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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"/>
<meta name="viewport" content="target-densitydpi=device-dpi" />
<meta name="HandheldFriendly" content="true"/>
<link rel="canonical" href="http://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;
}
div#sidebar {
background: #fff;
position: fixed;
top: 0; left: 0; bottom: 0;
width: 200px;
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;
}
div.container {
width: 550px;
margin: 40px 0 50px 260px;
}
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;
}
table, tr, td {
margin: 0; padding: 0;
}
td {
padding: 2px 12px 2px 0;
}
table .rule {
height: 1px;
background: #ccc;
margin: 5px 0;
}
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;
}
@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: 90%;
height: 120px;
left: 0;
top: -7px;
padding: 10px 0 10px 30px;
border: 0;
}
img#logo {
width: auto;
height: auto;
}
div.container {
margin: 0;
width: 100%;
}
p, div.container ul {
max-width: 98%;
overflow-x: scroll;
}
pre {
overflow: scroll;
}
}
</style>
</head>
<body>
<div id="sidebar" class="interface">
<a class="toc_title" href="#">
Underscore.js <span class="version">(1.6.0)</span>
</a>
<ul class="toc_section">
<li>» <a href="https://github.com/jashkenas/underscore">GitHub Repository</a></li>
<li>» <a href="docs/underscore.html">Annotated Source</a></li>
<li>» <a href="http://documentcloud.github.io/underscore-contrib/">Underscore-contrib</a></li>
</ul>
<a class="toc_title" href="#">
Introduction
</a>
<a class="toc_title" href="#collections">
Collections
</a>
<ul class="toc_section">
<li>- <a href="#each">each</a></li>
<li>- <a href="#map">map</a></li>
<li>- <a href="#reduce">reduce</a></li>
<li>- <a href="#reduceRight">reduceRight</a></li>
<li>- <a href="#find">find</a></li>
<li>- <a href="#filter">filter</a></li>
<li>- <a href="#where">where</a></li>
<li>- <a href="#findWhere">findWhere</a></li>
<li>- <a href="#reject">reject</a></li>
<li>- <a href="#every">every</a></li>
<li>- <a href="#some">some</a></li>
<li>- <a href="#contains">contains</a></li>
<li>- <a href="#invoke">invoke</a></li>
<li>- <a href="#pluck">pluck</a></li>
<li>- <a href="#max">max</a></li>
<li>- <a href="#min">min</a></li>
<li>- <a href="#sortBy">sortBy</a></li>
<li>- <a href="#groupBy">groupBy</a></li>
<li>- <a href="#indexBy">indexBy</a></li>
<li>- <a href="#countBy">countBy</a></li>
<li>- <a href="#shuffle">shuffle</a></li>
<li>- <a href="#sample">sample</a></li>
<li>- <a href="#toArray">toArray</a></li>
<li>- <a href="#size">size</a></li>
<li>- <a href="#partition">partition</a></li>
</ul>
<a class="toc_title" href="#arrays">
Arrays
</a>
<ul class="toc_section">
<li>- <a href="#first">first</a></li>
<li>- <a href="#initial">initial</a></li>
<li>- <a href="#last">last</a></li>
<li>- <a href="#rest">rest</a></li>
<li>- <a href="#compact">compact</a></li>
<li>- <a href="#flatten">flatten</a></li>
<li>- <a href="#without">without</a></li>
<li>- <a href="#union">union</a></li>
<li>- <a href="#intersection">intersection</a></li>
<li>- <a href="#difference">difference</a></li>
<li>- <a href="#uniq">uniq</a></li>
<li>- <a href="#zip">zip</a></li>
<li>- <a href="#object">object</a></li>
<li>- <a href="#indexOf">indexOf</a></li>
<li>- <a href="#lastIndexOf">lastIndexOf</a></li>
<li>- <a href="#sortedIndex">sortedIndex</a></li>
<li>- <a href="#range">range</a></li>
</ul>
<a class="toc_title" href="#functions">
Functions
</a>
<ul class="toc_section">
<li>- <a href="#bind">bind</a></li>
<li>- <a href="#bindAll">bindAll</a></li>
<li>- <a href="#partial">partial</a></li>
