Underscore is a utility-belt library for JavaScript that provides a lot of the functional programming support that you would expect in Prototype.js (or Ruby), but without extending any of the built-in JavaScript objects. It's the tie to go along with jQuery's tux.

Underscore provides 44-odd functions that support both the usual functional suspects: map, select, invoke — 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 JavaScript 1.6 compliant browsers will use the native implementations of forEach, map, filter, every, some and indexOf.

Underscore includes a complete Test & Benchmark Suite for your perusal.

The unabridged source code is available on GitHub.

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Table of Contents

each, map, reduce, detect, select, reject, all, any, include, invoke, pluck, max, min, sortBy, sortedIndex, toArray, size

first, last, compact, flatten, without, uniq, intersect, zip, indexOf, lastIndexOf

bind, bindAll, delay, defer, wrap, compose

keys, values, extend, clone, isEqual, isElement, isArray, isFunction, isUndefined

noConflict, uniqueId, template

Collection Functions (Arrays or Objects)

each_.each(list, iterator, [context]) Alias: forEach
Iterates over a list of elements, yielding each in turn to an iterator function. The iterator is bound to the context object, if one is passed. Each invocation of iterator is called with three arguments: (element, index, list). If list is a JavaScript object, iterator's arguments will be (value, key, list). If the list has an each method of its own, it will be used instead. Delegates to the native forEach function if it exists.

_.each([1, 2, 3], function(num){ alert(num); });
=> alerts each number in turn...

map_.map(list, iterator, [context])
Produces a new array of values by mapping each value in list through a transformation function (iterator). If the native map method exists, it will be used instead.

_.map([1, 2, 3], function(num){ return num * 3 });
=> [3, 6, 9]

reduce_.reduce(list, memo, iterator, [context]) Alias: inject
Also known as inject and foldl, reduce boils down a list of values into a single value. Memo is the initial state of the reduction, and each successive step of it should be returned by iterator.

var sum = _.reduce([1, 2, 3], 0, function(memo, num){ return memo + num });
=> 6

detect_.detect(list, iterator, [context])
Looks through each value in the list, returning the first one that passes a truth test (iterator). The function returns as soon as it finds the first acceptable element, and doesn't continue to traverse the list.

var even = _.detect([1, 2, 3, 4, 5, 6], function(num){ return num % 2 == 0; });
=> 2

select_.select(list, iterator, [context]) Alias: filter
Looks through each value in the list, returning an array of all the values that pass a truth test (iterator). Delegates to the native filter method, if it exists.

var evens = _.select([1, 2, 3, 4, 5, 6], function(num){ return num % 2 == 0; });
=> [2, 4, 6]

reject_.reject(list, iterator, [context])
Returns the values in list without the elements that the truth test (iterator) passes. The opposite of select.

var odds = _.reject([1, 2, 3, 4, 5, 6], function(num){ return num % 2 == 0; });
=> [1, 3, 5]

all_.all(list, [iterator], [context]) Alias: every
Returns true if all of the values in the list pass the iterator truth test. If an iterator is not provided, the truthy value of the element will be used instead. Delegates to the native method every, if present.

_.all([true, 1, null, 'yes']);
=> false

any_.any(list, [iterator], [context]) Alias: some
Returns true if any of the values in the list pass the iterator truth test. Short-circuits and stops traversing the list if a true element is found. Delegates to the native method some, if present.

_.any([null, 0, 'yes', false]);
=> true

include_.include(list, value)
Returns true if the value is present in the list, using === to test equality. Uses indexOf internally, if list is an Array.

_.include([1, 2, 3], 3);
=> true

invoke_.invoke(list, methodName, [*arguments])
Calls the method named by methodName on each value in the list. Any extra arguments passed to invoke will be forwarded on to the method invocation.

_.invoke([[5, 1, 7], [3, 2, 1]], 'sort');
=> [[1, 5, 7], [1, 2, 3]]

pluck_.pluck(list, propertyName)
An convenient version of what is perhaps the most common use-case for map: extracting a list of property values.

var stooges = [{name : 'moe', age : 40}, {name : 'larry', age : 50}, {name : 'curly', age : 60}];
_.pluck(stooges, 'name');
=> ["moe", "larry", "curly"]

max_.max(list, [iterator], [context])
Returns the maximum value in list. If iterator is passed, it will be used on each value to generate the criterion by which the value is ranked.

