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<h1>Underscore.js</h1>
<p>
<a href="http://github.com/documentcloud/underscore/">Underscore</a> is a
utility-belt library for JavaScript that provides a lot of the
functional programming support that you would expect in
<a href="http://prototypejs.org/api">Prototype.js</a>
(or <a href="http://www.ruby-doc.org/core/classes/Enumerable.html">Ruby</a>),
but without extending any of the built-in JavaScript objects. It's the
tie to go along with <a href="http://docs.jquery.com">jQuery</a>'s tux.
</p>
<p>
Underscore provides 60-odd functions that support both the usual
functional suspects: <b>map</b>, <b>select</b>, <b>invoke</b> &mdash;
as well as more specialized helpers: function binding, javascript
templating, deep equality testing, and so on. It delegates to built-in
functions, if present, so
<a href="https://developer.mozilla.org/en/New_in_JavaScript_1.6">JavaScript 1.6</a>
compliant browsers will use the
native implementations of <b>forEach</b>, <b>map</b>, <b>filter</b>,
<b>every</b>, <b>some</b> and <b>indexOf</b>.
</p>
<p>
A complete <a href="test/test.html">Test &amp; Benchmark Suite</a>
is included for your perusal.
</p>
<p>
The unabridged source code is
<a href="http://github.com/documentcloud/underscore/">available on GitHub</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>
<p>
<table>
<tr>
<td><a href="underscore.js">Development Version (0.5.0)</a></td>
<td><i>20kb, Uncompressed with Comments</i></td>
</tr>
<tr>
<td><a href="underscore-min.js">Production Version (0.5.0)</a></td>
<td><i>2kb, Packed and Gzipped</i></td>
</tr>
</table>
</p>
<h2 id="styles">Object-Oriented and Functional Styles</h2>
<p>
You can use Underscore in either an object-oriented or a functional style,
depending on your preference. The following two lines of code are
identical ways to double a list of numbers.
</p>
<pre>
_.map([1, 2, 3], function(n){ return n * 2; });
_([1, 2, 3]).map(function(n){ return n * 2; });</pre>
<p>
Using the object-oriented style allows you to chain together methods. Calling
<tt>chain</tt> on a wrapped object will cause all future method calls to
return wrapped objects as well. When you've finished the computation,
use <tt>value</tt> to retrieve the final value. Here's an example of chaining
together a <b>map/flatten/reduce</b>, in order to get the word count of
every word in a song.
</p>
<pre>
var lyrics = [
{line : 1, words : "I'm a lumberjack and I'm okay"},
{line : 2, words : "I sleep all night and I work all day"},
{line : 3, words : "He's a lumberjack and he's okay"},
{line : 4, words : "He sleeps all night and he works all day"}
];
_(lyrics).chain()
.map(function(line) { return line.words.split(' '); })
.flatten()
.reduce({}, function(counts, word) {
counts[word] = (counts[word] || 0) + 1;
return counts;
}).value();
=&gt; {lumberjack : 2, all : 4, night : 2 ... }</pre>
<p>
In addition, the
<a href="https://developer.mozilla.org/En/Core_JavaScript_1.5_Reference/Objects/Array">Array prototype's methods</a>
are proxied through the chained Underscore object, so you can slip a
<tt>reverse</tt> or a <tt>push</tt> into your chain, and continue to
modify the array.