<li>- <a href="#memoize">memoize</a></li>
<li>- <a href="#delay">delay</a></li>
<li>- <a href="#defer">defer</a></li>
<li>- <a href="#throttle">throttle</a></li>
<li>- <a href="#debounce">debounce</a></li>
<li>- <a href="#once">once</a></li>
<li>- <a href="#after">after</a></li>
<li>- <a href="#wrap">wrap</a></li>
<li>- <a href="#negate">negate</a></li>
<li>- <a href="#compose">compose</a></li>
</ul>
<a class="toc_title" href="#objects">
Objects
</a>
<ul class="toc_section">
<li>- <a href="#keys">keys</a></li>
<li>- <a href="#values">values</a></li>
<li>- <a href="#pairs">pairs</a></li>
<li>- <a href="#invert">invert</a></li>
<li>- <a href="#object-functions">functions</a></li>
<li>- <a href="#extend">extend</a></li>
<li>- <a href="#pick">pick</a></li>
<li>- <a href="#omit">omit</a></li>
<li>- <a href="#defaults">defaults</a></li>
<li>- <a href="#clone">clone</a></li>
<li>- <a href="#tap">tap</a></li>
<li>- <a href="#has">has</a></li>
<li>- <a href="#matches">matches</a></li>
<li>- <a href="#property">property</a></li>
<li>- <a href="#isEqual">isEqual</a></li>
<li>- <a href="#isEmpty">isEmpty</a></li>
<li>- <a href="#isElement">isElement</a></li>
<li>- <a href="#isArray">isArray</a></li>
<li>- <a href="#isObject">isObject</a></li>
<li>- <a href="#isArguments">isArguments</a></li>
<li>- <a href="#isFunction">isFunction</a></li>
<li>- <a href="#isString">isString</a></li>
<li>- <a href="#isNumber">isNumber</a></li>
<li>- <a href="#isFinite">isFinite</a></li>
<li>- <a href="#isBoolean">isBoolean</a></li>
<li>- <a href="#isDate">isDate</a></li>
<li>- <a href="#isRegExp">isRegExp</a></li>
<li>- <a href="#isNaN">isNaN</a></li>
<li>- <a href="#isNull">isNull</a></li>
<li>- <a href="#isUndefined">isUndefined</a></li>
</ul>
<a class="toc_title" href="#utility">
Utility
</a>
<ul class="toc_section">
<li>- <a href="#noConflict">noConflict</a></li>
<li>- <a href="#identity">identity</a></li>
<li>- <a href="#constant">constant</a></li>
<li>- <a href="#times">times</a></li>
<li>- <a href="#random">random</a></li>
<li>- <a href="#mixin">mixin</a></li>
<li>- <a href="#uniqueId">uniqueId</a></li>
<li>- <a href="#escape">escape</a></li>
<li>- <a href="#unescape">unescape</a></li>
<li>- <a href="#result">result</a></li>
<li>- <a href="#now">now</a></li>
<li>- <a href="#template">template</a></li>
</ul>
<a class="toc_title" href="#chaining">
Chaining
</a>
<ul class="toc_section">
<li>- <a href="#chain">chain</a></li>
<li>- <a href="#value">value</a></li>
</ul>
<a class="toc_title" href="#links">
Links
</a>
<a class="toc_title" href="#changelog">
Change Log
</a>
</div>
<div class="container">
<p id="introduction">
<img id="logo" src="docs/images/underscore.png" alt="Underscore.js" />
</p>
<p>
<a href="http://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="http://jquery.com">jQuery</a>'s tux and
<a href="http://backbonejs.org">Backbone</a>'s suspenders.
</p>
<p>
Underscore provides 80-odd functions that support both the usual
functional suspects: <b>map</b>, <b>filter</b>, <b>invoke</b> —
as well as more specialized helpers: function binding, javascript
templating, deep equality testing, and so on.
</p>
<p>
A complete <a href="test/">Test & Benchmark Suite</a>
is included for your perusal.
</p>
<p>
You may also read through the <a href="docs/underscore.html">annotated source code</a>.
</p>
<p>
Enjoying Underscore, and want to turn it up to 11? Try <a href="http://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>,
on Freenode in the <tt>#documentcloud</tt> channel,
or send tweets to <a href="https://twitter.com/documentcloud">@documentcloud</a>.
