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<!DOCTYPE HTML>
<html>
<head>
<meta http-equiv="content-type" content="text/html;charset=UTF-8" />
<meta http-equiv="X-UA-Compatible" content="chrome=1" />
<meta name="viewport" content="width=device-width, initial-scale=1.0"/>
<link rel="canonical" href="https://underscorejs.org/" />
<link rel="shortcut icon" href="favicon.ico" type="image/x-icon" />
<title>Underscore.js</title>
<style>
body {
font-size: 14px;
line-height: 22px;
background: #f4f4f4 url(docs/images/background.png);
color: #000;
font-family: Helvetica Neue, Helvetica, Arial;
}
.interface {
font-family: "Lucida Grande", "Lucida Sans Unicode", Helvetica, Arial, sans-serif !important;
}
.container{
margin-top:50px;
}
div#sidebar {
background: #fff;
position: fixed;
top: 0; left: 0; bottom: 0;
width: 220px;
overflow-y: auto;
overflow-x: hidden;
-webkit-overflow-scrolling: touch;
padding: 15px 0 30px 30px;
border-right: 1px solid #bbb;
box-shadow: 0 0 20px #ccc; -webkit-box-shadow: 0 0 20px #ccc; -moz-box-shadow: 0 0 20px #ccc;
}
a.toc_title, a.toc_title:visited {
display: block;
color: black;
font-weight: bold;
margin-top: 15px;
}
a.toc_title:hover {
text-decoration: underline;
}
#sidebar .version {
font-size: 10px;
font-weight: normal;
}
ul.toc_section {
font-size: 11px;
line-height: 14px;
margin: 5px 0 0 0;
padding-left: 0px;
list-style-type: none;
font-family: Lucida Grande;
}
.toc_section li {
cursor: pointer;
margin: 0 0 3px 0;
}
.toc_section li a {
text-decoration: none;
color: black;
}
.toc_section li a:hover {
text-decoration: underline;
}
input#function_filter {
width: 80%;
}
div.container {
width: 550px;
margin: 40px 0 50px 280px;
}
img#logo {
width: 396px;
height: 69px;
}
div.warning {
margin-top: 15px;
font: bold 11px Arial;
color: #770000;
}
p {
margin: 20px 0;
width: 550px;
}
a, a:visited {
color: #444;
}
a:active, a:hover {
color: #000;
}
h1, h2, h3, h4, h5, h6 {
padding-top: 20px;
}
h2 {
font-size: 20px;
}
b.header {
font-size: 16px;
line-height: 30px;
}
span.alias {
font-size: 14px;
font-style: italic;
margin-left: 20px;
}
a.source {
font-variant: small-caps;
margin-left: 20px;
}
a.source + a.source {
margin-left: unset;
}
table, tr, td {
margin: 0; padding: 0;
}
td {
padding: 2px 12px 2px 0;
}
table .rule {
height: 1px;
background: #ccc;
margin: 5px 0;
}
table .dl-link {
white-space: nowrap;
}
ul {
list-style-type: circle;
padding: 0 0 0 20px;
}
li {
margin-bottom: 10px;
}
code, pre, tt {
font-family: Monaco, Consolas, "Lucida Console", monospace;
font-size: 12px;
line-height: 18px;
font-style: normal;
}
tt {
padding: 0px 3px;
background: #fff;
border: 1px solid #ddd;
zoom: 1;
}
code {
margin-left: 20px;
}
pre {
font-size: 12px;
padding: 2px 0 2px 15px;
border-left: 5px solid #bbb;
margin: 0px 0 30px;
}
input[type=checkbox] {
position: absolute;
top: -9999px;
left: -9999px;
}
label {
position: fixed;
left:0;
display:none;
width: 1em;
height: 1.5em;
padding: .5em 0.5em 0;
font-size:1.1em;
color:#fff;
transition:color .3s ease-in-out;
cursor: pointer;
user-select:none;
margin:0;
background:rgba(0, 0, 0, 1);
}
.overlay {
height: 100%;
width: 0;
position: fixed;
z-index: 1;
top: 0;
left: 0;
background-color: rgb(0,0,0);
background-color: rgba(0,0,0, 0.9);
overflow-x: hidden;
transition: 0.5s;
}
@media only screen and (-webkit-min-device-pixel-ratio: 1.5) and (max-width: 640px),
only screen and (-o-min-device-pixel-ratio: 3/2) and (max-width: 640px),
only screen and (min-device-pixel-ratio: 1.5) and (max-width: 640px) {
img {
max-width: 100%;
}
div#sidebar {
-webkit-overflow-scrolling: initial;
position: relative;
width: 100%;
height: 100%;
left: 0;
padding: 20px 0 10px 45px;
border: 0;
}
img#logo {
width: auto;
height: auto;
}
div.container {
margin: 50px 0;
width: 100%;
}
p, div.container ul {
max-width: 98%;
overflow-x: scroll;
}
pre {
overflow: scroll;
}
}
@media screen and (max-width: 640px){
body{
line-height: 22px;
}
ul.toc_section {
font-size: 14px;
line-height: 15px;
