2 id="See_also">See also</h2>
<ul>
<li><a href="https://developer.mozilla.org/en-US/docs/JavaScript/Guide/Iterators_and_Generators" title="/en-US/docs/JavaScript/Guide/Iterators_and_Generators">Iterators and Generators</a></li>
<li><code><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/StopIteration">StopIteration</a></code></li>
</ul>
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