</p>
<h2>Table of Contents</h2>
<p>
<b>Collections</b>
<br />
<span class="methods"><a href="#each">each</a>, <a href="#map">map</a>,
<a href="#reduce">reduce</a>, <a href="#reduceRight">reduceRight</a>,
<a href="#detect">detect</a>, <a href="#select">select</a>,
<a href="#reject">reject</a>, <a href="#all">all</a>,
<a href="#any">any</a>, <a href="#include">include</a>,
<a href="#invoke">invoke</a>, <a href="#pluck">pluck</a>,
<a href="#max">max</a>, <a href="#min">min</a>,
<a href="#sortBy">sortBy</a>, <a href="#sortedIndex">sortedIndex</a>,
<a href="#toArray">toArray</a>, <a href="#size">size</a></span>
</p>
<p>
<b>Arrays</b>
<br />
<span class="methods"><a href="#first">first</a>, <a href="#rest">rest</a>, <a href="#last">last</a>,
<a href="#compact">compact</a>, <a href="#flatten">flatten</a>, <a href="#without">without</a>, <a href="#uniq">uniq</a>,
<a href="#intersect">intersect</a>, <a href="#zip">zip</a>, <a href="#indexOf">indexOf</a></span>,
<a href="#lastIndexOf">lastIndexOf</a>, <a href="#range">range</a></span>
</p>
<p>
<b>Functions</b>
<br />
<span class="methods"><a href="#bind">bind</a>, <a href="#bindAll">bindAll</a>, <a href="#delay">delay</a>,
<a href="#defer">defer</a>, <a href="#wrap">wrap</a></span>, <a href="#compose">compose</a></span>
</p>
<p>
<b>Objects</b>
<br />
<span class="methods"><a href="#keys">keys</a>, <a href="#values">values</a>,
<a href="#functions">functions</a>, <a href="#extend">extend</a>, <a href="#clone">clone</a>,
<a href="#isEqual">isEqual</a>, <a href="#isEmpty">isEmpty</a>, <a href="#isElement">isElement</a>,
<a href="#isArray">isArray</a>, <a href="#isFunction">isFunction</a>, <a href="#isString">isString</a>,
<a href="#isNumber">isNumber</a>, <a href="#isDate">isDate</a>, <a href="#isRegExp">isRegExp</a>
<a href="#isNaN">isNaN</a>, <a href="#isNull">isNull</a>,
<a href="#isUndefined">isUndefined</a>
</span>
</p>
<p>
<b>Utility</b>
<br />
<span class="methods"><a href="#noConflict">noConflict</a>,
<a href="#identity">identity</a>, <a href="#breakLoop">breakLoop</a></span>,
<a href="#uniqueId">uniqueId</a>, <a href="#template">template</a></span>
</p>
<p>
<b>Chaining</b>
<br />
<span class="methods"><a href="#chain">chain</a>, <a href="#value">value</a>
</p>
<div id="documentation">
<h2>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>. Use <a href="#breakLoop"><tt>breakLoop</tt></a>
to break out of the iteration. Delegates to the native
<b>forEach</b> function if it exists.
</p>
<pre>
_.each([1, 2, 3], function(num){ alert(num); });
=&gt; alerts each number in turn...</pre>
<p id="map">
<b class="header">map</b><code>_.map(list, iterator, [context])</code>
<br />
Produces a new array of values by mapping each value in <b>list</b>
through a transformation function (<b>iterator</b>). If the native
<b>map</b> method exists, it will be used instead.
</p>
<pre>
_.map([1, 2, 3], function(num){ return num * 3 });
=&gt; [3, 6, 9]</pre>
<p id="reduce">
<b class="header">reduce</b><code>_.reduce(list, memo, iterator, [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>.
</p>
<pre>
var sum = _.reduce([1, 2, 3], 0, function(memo, num){ return memo + num });
=&gt; 6
</pre>
<p id="reduceRight">
<b class="header">reduceRight</b><code>_.reduceRight(list, memo, iterator, [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); });
=&gt; [4, 5, 2, 3, 0, 1]
</pre>
<p id="detect">
<b class="header">detect</b><code>_.detect(list, iterator, [context])</code>
<br />
Looks through each value in the <b>list</b>, returning the first one that
passes a truth test (<b>iterator</b>). The function returns as
soon as it finds an acceptable element, and doesn't traverse the
entire list.
</p>
<pre>
var even = _.detect([1, 2, 3, 4, 5, 6], function(num){ return num % 2 == 0; });
=&gt; 2
</pre>
<p id="select">
<b class="header">select</b><code>_.select(list, iterator, [context])</code>
<span class="alias">Alias: <b>filter</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>iterator</b>). Delegates to the
native <b>filter</b> method, if it exists.
</p>
<pre>
var evens = _.select([1, 2, 3, 4, 5, 6], function(num){ return num % 2 == 0; });
=&gt; [2, 4, 6]
</pre>
<p id="reject">
<b class="header">reject</b><code>_.reject(list, iterator, [context])</code>
<br />
Returns the values in <b>list</b> without the elements that the truth
test (<b>iterator</b>) passes. The opposite of <b>select</b>.