font-family: Helvetica Neue, Helvetica, Arial;
}
label {
display:block;
top:0;
z-index:999;
}
label:after {
position: absolute;
right: .25em;
top: 5px;
content:"\2261";
font-size:1.8em;
}
label:hover,
input:checked ~ label {
color:#fff;
}
input:checked ~ #myNav{
width:0;
}
#myNav{
width: 100%;
}
.overlay-content {
position: relative;
top: 25%;
width: 100%;
}
}
@media screen and (prefers-color-scheme: dark) {
body {
background: #111 !important;
color: #c6c6c6;
}
div#sidebar {
background: #111;
border-right: 1px solid #666;
box-shadow: 0 0 20px #555;
-webkit-box-shadow: 0 0 20px #555;
-moz-box-shadow: 0 0 20px #555;
}
a.toc_title,
a.toc_title:visited {
color: #f4f4f4;
}
a.toc_title:hover {
text-decoration: underline;
}
.toc_section li a {
text-decoration: none;
color: #f4f4f4;
}
img#logo {
filter: invert(1) hue-rotate(180deg) brightness(0.9);
}
div.warning {
color: #CC6060;
}
a,
a:visited {
color: #f4f4f4;
}
a:active,
a:hover {
color: #ffffff;
}
table .rule {
background: #777;
}
tt {
background: #444;
border: 1px solid #777;
}
pre {
border-left: 5px solid #aaa;
}
}
</style>
</head>
<body>
<input type="checkbox" id="menu" checked>
<label for="menu" role="navigation" aria-label="Menu"></label>
<div id="myNav" class="overlay">
<div id="sidebar" class="interface overlay-content">
<a class="toc_title" href="#">
Underscore.js <span class="version">(1.13.8)</span>
</a>
<ul class="toc_section">
<li>» <a href="https://github.com/jashkenas/underscore">GitHub Repository</a></li>
<li>» <a href="docs/modules/index-all.html">Annotated Source (modular)</a></li>
<li>» <a href="docs/underscore-esm.html">Annotated Source (single read)</a></li>
<li>» <a href="https://documentcloud.github.io/underscore-contrib/">Underscore-contrib</a></li>
<li>» <a href="https://twitter.com/underscoredotjs">Twitter</a></li>
<li>» <a href="https://tidelift.com/subscription/pkg/npm-underscore?utm_source=npm-underscore&utm_medium=referral&utm_campaign=enterprise">Tidelift</a></li>
<li>» <a href="https://patreon.com/juliangonggrijp">Patreon</a></li>
</ul>
<input id="function_filter" placeholder="Filter" type="text" autofocus />
<div class="searchable_section">
<a class="toc_title" href="#">
Introduction
</a>
</div>
<div class="searchable_section">
<a class="toc_title" href="#collections">
Collections
</a>
<ul class="toc_section">
<li data-name="each" data-aliases="forEach">- <a href="#each">each</a></li>
<li data-name="map" data-aliases="collect">- <a href="#map">map</a></li>
<li data-name="reduce" data-aliases="inject foldl">- <a href="#reduce">reduce</a></li>
<li data-name="reduceRight" data-aliases="foldr">- <a href="#reduceRight">reduceRight</a></li>
<li data-name="find" data-aliases="detect">- <a href="#find">find</a></li>
<li data-name="filter" data-aliases="select">- <a href="#filter">filter</a></li>
<li data-name="where">- <a href="#where">where</a></li>
<li data-name="findWhere">- <a href="#findWhere">findWhere</a></li>
<li data-name="reject">- <a href="#reject">reject</a></li>
<li data-name="every" data-aliases="all">- <a href="#every">every</a></li>
<li data-name="some" data-aliases="any">- <a href="#some">some</a></li>
<li data-name="contains" data-aliases="include includes">- <a href="#contains">contains</a></li>
<li data-name="invoke">- <a href="#invoke">invoke</a></li>
<li data-name="pluck">- <a href="#pluck">pluck</a></li>
<li data-name="max">- <a href="#max">max</a></li>
<li data-name="min">- <a href="#min">min</a></li>
<li data-name="sortBy">- <a href="#sortBy">sortBy</a></li>
<li data-name="groupBy">- <a href="#groupBy">groupBy</a></li>
<li data-name="indexBy">- <a href="#indexBy">indexBy</a></li>
<li data-name="countBy">- <a href="#countBy">countBy</a></li>
<li data-name="shuffle">- <a href="#shuffle">shuffle</a></li>
<li data-name="sample">- <a href="#sample">sample</a></li>
<li data-name="toArray">- <a href="#toArray">toArray</a></li>
<li data-name="size">- <a href="#size">size</a></li>
<li data-name="partition">- <a href="#partition">partition</a></li>