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};

min_.min(list, [iterator], [context])
Returns the minimum value in list. If iterator is passed, it will be used on each value to generate the criterion by which the value is ranked.

var numbers = [10, 5, 100, 2, 1000];
=> 2

sortBy_.sortBy(list, iterator, [context])
Returns a sorted list, ranked by the results of running each value through iterator.

_.sortBy([1, 2, 3, 4, 5, 6], function(num){ return Math.sin(num); });
=> [5, 4, 6, 3, 1, 2]

sortedIndex_.sortedIndex(list, value, [iterator])
Uses a binary search to determine the index at which the value should be inserted into the list in order to maintain the list's sorted order. If an iterator is passed, it will be used to compute the sort ranking of each value.

_.sortedIndex([10, 20, 30, 40, 50], 35);
=> 3

Converts the list (anything that can be iterated over), into a real Array. Useful for transmuting the arguments object.

(function(){ return _.toArray(arguments).slice(0); })(1, 2, 3);
=> [1, 2, 3]

Return the number of values in the list.

_.size({one : 1, two : 2, three : 3});
=> 3

Array Functions

Convenience to return the first element of an array (identical to array[0]).

_.first([3, 2, 1]);
=> 3

Returns the last element of an array.

_.last([3, 2, 1]);
=> 1

Returns a copy of the array with all falsy values removed. In JavaScript, false, null, 0, "", undefined and NaN are all falsy.

_.compact([0, 1, false, 2, '', 3]);
=> [1, 2, 3]

Flattens a nested array (the nesting can be to any depth).

_.flatten([1, [2], [3, [[[4]]]]]);
=> [1, 2, 3, 4];

without_.without(array, [*values])
Returns a copy of the array with all instances of the values removed. === is used for the equality test.

_.without([1, 2, 1, 0, 3, 1, 4], 0, 1);
=> [2, 3, 4]

uniq_.uniq(array, [isSorted])
Produces a duplicate-free version of the array, using === to test object equality. If you know in advance that the array is sorted, passing true for isSorted will run a much faster algorithm.

_.uniq([1, 2, 1, 3, 1, 4]);
=> [1, 2, 3, 4]

Computes the list of values that are the intersection of all the arrays. Each value in the result is present in each of the arrays.

_.intersect([1, 2, 3], [101, 2, 1, 10], [2, 1]);
=> [1, 2]

Merges together the values of each of the arrays with the values at the corresponding position. Useful when you have separate data sources that are coordinated through matching array indexes.

_.zip(['moe', 'larry', 'curly'], [30, 40, 50], [true, false, false]);
=> [["moe", 30, true], ["larry", 40, false], ["curly", 50, false]]

indexOf_.indexOf(array, value)
Returns the index at which value can be found in the array, or -1 if value is not present in the array. Uses the native indexOf function unless it's missing.

_.indexOf([1, 2, 3], 2);
=> 1

lastIndexOf_.lastIndexOf(array, value)
Returns the index of the last occurrence of value in the array, or -1 if value is not present. Uses the native lastIndexOf function if possible.

_.lastIndexOf([1, 2, 3, 1, 2, 3], 2);
=> 4

Function (uh, ahem) Functions

bind_.bind(function, context, [*arguments])
Bind a function to a context object, meaning that whenever the function is called, the value of this will be the context. Optionally, bind arguments to the function to pre-fill them, also known as currying.

var func = function(greeting){ return greeting + ': ' + this.name };
func = _.bind(func, {name : 'moe'}, 'hi');
=> 'hi: moe'

bindAll_.bindAll(*methodNames, context)
Binds a number of methods on the context object, specified by methodNames, 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 this.

var buttonView = {
  label   : 'underscore', 
  onClick : function(){ alert('clicked: ' + this.label); },
  onHover : function(){ console.log('hovering: ' + this.label); }
_.bindAll('onClick', 'onHover', buttonView);
jQuery('#underscore_button').bind('click', buttonView.onClick);
=> When the button is clicked, this.label will have the correct value...

delay_.delay(function, wait, [*arguments])
Much like setTimeout, invokes function after wait milliseconds. If you pass the optional arguments, they will be forwarded on to the function when it is invoked.

var log = _.bind(console.log, console);
_.delay(log, 1000, 'logged later');
=> 'logged later' // Appears after one second.