</p>
<pre>
var odds = _.reject([1, 2, 3, 4, 5, 6], function(num){ return num % 2 == 0; });
=&gt; [1, 3, 5]
</pre>
<p id="all">
<b class="header">all</b><code>_.all(list, [iterator], [context])</code>
<span class="alias">Alias: <b>every</b></span>
<br />
Returns <i>true</i> if all of the values in the <b>list</b> pass the <b>iterator</b>
truth test. If an <b>iterator</b> is not provided, the truthy value of
the element will be used instead. Delegates to the native method <b>every</b>, if
present.
</p>
<pre>
_.all([true, 1, null, 'yes']);
=&gt; false
</pre>
<p id="any">
<b class="header">any</b><code>_.any(list, [iterator], [context])</code>
<span class="alias">Alias: <b>some</b></span>
<br />
Returns <i>true</i> if any of the values in the <b>list</b> pass the
<b>iterator</b> truth test. Short-circuits and stops traversing the list
if a true element is found. Delegates to the native method <b>some</b>,
if present.
</p>
<pre>
_.any([null, 0, 'yes', false]);
=&gt; true
</pre>
<p id="include">
<b class="header">include</b><code>_.include(list, value)</code>
<br />
Returns <i>true</i> if the <b>value</b> is present in the <b>list</b>, using
<i>===</i> to test equality. Uses <b>indexOf</b> internally, if <b>list</b>
is an Array.
</p>
<pre>
_.include([1, 2, 3], 3);
=&gt; 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');
=&gt; [[1, 5, 7], [1, 2, 3]]
</pre>
<p id="pluck">
<b class="header">pluck</b><code>_.pluck(list, propertyName)</code>
<br />
An 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');
=&gt; ["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 <b>iterator</b> is passed,
it will be used on each value to generate the criterion by which the
value is ranked.
</p>
<pre>
var stooges = [{name : 'moe', age : 40}, {name : 'larry', age : 50}, {name : 'curly', age : 60}];
_.max(stooges, function(stooge){ return stooge.age; });
=&gt; {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 <b>iterator</b> is passed,
it will be used on each value to generate the criterion by which the
value is ranked.
</p>
<pre>
var numbers = [10, 5, 100, 2, 1000];
_.min(numbers);
=&gt; 2
</pre>
<p id="sortBy">
<b class="header">sortBy</b><code>_.sortBy(list, iterator, [context])</code>
<br />
Returns a sorted <b>list</b>, ranked by the results of running each
value through <b>iterator</b>.
</p>
<pre>
_.sortBy([1, 2, 3, 4, 5, 6], function(num){ return Math.sin(num); });
=&gt; [5, 4, 6, 3, 1, 2]
</pre>
<p id="sortedIndex">
<b class="header">sortedIndex</b><code>_.sortedIndex(list, value, [iterator])</code>
<br />
Uses a binary search to determine the index at which the <b>value</b>
should be inserted into the <b>list</b> in order to maintain the <b>list</b>'s
sorted order. If an <b>iterator</b> is passed, it will be used to compute
the sort ranking of each value.
</p>
<pre>
_.sortedIndex([10, 20, 30, 40, 50], 35);
=&gt; 3
</pre>
<p id="toArray">
<b class="header">toArray</b><code>_.toArray(list)</code>
<br />
Converts the <b>list</b> (anything that can be iterated over), into a
real Array. Useful for transmuting the <b>arguments</b> object.
</p>
<pre>
(function(){ return _.toArray(arguments).slice(0); })(1, 2, 3);
=&gt; [1, 2, 3]
</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});
=&gt; 3
</pre>
<h2>Array Functions</h2>
<p>
<i>Note: All array functions will also work on the <b>arguments</b> object.</i>
</p>
<p id="first">
<b class="header">first</b><code>_.first(array, [n])</code>
<span class="alias">Alias: <b>head</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]);
=&gt; 5
</pre>
<p id="rest">
<b class="header">rest</b><code>_.rest(array, [index])</code>
<span class="alias">Alias: <b>tail</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]);
=&gt; [4, 3, 2, 1]
</pre>
<p id="last">
<b class="header">last</b><code>_.last(array)</code>
<br />
Returns the last element of an <b>array</b>.
</p>
<pre>
_.last([5, 4, 3, 2, 1]);
=&gt; 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]);
=&gt; [1, 2, 3]
</pre>
<p id="flatten">
<b class="header">flatten</b><code>_.flatten(array)</code>
<br />
Flattens a nested <b>array</b> (the nesting can be to any depth).