</ul>
</div>
<div class="searchable_section">
<a class="toc_title" href="#arrays">
Arrays
</a>
<ul class="toc_section">
<li data-name="first" data-aliases="head take">- <a href="#first">first</a></li>
<li data-name="initial">- <a href="#initial">initial</a></li>
<li data-name="last">- <a href="#last">last</a></li>
<li data-name="rest" data-aliases="tail drop">- <a href="#rest">rest</a></li>
<li data-name="compact">- <a href="#compact">compact</a></li>
<li data-name="flatten">- <a href="#flatten">flatten</a></li>
<li data-name="without">- <a href="#without">without</a></li>
<li data-name="union">- <a href="#union">union</a></li>
<li data-name="intersection">- <a href="#intersection">intersection</a></li>
<li data-name="difference">- <a href="#difference">difference</a></li>
<li data-name="uniq" data-aliases="unique">- <a href="#uniq">uniq</a></li>
<li data-name="zip">- <a href="#zip">zip</a></li>
<li data-name="unzip" data-aliases="transpose">- <a href="#unzip">unzip</a></li>
<li data-name="object">- <a href="#object">object</a></li>
<li data-name="chunk">- <a href="#chunk">chunk</a></li>
<li data-name="indexOf">- <a href="#indexOf">indexOf</a></li>
<li data-name="lastIndexOf">- <a href="#lastIndexOf">lastIndexOf</a></li>
<li data-name="sortedIndex">- <a href="#sortedIndex">sortedIndex</a></li>
<li data-name="findIndex">- <a href="#findIndex">findIndex</a></li>
<li data-name="findLastIndex">- <a href="#findLastIndex">findLastIndex</a></li>
<li data-name="range">- <a href="#range">range</a></li>
</ul>
</div>
<div class="searchable_section">
<a class="toc_title" href="#functions">
Functions
</a>
<ul class="toc_section">
<li data-name="bind">- <a href="#bind">bind</a></li>
<li data-name="bindAll">- <a href="#bindAll">bindAll</a></li>
<li data-name="partial">- <a href="#partial">partial</a></li>
<li data-name="memoize">- <a href="#memoize">memoize</a></li>
<li data-name="delay">- <a href="#delay">delay</a></li>
<li data-name="defer">- <a href="#defer">defer</a></li>
<li data-name="throttle">- <a href="#throttle">throttle</a></li>
<li data-name="debounce">- <a href="#debounce">debounce</a></li>
<li data-name="once">- <a href="#once">once</a></li>
<li data-name="after">- <a href="#after">after</a></li>
<li data-name="before">- <a href="#before">before</a></li>
<li data-name="wrap">- <a href="#wrap">wrap</a></li>
<li data-name="negate">- <a href="#negate">negate</a></li>
<li data-name="compose">- <a href="#compose">compose</a></li>
<li data-name="restArguments">- <a href="#restArguments">restArguments</a></li>
</ul>
</div>
<div class="searchable_section">
<a class="toc_title" href="#objects">
Objects
</a>
<ul class="toc_section">
<li data-name="keys">- <a href="#keys">keys</a></li>
<li data-name="allKeys">- <a href="#allKeys">allKeys</a></li>
<li data-name="values">- <a href="#values">values</a></li>
<li data-name="mapObject">- <a href="#mapObject">mapObject</a></li>
<li data-name="pairs">- <a href="#pairs">pairs</a></li>
<li data-name="invert">- <a href="#invert">invert</a></li>
<li data-name="create">- <a href="#create">create</a></li>
<li data-name="object-functions" data-aliases="methods">- <a href="#object-functions">functions</a></li>
<li data-name="findKey">- <a href="#findKey">findKey</a></li>
<li data-name="extend">- <a href="#extend">extend</a></li>
<li data-name="extendOwn" data-aliases="assign">- <a href="#extendOwn">extendOwn</a></li>
<li data-name="pick">- <a href="#pick">pick</a></li>
<li data-name="omit">- <a href="#omit">omit</a></li>
<li data-name="defaults">- <a href="#defaults">defaults</a></li>
<li data-name="clone">- <a href="#clone">clone</a></li>
<li data-name="tap">- <a href="#tap">tap</a></li>
<li data-name="toPath">- <a href="#toPath">toPath</a></li>
<li data-name="has">- <a href="#has">has</a></li>
<li data-name="get">- <a href="#get">get</a></li>
<li data-name="property">- <a href="#property">property</a></li>
<li data-name="propertyOf">- <a href="#propertyOf">propertyOf</a></li>
