Underscore.js

Object-Oriented and Functional Styles

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.

_.map([1, 2, 3], function(n){ return n * 2; });
_([1, 2, 3]).map(function(n){ return n * 2; });

Using the object-oriented style allows you to chain together methods. Calling chain on a wrapped object will cause all future method calls to return wrapped objects as well. When you've finished the computation, use value to retrieve the final value. Here's an example of chaining together a map/flatten/reduce, in order to get the word count of every word in a song.

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();

=> {lumberjack : 2, all : 4, night : 2 ... }

In addition, the Array prototype's methods are proxied through the chained Underscore object, so you can slip a reverse or a push into your chain, and continue to modify the array.

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). Delegates to the native forEach function if it exists.

_.each([1, 2, 3], function(num){ alert(num); });
=> alerts each number in turn...
_.each({one : 1, two : 2, three : 3}, function(num, key){ 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. If list is a JavaScript object, iterator's arguments will be (value, key, list).

_.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]

reduce_.reduce(list, iterator, memo, [context]) Aliases: inject, foldl
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], function(memo, num){ return memo + num; }, 0);
=> 6

reduceRight_.reduceRight(list, iterator, memo, [context]) Alias: foldr
The right-associative version of reduce. Delegates to the JavaScript 1.8 version of reduceRight, if it exists. Foldr is not as useful in JavaScript as it would be in a language with lazy evaluation.

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]

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 an acceptable element, and doesn't traverse the entire 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) Alias: contains
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];
_.min(numbers);
=> 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

toArray_.toArray(list)
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]

size_.size(list)
Return the number of values in the list.

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

Array Functions

Note: All array functions will also work on the arguments object.

first_.first(array, [n]) Alias: head
Returns the first element of an array. Passing n will return the first n elements of the array.

_.first([5, 4, 3, 2, 1]);
=> 5

rest_.rest(array, [index]) Alias: tail
Returns the rest of the elements in an array. Pass an index to return the values of the array from that index onward.

_.rest([5, 4, 3, 2, 1]);
=> [4, 3, 2, 1]

last_.last(array)
Returns the last element of an array.

_.last([5, 4, 3, 2, 1]);
=> 1

compact_.compact(array)
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]

flatten_.flatten(array)
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]) Alias: unique
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]

intersect_.intersect(*arrays)
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]

zip_.zip(*arrays)
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. If you're working with a matrix of nested arrays, zip.apply can transpose the matrix in a similar fashion.

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

indexOf_.indexOf(array, value, [isSorted])
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. If you're working with a large array, and you know that the array is already sorted, pass true for isSorted to use a faster binary search.

_.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

range_.range([start], stop, [step])
A function to create flexibly-numbered lists of integers, handy for each and map loops. start, if omitted, defaults to 0; step defaults to 1. Returns a list of integers from start to stop, incremented (or decremented) by step, exclusive.

_.range(10);
=> [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
_.range(1, 11);
=> [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
_.range(0, 30, 5);
=> [0, 5, 10, 15, 20, 25]
_.range(0, -10, -1);
=> [0, -1, -2, -3, -4, -5, -6, -7, -8, -9]
_.range(0);
=> []

Function (uh, ahem) Functions

bind_.bind(function, object, [*arguments])
Bind a function to an object, meaning that whenever the function is called, the value of this will be the object. 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');
func();
=> 'hi: moe'

bindAll_.bindAll(object, [*methodNames])
Binds a number of methods on the 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. If no methodNames are provided, all of the object's function properties will be bound to it.

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);
=> When the button is clicked, this.label will have the correct value...

memoize_.memoize(function, [hashFunction])
Memoizes a given function by caching the computed result. Useful for speeding up slow-running computations. If passed an optional hashFunction, it will be used to compute the hash key for storing the result, based on the arguments to the original function. The default hashFunction just uses the first argument to the memoized function as the key.

var fibonacci = function(n) {
  return n < 2 ? n : fibonacci(n - 1) + fibonacci(n - 2);
};
var fastFibonacci = _.memoize(fibonacci);

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.

defer_.defer(function)
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.

throttle_.throttle(function, wait)
Returns a throttled version of the function, that, when invoked repeatedly, will only actually call the wrapped function at most once per every wait milliseconds. Useful for rate-limiting events that occur faster than you can keep up with.

var throttled = _.throttle(updatePosition, 100);
$(window).scroll(throttled);

debounce_.debounce(function, wait)
Repeated calls to a debounced function will postpone it's execution until after wait milliseconds have elapsed. Useful for implementing behavior that should only happen after the input has stopped arriving. For example: rendering a preview of a Markdown comment, recalculating a layout after the window has stopped being resized...