</p>
<pre>
_.flatten([1, [2], [3, [[[4]]]]]);
=&gt; [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. <i>===</i> is used for the equality test.
</p>
<pre>
_.without([1, 2, 1, 0, 3, 1, 4], 0, 1);
=&gt; [2, 3, 4]
</pre>
<p id="uniq">
<b class="header">uniq</b><code>_.uniq(array, [isSorted])</code>
<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.
</p>
<pre>
_.uniq([1, 2, 1, 3, 1, 4]);
=&gt; [1, 2, 3, 4]
</pre>
<p id="intersect">
<b class="header">intersect</b><code>_.intersect(*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>
_.intersect([1, 2, 3], [101, 2, 1, 10], [2, 1]);
=&gt; [1, 2]
</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.
</p>
<pre>
_.zip(['moe', 'larry', 'curly'], [30, 40, 50], [true, false, false]);
=&gt; [["moe", 30, true], ["larry", 40, false], ["curly", 50, false]]
</pre>
<p id="indexOf">
<b class="header">indexOf</b><code>_.indexOf(array, value)</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>. Uses the native
<b>indexOf</b> function unless it's missing.
</p>
<pre>
_.indexOf([1, 2, 3], 2);
=&gt; 1
</pre>
<p id="lastIndexOf">
<b class="header">lastIndexOf</b><code>_.lastIndexOf(array, value)</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. Uses the native <b>lastIndexOf</b>
function if possible.
</p>
<pre>
_.lastIndexOf([1, 2, 3, 1, 2, 3], 2);
=&gt; 4
</pre>
<p id="range">
<b class="header">range</b><code>_.range([start], stop, [step])</code>
<br />
A function to create flexibly-numbered lists of integers, handy for
<tt>each</tt> and <tt>map</tt> loops. <b>start</b>, if omitted, defaults
to <i>0</i>; <b>step</b> defaults to <i>1</i>. Returns a list of integers
from <b>start</b> to <b>stop</b>, incremented (or decremented) by <b>step</b>,
exclusive.
</p>
<pre>
_.range(10);
=&gt; [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
_.range(1, 11);
=&gt; [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
_.range(0, 30, 5);
=&gt; [0, 5, 10, 15, 20, 25]
_.range(0, -10, -1);
=&gt; [0, -1, -2, -3, -4, -5, -6, -7, -8, -9]
_.range(0);
=&gt; []
</pre>
<h2>Function (uh, ahem) Functions</h2>
<p id="bind">
<b class="header">bind</b><code>_.bind(function, object, [*arguments])</code>
<br />
Bind a <b>function</b> to an <b>object</b>, meaning that whenever
the function is called, the value of <i>this</i> will be the <b>object</b>.
Optionally, bind <b>arguments</b> to the <b>function</b> to pre-fill them,
also known as <b>currying</b>.
</p>
<pre>
var func = function(greeting){ return greeting + ': ' + this.name };
func = _.bind(func, {name : 'moe'}, 'hi');
func();
=&gt; 'hi: moe'
</pre>
<p id="bindAll">
<b class="header">bindAll</b><code>_.bindAll(object, [*methodNames])</code>
<br />
Binds a number of methods on the <b>object</b>, specified by
<b>methodNames</b>, to be run in the context of that object whenever they
are invoked. Very handy for binding functions that are going to be used
as event handlers, which would otherwise be invoked with a fairly useless
<i>this</i>. If no <b>methodNames</b> are provided, all of the object's
function properties will be bound to it.
</p>
<pre>
var buttonView = {
label : 'underscore',
onClick : function(){ alert('clicked: ' + this.label); },
onHover : function(){ console.log('hovering: ' + this.label); }
};
_.bindAll(buttonView);
jQuery('#underscore_button').bind('click', buttonView.onClick);
=&gt; When the button is clicked, this.label will have the correct value...
</pre>
<p id="delay">
<b class="header">delay</b><code>_.delay(function, wait, [*arguments])</code>
<br />
Much like <b>setTimeout</b>, invokes <b>function</b> after <b>wait</b>
milliseconds. If you pass the optional <b>arguments</b>, they will be
forwarded on to the <b>function</b> when it is invoked.