<li data-name="matcher">- <a href="#matcher">matcher</a></li>
<li data-name="isEqual">- <a href="#isEqual">isEqual</a></li>
<li data-name="isMatch">- <a href="#isMatch">isMatch</a></li>
<li data-name="isEmpty">- <a href="#isEmpty">isEmpty</a></li>
<li data-name="isElement">- <a href="#isElement">isElement</a></li>
<li data-name="isArray">- <a href="#isArray">isArray</a></li>
<li data-name="isObject">- <a href="#isObject">isObject</a></li>
<li data-name="isArguments">- <a href="#isArguments">isArguments</a></li>
<li data-name="isFunction">- <a href="#isFunction">isFunction</a></li>
<li data-name="isString">- <a href="#isString">isString</a></li>
<li data-name="isNumber">- <a href="#isNumber">isNumber</a></li>
<li data-name="isFinite">- <a href="#isFinite">isFinite</a></li>
<li data-name="isBoolean">- <a href="#isBoolean">isBoolean</a></li>
<li data-name="isDate">- <a href="#isDate">isDate</a></li>
<li data-name="isRegExp">- <a href="#isRegExp">isRegExp</a></li>
<li data-name="isError">- <a href="#isError">isError</a></li>
<li data-name="isSymbol">- <a href="#isSymbol">isSymbol</a></li>
<li data-name="isMap">- <a href="#isMap">isMap</a></li>
<li data-name="isWeakMap">- <a href="#isWeakMap">isWeakMap</a></li>
<li data-name="isSet">- <a href="#isSet">isSet</a></li>
<li data-name="isWeakSet">- <a href="#isWeakSet">isWeakSet</a></li>
<li data-name="isArrayBuffer">- <a href="#isArrayBuffer">isArrayBuffer</a></li>
<li data-name="isDataView">- <a href="#isDataView">isDataView</a></li>
<li data-name="isTypedArray" data-aliases="isInt8Array isUint8Array isUint8ClampedArray isInt16Array isUint16Array isInt32Array isUint32Array isBigInt64Array isBigUint64Array isFloat32Array isFloat64Array">- <a href="#isTypedArray">isTypedArray</a></li>
<li data-name="isNaN">- <a href="#isNaN">isNaN</a></li>
<li data-name="isNull">- <a href="#isNull">isNull</a></li>
<li data-name="isUndefined">- <a href="#isUndefined">isUndefined</a></li>
</ul>
</div>
<div class="searchable_section">
<a class="toc_title" href="#utility">
Utility
</a>
<ul class="toc_section">
<li data-name="noConflict">- <a href="#noConflict">noConflict</a></li>
<li data-name="identity">- <a href="#identity">identity</a></li>
<li data-name="constant">- <a href="#constant">constant</a></li>
<li data-name="noop">- <a href="#noop">noop</a></li>
<li data-name="times">- <a href="#times">times</a></li>
<li data-name="random">- <a href="#random">random</a></li>
<li data-name="mixin">- <a href="#mixin">mixin</a></li>
<li data-name="iteratee">- <a href="#iteratee">iteratee</a></li>
<li data-name="uniqueId">- <a href="#uniqueId">uniqueId</a></li>
<li data-name="escape">- <a href="#escape">escape</a></li>
<li data-name="unescape">- <a href="#unescape">unescape</a></li>
<li data-name="result">- <a href="#result">result</a></li>
<li data-name="now">- <a href="#now">now</a></li>
<li data-name="template">- <a href="#template">template</a></li>
<li data-name="version">- <a href="#version">version</a></li>
</ul>
</div>
<div class="searchable_section">
<a class="toc_title" href="#oop" data-name="oop" data-aliases="_() ()">
OOP Style
</a>
</div>
<div class="searchable_section">
<a class="toc_title" href="#chaining">
Chaining
</a>
<ul class="toc_section">
<li data-name="chain">- <a href="#chain">chain</a></li>
<li data-name="value">- <a href="#value">value</a></li>
</ul>
</div>
<div class="searchable_section">
<a class="toc_title" href="#links">
Links
</a>
</div>
<div class="searchable_section">
<a class="toc_title" href="#notes">
Notes
</a>
</div>
<div class="searchable_section">
<a class="toc_title" href="#changelog">
Change Log
</a>
</div>
</div>
</div>
<div class="container">
<p id="introduction">
<img id="logo" src="docs/images/underscore.png" alt="Underscore.js" />
</p>
<p>
<a href="https://github.com/jashkenas/underscore/">Underscore</a>
is a JavaScript library that provides a whole mess of useful functional
programming helpers without extending any built-in objects.