</p>
<p>
<i>Underscore is an open-source component of <a href="http://documentcloud.org/">DocumentCloud</a>.</i>
</p>
<h2>Downloads <i style="padding-left: 12px; font-size:12px;">(Right-click, and use "Save As")</i></h2>
<table>
<tr>
<td><a href="underscore.js">Development Version (1.6.0)</a></td>
<td><i>44kb, Uncompressed with Plentiful Comments</i></td>
</tr>
<tr>
<td><a href="underscore-min.js">Production Version (1.6.0)</a></td>
<td>
<i>5.0kb, Minified and Gzipped</i>
<small>(<a href="underscore-min.map">Source Map</a>)</small>
</td>
</tr>
<tr>
<td colspan="2"><div class="rule"></div></td>
</tr>
<tr>
<td><a href="https://raw.github.com/jashkenas/underscore/master/underscore.js">Edge Version</a></td>
<td><i>Unreleased, current <tt>master</tt>, use at your own risk</i></td>
</tr>
</table>
<h2>Installation</h2>
<ul>
<li>
<b>Node.js</b> <tt>npm install underscore</tt>
</li>
<li>
<b>Require.js</b> <tt>require(["underscore"], ...</tt>
</li>
<li>
<b>Bower</b> <tt>bower install underscore</tt>
</li>
<li>
<b>Component</b> <tt>component install jashkenas/underscore</tt>
</li>
</ul>
<div id="documentation">
<h2 id="collections">Collection Functions (Arrays or Objects)</h2>
<p id="each">
<b class="header">each</b><code>_.each(list, iterator, [context])</code>
<span class="alias">Alias: <b>forEach</b></span>
<br />
Iterates over a <b>list</b> of elements, yielding each in turn to an <b>iterator</b>
function. The <b>iterator</b> is bound to the <b>context</b> object, if one is
passed. Each invocation of <b>iterator</b> is called with three arguments:
<tt>(element, index, list)</tt>. If <b>list</b> is a JavaScript object, <b>iterator</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, iterator, [context])</code>
<span class="alias">Alias: <b>collect</b></span>
<br />
Produces a new array of values by mapping each value in <b>list</b>
through a transformation function (<b>iterator</b>). If <b>list</b>
is a JavaScript object, <b>iterator</b>'s arguments will be
<tt>(value, key, 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]</pre>
<p id="reduce">
<b class="header">reduce</b><code>_.reduce(list, iterator, memo, [context])</code>
<span class="alias">Aliases: <b>inject, foldl</b></span>
<br />
Also known as <b>inject</b> and <b>foldl</b>, <b>reduce</b> 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>iterator</b>. The iterator 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>
<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, iterator, memo, [context])</code>
<span class="alias">Alias: <b>foldr</b></span>
<br />
The right-associative version of <b>reduce</b>. Delegates to the
JavaScript 1.8 version of <b>reduceRight</b>, if it exists. <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>
<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.
</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>
<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>).
</p>
<pre>
var evens = _.filter([1, 2, 3, 4, 5, 6], function(num){ return num % 2 == 0; });
=> [2, 4, 6]
</pre>
<p id="where">
<b class="header">where</b><code>_.where(list, properties)</code>
<br />
Looks through each value in the <b>list</b>, returning an array of all
the values that contain all of 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="findWhere">
<b class="header">findWhere</b><code>_.findWhere(list, properties)</code>
<br />
Looks through the <b>list</b> and returns the <i>first</i> value that matches
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="reject">
<b class="header">reject</b><code>_.reject(list, predicate, [context])</code>
<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>.
</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>
<br />
Returns <i>true</i> if all of the values in the <b>list</b> pass the
<b>predicate</b> truth test.
</p>
<pre>
_.every([true, 1, null, 'yes'], _.identity);
=> false
</pre>
<p id="some">
<b class="header">some</b><code>_.some(list, [predicate], [context])</code>
<span class="alias">Alias: <b>any</b></span>
<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.
</p>
<pre>
_.some([null, 0, 'yes', false]);
=> true
</pre>
<p id="contains">
<b class="header">contains</b><code>_.contains(list, value)</code>
<span class="alias">Alias: <b>include</b></span>
<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.
</p>
<pre>
_.contains([1, 2, 3], 3);
=> true
</pre>
<p id="invoke">
<b class="header">invoke</b><code>_.invoke(list, methodName, *arguments)</code>
<br />
Calls the method named by <b>methodName</b> on each value in the <b>list</b>.
Any extra arguments passed to <b>invoke</b> will be forwarded on to the
method invocation.
</p>
<pre>
_.invoke([[5, 1, 7], [3, 2, 1]], 'sort');
=> [[1, 5, 7], [1, 2, 3]]
</pre>
<p id="pluck">
<b class="header">pluck</b><code>_.pluck(list, propertyName)</code>
<br />
A convenient version of what is perhaps the most common use-case for
<b>map</b>: extracting a list of property values.