</p>
<pre>
var log = _.bind(console.log, console);
_.delay(log, 1000, 'logged later');
=&gt; 'logged later' // Appears after one second.
</pre>
<p id="defer">
<b class="header">defer</b><code>_.defer(function)</code>
<br />
Defers invoking the <b>function</b> until the current call stack has cleared,
similar to using <b>setTimeout</b> with a delay of 0. Useful for performing
expensive computations or HTML rendering in chunks without blocking the UI thread
from updating.
</p>
<pre>
_.defer(function(){ alert('deferred'); });
// Returns from the function before the alert runs.
</pre>
<p id="wrap">
<b class="header">wrap</b><code>_.wrap(function, wrapper)</code>
<br />
Wraps the first <b>function</b> inside of the <b>wrapper</b> function,
passing it as the first argument. This allows the <b>wrapper</b> to
execute code before and after the <b>function</b> runs, adjust the arguments,
and execute it conditionally.
</p>
<pre>
var hello = function(name) { return "hello: " + name; };
hello = _.wrap(hello, function(func) {
return "before, " + func("moe") + ", after";
});
hello();
=&gt; 'before, hello: moe, after'
</pre>
<p id="compose">
<b class="header">compose</b><code>_.compose(*functions)</code>
<br />
Returns the composition of a list of <b>functions</b>, where each function
consumes the return value of the function that follows. In math terms,
composing the functions <i>f()</i>, <i>g()</i>, and <i>h()</i> produces
<i>f(g(h()))</i>.
</p>
<pre>
var greet = function(name){ return "hi: " + name; };
var exclaim = function(statement){ return statement + "!"; };
var welcome = _.compose(greet, exclaim);
welcome('moe');
=&gt; 'hi: moe!'
</pre>
<h2>Object Functions</h2>
<p id="keys">
<b class="header">keys</b><code>_.keys(object)</code>
<br />
Retrieve all the names of the <b>object</b>'s properties.
</p>
<pre>
_.keys({one : 1, two : 2, three : 3});
=&gt; ["one", "two", "three"]
</pre>
<p id="values">
<b class="header">values</b><code>_.values(object)</code>
<br />
Return all of the values of the <b>object</b>'s properties.
</p>
<pre>
_.values({one : 1, two : 2, three : 3});
=&gt; [1, 2, 3]
</pre>
<p id="functions">
<b class="header">functions</b><code>_.functions(object)</code>
<span class="alias">Alias: <b>methods</b></span>
<br />
Returns a sorted list of the names of every method in an object &mdash;
that is to say, the name of every function property of the object.
</p>
<pre>
_.functions(_);
=&gt; ["all", "any", "bind", "bindAll", "clone", "compact", "compose" ...
</pre>
<p id="extend">
<b class="header">extend</b><code>_.extend(destination, source)</code>
<br />
Copy all of the properties in the <b>source</b> object over to the
<b>destination</b> object.
</p>
<pre>
_.extend({name : 'moe'}, {age : 50});
=&gt; {name : 'moe', age : 50}
</pre>
<p id="clone">
<b class="header">clone</b><code>_.clone(object)</code>
<br />
Create a shallow-copied clone of the <b>object</b>. Any nested objects
or arrays will be copied by reference, not duplicated.
</p>
<pre>
_.clone({name : 'moe'});
=&gt; {name : 'moe'};
</pre>
<p id="isEqual">
<b class="header">isEqual</b><code>_.isEqual(object, other)</code>
<br />
Performs an optimized deep comparison between the two objects, to determine
if they should be considered equal.
</p>
<pre>
var moe = {name : 'moe', luckyNumbers : [13, 27, 34]};
var clone = {name : 'moe', luckyNumbers : [13, 27, 34]};
moe == clone;
=&gt; false
_.isEqual(moe, clone);
=&gt; true
</pre>
<p id="isEmpty">
<b class="header">isEmpty</b><code>_.isEmpty(object)</code>
<br />
Returns <i>true</i> if <b>object</b> contains no values.
</p>
<pre>
_.isEmpty([1, 2, 3]);
=&gt; false
_.isEmpty({});
=&gt; true
</pre>
<p id="isElement">
<b class="header">isElement</b><code>_.isElement(object)</code>
<br />
Returns <i>true</i> if <b>object</b> is a DOM element.