It’s the answer to the question: “If I sit down in front of a
blank HTML page, and want to start being productive immediately, what do I need?”
… and the tie to go along with
<a href="https://jquery.com">jQuery</a>'s tux and
<a href="https://backbonejs.org">Backbone</a>'s suspenders.
</p>
<p>
Underscore provides over 100 functions that support both your favorite
workaday functional helpers: <b>map</b>, <b>filter</b>, <b>invoke</b> —
as well as more specialized goodies: function binding, javascript
templating, creating quick indexes, deep equality testing, and so on.
</p>
<p>
A complete <a href="test/">Test Suite</a>
is included for your perusal.
</p>
<p>
You may also read through the <a
href="docs/underscore-esm.html">annotated source code</a>. There is a <a
href="docs/modules/index-all.html">modular version</a> with clickable
import references as well.
</p>
<p>
You may choose between monolithic and modular imports. There is a quick
summary of the options below, as well as a more comprehensive
discussion in <a
href="https://juliangonggrijp.com/article/introducing-modular-underscore.html"
>the article</a>.
</p>
<p>
Enjoying Underscore, and want to <i>turn it up to 11?</i> Try <a href="https://documentcloud.github.io/underscore-contrib/">Underscore-contrib</a>.
</p>
<p>
The project is
<a href="https://github.com/jashkenas/underscore">hosted on GitHub</a>.
You can report bugs and discuss features on the
<a href="https://github.com/jashkenas/underscore/issues">issues page</a>
or chat in the <a href="https://gitter.im/jashkenas/underscore">Gitter</a>
channel.
</p>
<p>
You can support the project by donating on
<a href="https://patreon.com/juliangonggrijp">Patreon</a>.
Enterprise coverage is available as part of the <a
href="https://tidelift.com/subscription/pkg/npm-underscore?utm_source=npm-underscore&utm_medium=referral&utm_campaign=enterprise"
>Tidelift Subscription</a>.
</p>
<p>
<i>Underscore is an open-source component of <a href="https://documentcloud.org/">DocumentCloud</a>.</i>
</p>
<h2>v1.13.8 Downloads <i style="padding-left: 12px; font-size:12px;">(Right-click, and use "Save As")</i></h2>
<table>
<tr>
<td class="dl-link"><a href="underscore-esm.js">ESM (Development)</a></td>
<td>
<i>65.9 KB, Uncompressed with Plentiful Comments</i>
<small>(<a href="underscore-esm.js.map">Source Map</a>)</small>
</td>
</tr>
<tr>
<td class="dl-link"><a href="underscore-esm-min.js">ESM (Production)</a></td>
<td>
<i>8.59 KB, Minified and Gzipped</i>
<small>(<a href="underscore-esm-min.js.map">Source Map</a>)</small>
</td>
</tr>
<tr>
<td class="dl-link"><a href="underscore-umd.js">UMD (Development)</a></td>
<td>
<i>68.4 KB, Uncompressed with Bountiful Comments</i>
<small>(<a href="underscore-umd.js.map">Source Map</a>)</small>
</td>
</tr>
<tr>
<td class="dl-link"><a href="underscore-umd-min.js">UMD (Production)</a></td>
<td>
<i>7.48 KB, Minified and Gzipped</i>
<small>(<a href="underscore-umd-min.js.map">Source Map</a>)</small>
</td>
</tr>
<tr>
<td colspan="2"><div class="rule"></div></td>
</tr>
<tr>
<td class="dl-link"><a href="https://raw.github.com/jashkenas/underscore/master/underscore-esm.js">Edge ESM</a></td>
<td><i>Unreleased, current <tt>master</tt>, use by your own judgement and at your own risk</i></td>
</tr>
<tr>
<td class="dl-link"><a href="https://raw.github.com/jashkenas/underscore/master/underscore-umd.js">Edge UMD</a></td>