</p>
<pre>
var stooges = [{name: 'moe', age: 40}, {name: 'larry', age: 50}, {name: 'curly', age: 60}];
_.pluck(stooges, 'name');
=> ["moe", "larry", "curly"]
</pre>
<p id="max">
<b class="header">max</b><code>_.max(list, [iterator], [context])</code>
<br />
Returns the maximum value in <b>list</b>. If an <b>iterator</b>
function is provided, it will be used on each value to generate the
criterion by which the value is ranked. <i>-Infinity</i> is returned
if <b>list</b> is empty, so an <a href="#isEmpty">isEmpty</a> guard
may be required.
</p>
<pre>
var stooges = [{name: 'moe', age: 40}, {name: 'larry', age: 50}, {name: 'curly', age: 60}];
_.max(stooges, function(stooge){ return stooge.age; });
=> {name: 'curly', age: 60};
</pre>
<p id="min">
<b class="header">min</b><code>_.min(list, [iterator], [context])</code>
<br />
Returns the minimum value in <b>list</b>. If an <b>iterator</b>
function is provided, it will be used on each value to generate the
criterion by which the value is ranked. <i>Infinity</i> is returned
if <b>list</b> is empty, so an <a href="#isEmpty">isEmpty</a> guard
may be required.
</p>
<pre>
var numbers = [10, 5, 100, 2, 1000];
_.min(numbers);
=> 2
</pre>
<p id="sortBy">
<b class="header">sortBy</b><code>_.sortBy(list, iterator, [context])</code>
<br />
Returns a (stably) sorted copy of <b>list</b>, ranked in ascending
order by the results of running each value through <b>iterator</b>.
Iterator may also be the string name of the property to sort by (eg.
<tt>length</tt>).
</p>
<pre>
_.sortBy([1, 2, 3, 4, 5, 6], function(num){ return Math.sin(num); });
=> [5, 4, 6, 3, 1, 2]
</pre>
<p id="groupBy">
<b class="header">groupBy</b><code>_.groupBy(list, iterator, [context])</code>
<br />
Splits a collection into sets, grouped by the result of running each
value through <b>iterator</b>. If <b>iterator</b> is a string instead of
a function, groups by the property named by <b>iterator</b> on each of
the values.
</p>
<pre>
_.groupBy([1.3, 2.1, 2.4], function(num){ return Math.floor(num); });
=> {1: [1.3], 2: [2.1, 2.4]}
_.groupBy(['one', 'two', 'three'], 'length');
=> {3: ["one", "two"], 5: ["three"]}
</pre>
<p id="indexBy">
<b class="header">indexBy</b><code>_.indexBy(list, iterator, [context])</code>
<br />
Given a <b>list</b>, and an <b>iterator</b> function that returns a
key for each element in the list (or a property name),
returns an object with an index of each item.
Just like <a href="#groupBy">groupBy</a>, but for when you know your
keys are unique.
</p>
<pre>
var stooges = [{name: 'moe', age: 40}, {name: 'larry', age: 50}, {name: 'curly', age: 60}];
_.indexBy(stooges, 'age');
=> {
"40": {name: 'moe', age: 40},
"50": {name: 'larry', age: 50},
"60": {name: 'curly', age: 60}
}
</pre>
<p id="countBy">
<b class="header">countBy</b><code>_.countBy(list, iterator, [context])</code>
<br />
Sorts a list into groups and returns a count for the number of objects
in each group.
Similar to <tt>groupBy</tt>, but instead of returning a list of values,
returns a count for the number of values in that group.
</p>
<pre>
_.countBy([1, 2, 3, 4, 5], function(num) {
return num % 2 == 0 ? 'even': 'odd';
});
=> {odd: 3, even: 2}
</pre>
<p id="shuffle">
<b class="header">shuffle</b><code>_.shuffle(list)</code>
<br />
Returns a shuffled copy of the <b>list</b>, using a version of the
<a href="http://en.wikipedia.org/wiki/Fisher%E2%80%93Yates_shuffle">Fisher-Yates shuffle</a>.
</p>
<pre>
_.shuffle([1, 2, 3, 4, 5, 6]);
=> [4, 1, 6, 3, 5, 2]
</pre>
<p id="sample">
<b class="header">sample</b><code>_.sample(list, [n])</code>
<br />
Produce a random sample from the <b>list</b>. Pass a number to
return <b>n</b> random elements from the list. Otherwise a single random
item will be returned.