</p>
<pre>
_.isElement(jQuery('body')[0]);
=&gt; true
</pre>
<p id="isArray">
<b class="header">isArray</b><code>_.isArray(object)</code>
<br />
Returns <i>true</i> if <b>object</b> is an Array.
</p>
<pre>
(function(){ return _.isArray(arguments); })();
=&gt; false
_.isArray([1,2,3]);
=&gt; true
</pre>
<p id="isFunction">
<b class="header">isFunction</b><code>_.isFunction(object)</code>
<br />
Returns <i>true</i> if <b>object</b> is a Function.
</p>
<pre>
_.isFunction(alert);
=&gt; true
</pre>
<p id="isString">
<b class="header">isString</b><code>_.isString(object)</code>
<br />
Returns <i>true</i> if <b>object</b> is a String.
</p>
<pre>
_.isFunction("moe");
=&gt; true
</pre>
<p id="isNumber">
<b class="header">isNumber</b><code>_.isNumber(object)</code>
<br />
Returns <i>true</i> if <b>object</b> is a Number.
</p>
<pre>
_.isNumber(8.4 * 5);
=&gt; true
</pre>
<p id="isDate">
<b class="header">isDate</b><code>_.isDate(object)</code>
<br />
Returns <i>true</i> if <b>object</b> is a Date.
</p>
<pre>
_.isDate(new Date());
=&gt; true
</pre>
<p id="isRegExp">
<b class="header">isRegExp</b><code>_.isRegExp(object)</code>
<br />
Returns <i>true</i> if <b>object</b> is a RegExp.
</p>
<pre>
_.isRegExp(/moe/);
=&gt; true
</pre>
<p id="isNaN">
<b class="header">isNaN</b><code>_.isNaN(object)</code>
<br />
Returns <i>true</i> if <b>object</b> is <i>NaN</i>.<br /> Note: this is not
the same as the native <b>isNaN</b> function, which will also return
true if the variable is <i>undefined</i>.
</p>
<pre>
_.isNaN(NaN);
=&gt; true
isNaN(undefined);
=&gt; true
_.isNaN(undefined);
=&gt; false
</pre>
<p id="isNull">
<b class="header">isNull</b><code>_.isNull(object)</code>
<br />
Returns <i>true</i> if the value of <b>object</b> is <i>null</i>.
</p>
<pre>
_.isNull(null);
=&gt; true
_.isNull(undefined);
=&gt; false
</pre>
<p id="isUndefined">
<b class="header">isUndefined</b><code>_.isUndefined(variable)</code>
<br />
Returns <i>true</i> if <b>variable</b> is <i>undefined</i>.
</p>
<pre>
_.isUndefined(window.missingVariable);
=&gt; true
</pre>
<h2>Utility Functions</h2>
<p id="noConflict">
<b class="header">noConflict</b><code>_.noConflict()</code>
<br />
Give control of the "_" variable back to its previous owner. Returns
a reference to the <b>Underscore</b> object.
</p>
<pre>
var underscore = _.noConflict();</pre>
<p id="identity">
<b class="header">identity</b><code>_.identity(value)</code>
<br />
Returns the same value that is used as the argument. In math:
<tt>f(x) = x</tt><br />
This function looks useless, but is used throughout Underscore as
a default iterator.
</p>
<pre>
var moe = {name : 'moe'};
moe === _.identity(moe);
=&gt; true</pre>
<p id="breakLoop">
<b class="header">breakLoop</b><code>_.breakLoop()</code>
<br />
Breaks out of the current loop iteration. Similar to the <tt>break</tt>
keyword in regular "for" loop, but works within an iterator function.
Uses the native <tt>StopIteration</tt> object in JavaScript 1.7 compliant
browsers.
</p>
<pre>
var result = null;
_.each([1, 2, 3], function(num) {
if ((result = num) == 2) _.breakLoop();
});
result;
=&gt; 2</pre>
<p id="uniqueId">
<b class="header">uniqueId</b><code>_.uniqueId([prefix])</code>
<br />
Generate a globally-unique id for client-side models or DOM elements
that need one. If <b>prefix</b> is passed, the id will be appended to it.