<td><i>Unreleased, current <tt>master</tt>, use if you’re feeling lucky</i></td>
</tr>
</table>
<h2>v1.13.8 CDN URLs <i style="padding-left: 12px; font-size:12px;">(Use with <tt><script src="..."></script></tt>)</i></h2>
<ul>
<li>
<tt>https://cdn.jsdelivr.net/npm/[email protected]/underscore-umd-min.js</tt>
</li>
<li>
<tt>https://cdn.jsdelivr.net/npm/[email protected]/underscore-esm-min.js</tt>
</li>
<li>
<tt>https://unpkg.com/[email protected]/underscore-umd-min.js</tt>
</li>
<li>
<tt>https://unpkg.com/[email protected]/underscore-esm-min.js</tt>
</li>
<li>
<tt>https://cdnjs.cloudflare.com/ajax/libs/underscore.js/1.13.8/underscore-umd-min.js</tt>
</li>
<li>
<tt>https://cdnjs.cloudflare.com/ajax/libs/underscore.js/1.13.8/underscore-esm-min.js</tt>
</li>
</ul>
<p>
<i>In most cases, you can replace the version number above by
<tt>latest</tt> so that your embed will automatically use the latest
version, or <tt>stable</tt> if you want to delay updating until an update
has proven to be free of accidental breaking changes. Example:</i><br>
<tt>https://cdn.jsdelivr.net/npm/underscore@latest/underscore-umd-min.js</tt>
</p>
<h2>Package Installation</h2>
<ul>
<li>
<b>Node.js</b> <tt>npm install underscore</tt>
</li>
<li>
<b>Meteor.js</b> <tt>meteor add underscore</tt>
</li>
<li>
<b>Bower</b> <tt>bower install underscore</tt>
</li>
</ul>
<p>
<i>If you are hardcoding the path to the file within the package and you
are unsure which build to use, it is very likely that you need
<tt>underscore-umd.js</tt> or the minified variant
<tt>underscore-umd-min.js</tt>.</i>
</p>
<h2>Monolithic Import (recommended)</h2>
<ul>
<li>
<b>ESM</b> <tt>import _, { map } from 'underscore';</tt>
</li>
<li>
<b>AMD</b> <tt>require(['underscore'], ...)</tt>
</li>
<li>
<b>CommonJS</b> <tt>var _ = require('underscore');</tt>
</li>
<li>
<b>ExtendScript</b> <tt>#include "underscore-umd.js"</tt>
</li>
</ul>
<h2>Modular Import</h2>
<ul>
<li>
<b>ESM</b> <tt>import map from 'underscore/modules/map.js'</tt>
</li>
<li>
<b>AMD</b> <tt>require(['underscore/amd/map.js'], ...)</tt>
</li>
<li>
<b>CommonJS</b> <tt>var map = require('underscore/cjs/map.js');</tt>
</li>
</ul>
<p>
<i>For functions with multiple aliases, the file name of the module is
always the <b>first</b> name that appears in the documentation. For
example, <tt>_.reduce</tt>/<tt>_.inject</tt>/<tt>_.foldl</tt> is exported
from <tt>underscore/modules/reduce.js</tt>. Modular usage is mostly
recommended for creating a customized build of Underscore.</i>
</p>
<h2 id=compatibility>Engine Compatibility</h2>
<p>
Underscore 1.x is backwards compatible with any engine that fully
supports ES3, while also utilizing newer features when available, such as
<tt>Object.keys</tt>, typed arrays and ES modules. We routinely run our
unittests against the JavaScript engines listed below:
</p>
<ul>
<li>Chrome 26–latest</li>
<li>Edge 13, 18 and latest</li>
<li>Firefox 11–latest</li>
<li>Internet Explorer 9–11</li>
<li>Node.js 8–latest LTS</li>
<li>Safari 8–latest</li>
</ul>
<p>
In addition:
</p>
<ul>
<li>
We have recent confirmation that the library is compatible with Adobe
ExtendScript.
</li>
<li>
There is support code present for IE 8, which we will retain in
future Underscore 1.x updates.
</li>
<li>
Patches to enhance support for other ES3-compatible environments
are always welcome.
</li>
</ul>
<p>
Underscore 2.x will likely remove support for some outdated environments.