</p>
<pre>
_.sample([1, 2, 3, 4, 5, 6]);
=> 4
_.sample([1, 2, 3, 4, 5, 6], 3);
=> [1, 6, 2]
</pre>
<p id="toArray">
<b class="header">toArray</b><code>_.toArray(list)</code>
<br />
Creates a real Array from the <b>list</b> (anything that can be
iterated over). Useful for transmuting the <b>arguments</b> object.
</p>
<pre>
(function(){ return _.toArray(arguments).slice(1); })(1, 2, 3, 4);
=> [2, 3, 4]
</pre>
<p id="size">
<b class="header">size</b><code>_.size(list)</code>
<br />
Return the number of values in the <b>list</b>.
</p>
<pre>
_.size({one: 1, two: 2, three: 3});
=> 3
</pre>
<p id="partition">
<b class="header">partition</b><code>_.partition(array, predicate)</code>
<br />
Split <b>array</b> into two arrays: one whose elements all satisfy
<b>predicate</b> and one whose elements all do not satisfy <b>predicate</b>.
</p>
<pre>
_.partition([0, 1, 2, 3, 4, 5], isOdd);
=> [[1, 3, 5], [0, 2, 4]]
</pre>
<h2 id="arrays">Array Functions</h2>
<p>
<i>
Note: All array functions will also work on the <b>arguments</b> object.
However, Underscore functions are not designed to work on "sparse" arrays.
</i>
</p>
<p id="first">
<b class="header">first</b><code>_.first(array, [n])</code>
<span class="alias">Alias: <b>head</b>, <b>take</b></span>
<br />
Returns the first element of an <b>array</b>. Passing <b>n</b> will
return the first <b>n</b> elements of the array.
</p>
<pre>
_.first([5, 4, 3, 2, 1]);
=> 5
</pre>
<p id="initial">
<b class="header">initial</b><code>_.initial(array, [n])</code>
<br />
Returns everything but the last entry of the array. Especially useful on
the arguments object. Pass <b>n</b> to exclude the last <b>n</b> elements
from the result.
</p>
<pre>
_.initial([5, 4, 3, 2, 1]);
=> [5, 4, 3, 2]
</pre>
<p id="last">
<b class="header">last</b><code>_.last(array, [n])</code>
<br />
Returns the last element of an <b>array</b>. Passing <b>n</b> will return
the last <b>n</b> elements of the array.
</p>
<pre>
_.last([5, 4, 3, 2, 1]);
=> 1
</pre>
<p id="rest">
<b class="header">rest</b><code>_.rest(array, [index])</code>
<span class="alias">Alias: <b>tail, drop</b></span>
<br />
Returns the <b>rest</b> of the elements in an array. Pass an <b>index</b>
to return the values of the array from that index onward.
</p>
<pre>
_.rest([5, 4, 3, 2, 1]);
=> [4, 3, 2, 1]
</pre>
<p id="compact">
<b class="header">compact</b><code>_.compact(array)</code>
<br />
Returns a copy of the <b>array</b> with all falsy values removed.
In JavaScript, <i>false</i>, <i>null</i>, <i>0</i>, <i>""</i>,
<i>undefined</i> and <i>NaN</i> are all falsy.
</p>
<pre>
_.compact([0, 1, false, 2, '', 3]);
=> [1, 2, 3]
</pre>
<p id="flatten">
<b class="header">flatten</b><code>_.flatten(array, [shallow])</code>
<br />
Flattens a nested <b>array</b> (the nesting can be to any depth). If you
pass <b>shallow</b>, the array will only be flattened a single level.
</p>
<pre>
_.flatten([1, [2], [3, [[4]]]]);
=> [1, 2, 3, 4];
_.flatten([1, [2], [3, [[4]]]], true);
=> [1, 2, 3, [[4]]];
</pre>
<p id="without">
<b class="header">without</b><code>_.without(array, *values)</code>
<br />
Returns a copy of the <b>array</b> with all instances of the <b>values</b>
removed.
</p>
<pre>
_.without([1, 2, 1, 0, 3, 1, 4], 0, 1);
=> [2, 3, 4]
</pre>
<p id="union">
<b class="header">union</b><code>_.union(*arrays)</code>
<br />
Computes the union of the passed-in <b>arrays</b>: the list of unique items,
in order, that are present in one or more of the <b>arrays</b>.