</p>
<pre>
_.uniqueId('contact_');
=&gt; 'contact_104'
</pre>
<p id="template">
<b class="header">template</b><code>_.template(templateString, [context])</code>
<br />
Compiles JavaScript templates into functions that can be evaluated
for rendering. Useful for rendering complicated bits of HTML from JSON
data sources. Template functions can both interpolate variables, using<br />
<i>&lt;%= &hellip; %&gt;</i>, as well as execute arbitrary JavaScript code, with
<i>&lt;% &hellip; %&gt;</i>. When you evaluate a template function, pass in a
<b>context</b> object that has properties corresponding to the template's free
variables. If you're writing a one-off, you can pass the <b>context</b>
object as the second parameter to <b>template</b> in order to render
immediately instead of returning a template function.
</p>
<pre>
var compiled = _.template("hello: &lt;%= name %&gt;");
compiled({name : 'moe'});
=&gt; "hello: moe"
var list = "&lt;% _.each(people, function(name) { %&gt; &lt;li&gt;&lt;%= name %&gt;&lt;/li&gt; &lt;% }); %&gt;";
_.template(list, {people : ['moe', 'curly', 'larry']});
=&gt; "&lt;li&gt;moe&lt;/li&gt;&lt;li&gt;curly&lt;/li&gt;&lt;li&gt;larry&lt;/li&gt;"
</pre>
<h2>Chaining</h2>
<p id="chain">
<b class="header">chain</b><code>_(obj).chain()</code>
<br />
Returns a wrapped object. Calling methods on this object will continue
to return wrapped objects until <tt>value</tt> is used. (
<a href="#styles">A more realistic example.</a>)
</p>
<pre>
var stooges = [{name : 'curly', age : 25}, {name : 'moe', age : 21}, {name : 'larry', age : 23}];
var youngest = _(stooges).chain()
.sortBy(function(stooge){ return stooge.age; })
.map(function(stooge){ return stooge.name + ' is ' + stooge.age; })
.first()
.value();
=&gt; "moe is 21"
</pre>
<p id="value">
<b class="header">value</b><code>_(obj).value()</code>
<br />
Extracts the value of a wrapped object.
</p>
<pre>
_([1, 2, 3]).value();
=&gt; [1, 2, 3]
</pre>
<h2>Links &amp; Suggested Reading</h2>
<p>
<a href="http://mirven.github.com/underscore.lua/">Underscore.lua</a>,
a Lua port of the functions that are applicable in both languages.
Includes OOP-wrapping and chaining.
The <a href="http://github.com/mirven/underscore.lua">source</a> is
available on GitHub.
</p>
<p>
Ruby's <a href="http://ruby-doc.org/core/classes/Enumerable.html">Enumerable</a> module.
</p>
<p>
<a href="http://www.prototypejs.org/">Prototype.js</a>, which provides
JavaScript with collection functions in the manner closest to Ruby's Enumerable.
</p>
<p>
Oliver Steele's
<a href="http://osteele.com/sources/javascript/functional/">Functional JavaScript</a>,
which includes comprehensive higher-order function support as well as string lambdas.
</p>
<p>
Python's <a href="http://docs.python.org/library/itertools.html">itertools</a>.
</p>
<h2>Change Log</h2>
<p>
<b class="header">0.5.0</b><br />
<b>[API Changes]</b> <tt>_.bindAll</tt> now takes the context object as
its first parameter. If no method names are passed, all of the context
object's methods are bound to it, enabling chaining and easier binding.
<tt>_.functions</tt> now takes a single argument and returns the names
of its Function properties. Calling <tt>_.functions(_)</tt> will get you
the previous behavior.
Added <tt>_.isRegExp</tt> so that <tt>isEqual</tt> can now test for RegExp equality.
All of the "is" functions have been shrunk down into a single definition.
<a href="http://github.com/grayrest/">Karl Guertin</a> contributed patches.
</p>
<p>
<b class="header">0.4.7</b><br />
Added <tt>isDate</tt>, <tt>isNaN</tt>, and <tt>isNull</tt>, for completeness.
Optimizations for <tt>isEqual</tt> when checking equality between Arrays
or Dates. <tt>_.keys</tt> is now <small><i><b>25%&ndash;2X</b></i></small> faster (depending on your
browser) which speeds up the functions that rely on it, such as <tt>_.each</tt>.