</p>
<div id="documentation">
<h2 id="collections">Collection Functions (Arrays or Objects)</h2>
<p id="each">
<b class="header">each</b><code>_.each(list, iteratee, [context])</code>
<span class="alias">Alias: <b>forEach</b></span>
<a href="docs/modules/each.html" class="source">source</a>
<br />
Iterates over a <b>list</b> of elements, yielding each in turn to an
<b>iteratee</b> function.
The <b>iteratee</b> is bound to the <b>context</b> object, if one is
passed. Each invocation of <b>iteratee</b> is called with three arguments:
<tt>(element, index, list)</tt>. If <b>list</b> is a JavaScript object, <b>iteratee</b>'s
arguments will be <tt>(value, key, list)</tt>. Returns the <b>list</b> for chaining.
</p>
<pre>
_.each([1, 2, 3], alert);
=> alerts each number in turn...
_.each({one: 1, two: 2, three: 3}, alert);
=> alerts each number value in turn...</pre>
<p>
<i>
Note: Collection functions work on arrays, objects, and
array-like objects such as</i> <tt>arguments</tt>, <tt>NodeList</tt><i>
and similar. But it works by duck-typing, so avoid passing objects with
a numeric <tt>length</tt> property. It's also good to note that an
<tt>each</tt> loop cannot be broken out of — to break, use <b>_.find</b>
instead.
</i>
</p>
<p id="map">
<b class="header">map</b><code>_.map(list, iteratee, [context])</code>
<span class="alias">Alias: <b>collect</b></span>
<a href="docs/modules/map.html" class="source">source</a>
<br />
Produces a new array of values by mapping each value in <b>list</b>
through a transformation function (<a href="#iteratee"><b>iteratee</b></a>).
The iteratee is passed three arguments: the <tt>value</tt>,
then the <tt>index</tt> (or <tt>key</tt>) of the iteration,
and finally a reference to the entire <tt>list</tt>.
</p>
<pre>
_.map([1, 2, 3], function(num){ return num * 3; });
=> [3, 6, 9]
_.map({one: 1, two: 2, three: 3}, function(num, key){ return num * 3; });
=> [3, 6, 9]
_.map([[1, 2], [3, 4]], _.first);
=> [1, 3]</pre>
<p id="reduce">
<b class="header">reduce</b><code>_.reduce(list, iteratee, [memo], [context])</code>
<span class="alias">Aliases: <b>inject</b>, <b>foldl</b></span>
<a href="docs/modules/reduce.html" class="source">source</a>
<br />
Also known as <b>inject</b> and <b>foldl</b>, reduce boils down a <b>list</b> of values into a single value.
<b>Memo</b> is the initial state of the reduction, and each successive step of it should be returned by
<b>iteratee</b>. The iteratee is passed four arguments: the <tt>memo</tt>, then the <tt>value</tt> and
<tt>index</tt> (or key) of the iteration, and finally a reference to the entire <tt>list</tt>.
</p>
<p>
If no memo is passed to the initial invocation of reduce, the iteratee is not invoked on the first element
of the list. The first element is instead passed as the memo in the invocation of the iteratee on the next
element in the list.
</p>
<pre>
var sum = _.reduce([1, 2, 3], function(memo, num){ return memo + num; }, 0);
=> 6
</pre>
<p id="reduceRight">
<b class="header">reduceRight</b><code>_.reduceRight(list, iteratee, [memo], [context])</code>
<span class="alias">Alias: <b>foldr</b></span>
<a href="docs/modules/reduceRight.html" class="source">source</a>
<br />
The right-associative version of <b>reduce</b>. <b>Foldr</b>
is not as useful in JavaScript as it would be in a language with lazy
evaluation.
</p>
<pre>
var list = [[0, 1], [2, 3], [4, 5]];
var flat = _.reduceRight(list, function(a, b) { return a.concat(b); }, []);
=> [4, 5, 2, 3, 0, 1]
</pre>
<p id="find">
<b class="header">find</b><code>_.find(list, predicate, [context])</code>
<span class="alias">Alias: <b>detect</b></span>
<a href="docs/modules/find.html" class="source">source</a>
<br />
Looks through each value in the <b>list</b>, returning the first one that
passes a truth test (<b>predicate</b>), or <tt>undefined</tt> if no value
passes the test. The function returns as
soon as it finds an acceptable element, and doesn't traverse the
entire list.
<b>predicate</b> is transformed through <a href="#iteratee"><b>iteratee</b></a>
to facilitate shorthand syntaxes.