</p>
<pre>
_.union([1, 2, 3], [101, 2, 1, 10], [2, 1]);
=> [1, 2, 3, 101, 10]
</pre>
<p id="intersection">
<b class="header">intersection</b><code>_.intersection(*arrays)</code>
<br />
Computes the list of values that are the intersection of all the <b>arrays</b>.
Each value in the result is present in each of the <b>arrays</b>.
</p>
<pre>
_.intersection([1, 2, 3], [101, 2, 1, 10], [2, 1]);
=> [1, 2]
</pre>
<p id="difference">
<b class="header">difference</b><code>_.difference(array, *others)</code>
<br />
Similar to <b>without</b>, but returns the values from <b>array</b> that
are not present in the <b>other</b> arrays.
</p>
<pre>
_.difference([1, 2, 3, 4, 5], [5, 2, 10]);
=> [1, 3, 4]
</pre>
<p id="uniq">
<b class="header">uniq</b><code>_.uniq(array, [isSorted], [iterator])</code>
<span class="alias">Alias: <b>unique</b></span>
<br />
Produces a duplicate-free version of the <b>array</b>, using <i>===</i> to test
object equality. If you know in advance that the <b>array</b> is sorted,
passing <i>true</i> for <b>isSorted</b> will run a much faster algorithm.
If you want to compute unique items based on a transformation, pass an
<b>iterator</b> function.
</p>
<pre>
_.uniq([1, 2, 1, 3, 1, 4]);
=> [1, 2, 3, 4]
</pre>
<p id="zip">
<b class="header">zip</b><code>_.zip(*arrays)</code>
<br />
Merges together the values of each of the <b>arrays</b> with the
values at the corresponding position. Useful when you have separate
data sources that are coordinated through matching array indexes.
If you're working with a matrix of nested arrays, <tt>_.zip.apply</tt>
can transpose the matrix in a similar fashion.
</p>
<pre>
_.zip(['moe', 'larry', 'curly'], [30, 40, 50], [true, false, false]);
=> [["moe", 30, true], ["larry", 40, false], ["curly", 50, false]]
_.zip.apply(_, arrayOfRowsOfData);
=> arrayOfColumnsOfData
</pre>
<p id="object">
<b class="header">object</b><code>_.object(list, [values])</code>
<br />
Converts arrays into objects. Pass either a single list of
<tt>[key, value]</tt> pairs, or a list of keys, and a list of values.
If duplicate keys exist, the last value wins.
</p>
<pre>
_.object(['moe', 'larry', 'curly'], [30, 40, 50]);
=> {moe: 30, larry: 40, curly: 50}
_.object([['moe', 30], ['larry', 40], ['curly', 50]]);
=> {moe: 30, larry: 40, curly: 50}
</pre>
<p id="indexOf">
<b class="header">indexOf</b><code>_.indexOf(array, value, [isSorted])</code>
<br />
Returns the index at which <b>value</b> can be found in the <b>array</b>,
or <i>-1</i> if value is not present in the <b>array</b>. If you're working with a
large array, and you know that the array is already sorted, pass <tt>true</tt>
for <b>isSorted</b> to use a faster binary search ... or, pass a number as
the third argument in order to look for the first matching value in the
array after the given index.
</p>
<pre>
_.indexOf([1, 2, 3], 2);
=> 1
</pre>
<p id="lastIndexOf">
<b class="header">lastIndexOf</b><code>_.lastIndexOf(array, value, [fromIndex])</code>
<br />
Returns the index of the last occurrence of <b>value</b> in the <b>array</b>,
or <i>-1</i> if value is not present. Pass <b>fromIndex</b> to start your search at a
given index.
</p>
<pre>
_.lastIndexOf([1, 2, 3, 1, 2, 3], 2);
=> 4
</pre>
<p id="sortedIndex">
<b class="header">sortedIndex</b><code>_.sortedIndex(list, value, [iterator], [context])</code>
<br />
Uses a binary search to determine the index at which the <b>value</b>
<i>should</i> be inserted into the <b>list</b> in order to maintain the <b>list</b>'s
sorted order. If an <b>iterator</b> function is provided, it will be used to compute
the sort ranking of each value, including the <b>value</b> you pass.
Iterator may also be the string name of the property to sort by (eg. <tt>length</tt>).
</p>
<pre>
_.sortedIndex([10, 20, 30, 40, 50], 35);
=> 3
var stooges = [{name: 'moe', age: 40}, {name: 'curly', age: 60}];