</p>
<p>
<b class="header">0.4.6</b><br />
Added the <tt>range</tt> function, a port of the
<a href="http://docs.python.org/library/functions.html#range">Python
function of the same name</a>, for generating flexibly-numbered lists
of integers. Original patch contributed by
<a href="http://github.com/kylichuku">Kirill Ishanov</a>.
</p>
<p>
<b class="header">0.4.5</b><br />
Added <tt>rest</tt> for Arrays and arguments objects, and aliased
<tt>first</tt> as <tt>head</tt>, and <tt>rest</tt> as <tt>tail</tt>,
thanks to <a href="http://github.com/lukesutton/">Luke Sutton</a>'s patches.
Added tests ensuring that all Underscore Array functions also work on
<i>arguments</i> objects.
</p>
<p>
<b class="header">0.4.4</b><br />
Added <tt>isString</tt>, and <tt>isNumber</tt>, for consistency. Fixed
<tt>_.isEqual(NaN, NaN)</tt> to return <i>true</i> (which is debatable).
</p>
<p>
<b class="header">0.4.3</b><br />
Started using the native <tt>StopIteration</tt> object in browsers that support it.
Fixed Underscore setup for CommonJS environments.
</p>
<p>
<b class="header">0.4.2</b><br />
Renamed the unwrapping function to <tt>value</tt>, for clarity.
</p>
<p>
<b class="header">0.4.1</b><br />
Chained Underscore objects now support the Array prototype methods, so
that you can perform the full range of operations on a wrapped array
without having to break your chain. Added a <tt>breakLoop</tt> method
to <b>break</b> in the middle of any Underscore iteration. Added an
<tt>isEmpty</tt> function that works on arrays and objects.
</p>
<p>
<b class="header">0.4.0</b><br />
All Underscore functions can now be called in an object-oriented style,
like so: <tt>_([1, 2, 3]).map(...);</tt>. Original patch provided by
<a href="http://macournoyer.com/">Marc-André Cournoyer</a>.
Wrapped objects can be chained through multiple
method invocations. A <a href="#functions"><tt>functions</tt></a> method
was added, providing a sorted list of all the functions in Underscore.
</p>
<p>
<b class="header">0.3.3</b><br />
Added the JavaScript 1.8 function <tt>reduceRight</tt>. Aliased it
as <tt>foldr</tt>, and aliased <tt>reduce</tt> as <tt>foldl</tt>.
</p>
<p>
<b class="header">0.3.2</b><br />
Now runs on stock <a href="http://www.mozilla.org/rhino/">Rhino</a>
interpreters with: <tt>load("underscore.js")</tt>.
Added <a href="#identity"><tt>identity</tt></a> as a utility function.
</p>
<p>
<b class="header">0.3.1</b><br />
All iterators are now passed in the original collection as their third
argument, the same as JavaScript 1.6's <b>forEach</b>. Iterating over
objects is now called with <tt>(value, key, collection)</tt>, for details
see <a href="#each"><tt>_.each</tt></a>.
</p>
<p>
<b class="header">0.3.0</b><br />
Added <a href="http://github.com/dmitryBaranovskiy">Dmitry Baranovskiy</a>'s
comprehensive optimizations, merged in
<a href="http://github.com/kriskowal/">Kris Kowal</a>'s patches to make Underscore
<a href="http://wiki.commonjs.org/wiki/CommonJS">CommonJS</a> and
<a href="http://narwhaljs.org/">Narwhal</a> compliant.
</p>
<p>
<b class="header">0.2.0</b><br />
Added <tt>compose</tt> and <tt>lastIndexOf</tt>, renamed <tt>inject</tt> to
<tt>reduce</tt>, added aliases for <tt>inject</tt>, <tt>filter</tt>,
<tt>every</tt>, <tt>some</tt>, and <tt>forEach</tt>.
</p>
<p>
<b class="header">0.1.1</b><br />
Added <tt>noConflict</tt>, so that the "Underscore" object can be assigned to
other variables.
</p>
<p>
<b class="header">0.1.0</b><br />
Initial release of Underscore.js.
</p>
<p>
<a href="http://documentcloud.org/" title="A DocumentCloud Project" style="background:none;">
<img src="http://jashkenas.s3.amazonaws.com/images/a_documentcloud_project.png" alt="A DocumentCloud Project" />
</a>
</p>
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