</p>
<pre>
var even = _.find([1, 2, 3, 4, 5, 6], function(num){ return num % 2 == 0; });
=> 2
</pre>
<p id="filter">
<b class="header">filter</b><code>_.filter(list, predicate, [context])</code>
<span class="alias">Alias: <b>select</b></span>
<a href="docs/modules/filter.html" class="source">source</a>
<br />
Looks through each value in the <b>list</b>, returning an array of all
the values that pass a truth test (<b>predicate</b>).
<b>predicate</b> is transformed through <a href="#iteratee"><b>iteratee</b></a>
to facilitate shorthand syntaxes.
</p>
<pre>
var evens = _.filter([1, 2, 3, 4, 5, 6], function(num){ return num % 2 == 0; });
=> [2, 4, 6]
</pre>
<p id="findWhere">
<b class="header">findWhere</b><code>_.findWhere(list, properties)</code>
<a href="docs/modules/findWhere.html" class="source">source</a>
<br />
Looks through the <b>list</b> and returns the <i>first</i> value that <a href="#matches">matches</a>
all of the key-value pairs listed in <b>properties</b>.
</p>
<p>
If no match is found, or if <b>list</b> is empty, <i>undefined</i> will be
returned.
</p>
<pre>
_.findWhere(publicServicePulitzers, {newsroom: "The New York Times"});
=> {year: 1918, newsroom: "The New York Times",
reason: "For its public service in publishing in full so many official reports,
documents and speeches by European statesmen relating to the progress and
conduct of the war."}
</pre>
<p id="where">
<b class="header">where</b><code>_.where(list, properties)</code>
<a href="docs/modules/where.html" class="source">source</a>
<br />
Looks through each value in the <b>list</b>, returning an array of all
the values that <a href="#matches">matches</a> the key-value pairs listed in <b>properties</b>.
</p>
<pre>
_.where(listOfPlays, {author: "Shakespeare", year: 1611});
=> [{title: "Cymbeline", author: "Shakespeare", year: 1611},
{title: "The Tempest", author: "Shakespeare", year: 1611}]
</pre>
<p id="reject">
<b class="header">reject</b><code>_.reject(list, predicate, [context])</code>
<a href="docs/modules/reject.html" class="source">source</a>
<br />
Returns the values in <b>list</b> without the elements that the truth
test (<b>predicate</b>) passes. The opposite of <b>filter</b>.
<b>predicate</b> is transformed through <a href="#iteratee"><b>iteratee</b></a>
to facilitate shorthand syntaxes.
</p>
<pre>
var odds = _.reject([1, 2, 3, 4, 5, 6], function(num){ return num % 2 == 0; });
=> [1, 3, 5]
</pre>
<p id="every">
<b class="header">every</b><code>_.every(list, [predicate], [context])</code>
<span class="alias">Alias: <b>all</b></span>
<a href="docs/modules/every.html" class="source">source</a>
<br />
Returns <i>true</i> if all of the values in the <b>list</b> pass the
<b>predicate</b> truth test. Short-circuits and stops traversing the list
if a false element is found.
<b>predicate</b> is transformed through <a href="#iteratee"><b>iteratee</b></a>
to facilitate shorthand syntaxes.
</p>
<pre>
_.every([2, 4, 5], function(num) { return num % 2 == 0; });
=> false
</pre>
<p id="some">
<b class="header">some</b><code>_.some(list, [predicate], [context])</code>
<span class="alias">Alias: <b>any</b></span>
<a href="docs/modules/some.html" class="source">source</a>
<br />
Returns <i>true</i> if any of the values in the <b>list</b> pass the
<b>predicate</b> truth test. Short-circuits and stops traversing the list
if a true element is found.
<b>predicate</b> is transformed through <a href="#iteratee"><b>iteratee</b></a>
to facilitate shorthand syntaxes.
</p>
<pre>
_.some([null, 0, 'yes', false]);
=> true
</pre>
<p id="contains">
<b class="header">contains</b><code>_.contains(list, value, [fromIndex])</code>
<span class="alias">Aliases: <b>include</b>, <b>includes</b></span>
<a href="docs/modules/contains.html" class="source">source</a>
<br />
Returns <i>true</i> if the <b>value</b> is present in the <b>list</b>.
Uses <b>indexOf</b> internally, if <b>list</b> is an Array.
Use <b>fromIndex</b> to start your search at a given index.
</p>
<pre>
_.contains([1, 2, 3], 3);
=> true
</pre>
<p id="invoke">
<b class="header">invoke</b><code>_.invoke(list, methodName, *arguments)</code>