Defers invoking the function until the current call stack has cleared, similar to using setTimeout with a delay of 0. Useful for performing expensive computations or HTML rendering in chunks without blocking the UI thread from updating.

_.defer(function(){ alert('deferred'); });
// Returns from the function before the alert runs.

wrap_.wrap(function, wrapper)
Wraps the first function inside of the wrapper function, passing it as the first argument. This allows the wrapper to execute code before and after the function runs, adjust the arguments, and execute it conditionally.

var hello = function(name) { return "hello: " + name; };
hello = _.wrap(hello, function(func) {
  return "before, " + func("moe") + ", after";
=> 'before, hello: moe, after'

Returns the composition of a list of functions, where each function consumes the return value of the function that follows. In math terms, composing the functions f(), g(), and h() produces f(g(h())).

var greet    = function(name){ return "hi: " + name; };
var exclaim  = function(statement){ return statement + "!"; };
var welcome = _.compose(greet, exclaim);
=> 'hi: moe!'

Object Functions

Retrieve all the names of the object's properties.

_.keys({one : 1, two : 2, three : 3});
=> ["one", "two", "three"]

Return all of the values of the object's properties.

_.values({one : 1, two : 2, three : 3});
=> [1, 2, 3]

extend_.extend(destination, source)
Copy all of the properties in the source object over to the destination object.

_.extend({name : 'moe'}, {age : 50});
=> {name : 'moe', age : 50}

Create a shallow-copied clone of the object. Any nested objects or arrays will be copied by reference, not duplicated.

_.clone({name : 'moe'});
=> {name : 'moe'};

isEqual_.isEqual(object, other)
Performs an optimized deep comparison between the two objects, to determine if they should be considered equal.

var moe   = {name : 'moe', luckyNumbers : [13, 27, 34]};
var clone = {name : 'moe', luckyNumbers : [13, 27, 34]};
moe == clone;
=> false
_.isEqual(moe, clone);
=> true

Returns true if object is a DOM element.

=> true

Returns true if object is an Array.

(function(){ return _.isArray(arguments); })();
=> false
=> true

Returns true if object is a Function.

=> true

Returns true if variable is undefined.

=> true

Utility Functions

Give control of the "_" variable back to its previous owner. Returns a reference to the Underscore object.

var underscore = _.noConflict();

Generate a globally-unique id for client-side models or DOM elements that need one. If prefix is passed, the id will be appended to it.

=> 'contact_104'

template_.template(templateString, [context])
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
<%= … %>, as well as execute arbitrary JavaScript code, with <% … %>. When you evaluate a template function, pass in a context object that has properties corresponding to the template's free variables. If you're writing a one-off, you can pass the context object as the second parameter to template in order to render immediately instead of returning a template function.

var compiled = _.template("hello: <%= name %>");
compiled({name : 'moe'});
=> "hello: moe"

var list = "<% _.each(people, function(name) { %> <li><%= name %></li> <% }); %>";
_.template(list, {people : ['moe', 'curly', 'larry']});
=> "<li>moe</li><li>curly</li><li>larry</li>"

Change Log

All iterators are now passed in the original collection as their third argument, the same as JavaScript 1.6's forEach. Iterating over objects is now called with (value, key, collection), for details see _.each.

Added Dmitry Baranovskiy's comprehensive optimizations, merged in Kris Kowal's patches to make Underscore CommonJS and Narwhal compliant.

Added compose and lastIndexOf, renamed inject to reduce, added aliases for inject, filter, every, some, and forEach.

Added noConflict, so that the "Underscore" object can be assigned to other variables.

Initial release of Underscore.js.

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