var lazyLayout = _.debounce(calculateLayout, 300);
$(window).resize(lazyLayout);

once_.once(function)
Creates a version of the function that can only be called one time. Repeated calls to the modified function will have no effect, returning the value from the original call. Useful for initialization functions, instead of having to set a boolean flag and then check it later.

var initialize = _.once(createApplication);
initialize();
initialize();
// Application is only created once.

after_.after(count, function)
Creates a version of the function that will only be run after first being called count times. Useful for grouping asynchronous responses, where you want to be sure that all the async calls have finished, before proceeding.

var renderNotes = _.after(notes.length, render);
_.each(notes, function(note) {
  note.asyncSave({success: renderNotes}); 
});
// renderNotes is run once, after all notes have saved.

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";
});
hello();
=> 'before, hello: moe, after'

compose_.compose(*functions)
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(exclaim, greet);
welcome('moe');
=> 'hi: moe!'

Object Functions

keys_.keys(object)
Retrieve all the names of the object's properties.

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

values_.values(object)
Return all of the values of the object's properties.

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

functions_.functions(object) Alias: methods
Returns a sorted list of the names of every method in an object — that is to say, the name of every function property of the object.

_.functions(_);
=> ["all", "any", "bind", "bindAll", "clone", "compact", "compose" ...

extend_.extend(destination, *sources)
Copy all of the properties in the source objects over to the destination object. It's in-order, to the last source will override properties of the same name in previous arguments.

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

defaults_.defaults(object, *defaults)
Fill in missing properties in object with default values from the defaults objects. As soon as the property is filled, further defaults will have no effect.

var iceCream = {flavor : "chocolate"};
_.defaults(iceCream, {flavor : "vanilla", sprinkles : "lots"});
=> {flavor : "chocolate", sprinkles : "lots"}

clone_.clone(object)
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'};

tap_.tap(object, interceptor)
Invokes interceptor with the object, and then returns object. The primary purpose of this method is to "tap into" a method chain, in order to perform operations on intermediate results within the chain.

_([1,2,3,200]).chain().
  select(function(num) { return num % 2 == 0; }).
  tap(console.log).
  map(function(num) { return num * num }).
  value();
=> [2, 200]
=> [4, 40000]

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

isEmpty_.isEmpty(object)
Returns true if object contains no values.

_.isEmpty([1, 2, 3]);
=> false
_.isEmpty({});
=> true

isElement_.isElement(object)
Returns true if object is a DOM element.

_.isElement(jQuery('body')[0]);
=> true

isArray_.isArray(object)
Returns true if object is an Array.

(function(){ return _.isArray(arguments); })();
=> false
_.isArray([1,2,3]);
=> true

isArguments_.isArguments(object)
Returns true if object is an Arguments object.

(function(){ return _.isArguments(arguments); })(1, 2, 3);
=> true
_.isArguments([1,2,3]);
=> false

isFunction_.isFunction(object)
Returns true if object is a Function.

_.isFunction(alert);
=> true

isString_.isString(object)
Returns true if object is a String.

_.isString("moe");
=> true

isNumber_.isNumber(object)
Returns true if object is a Number.

_.isNumber(8.4 * 5);
=> true

isBoolean_.isBoolean(object)
Returns true if object is either true or false.

_.isBoolean(null);
=> false

isDate_.isDate(object)
Returns true if object is a Date.

_.isDate(new Date());
=> true

isRegExp_.isRegExp(object)
Returns true if object is a RegExp.

_.isRegExp(/moe/);
=> true

isNaN_.isNaN(object)
Returns true if object is NaN.
Note: this is not the same as the native isNaN function, which will also return true if the variable is undefined.

_.isNaN(NaN);
=> true
isNaN(undefined);
=> true
_.isNaN(undefined);
=> false

isNull_.isNull(object)
Returns true if the value of object is null.

_.isNull(null);
=> true
_.isNull(undefined);
=> false

isUndefined_.isUndefined(variable)
Returns true if variable is undefined.

_.isUndefined(window.missingVariable);
=> true

Utility Functions

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

var underscore = _.noConflict();

identity_.identity(value)
Returns the same value that is used as the argument. In math: f(x) = x
This function looks useless, but is used throughout Underscore as a default iterator.

var moe = {name : 'moe'};
moe === _.identity(moe);
=> true

times_.times(n, iterator)
Invokes the given iterator function n times.

_(3).times(function(){ genie.grantWish(); });

mixin_.mixin(object)
Allows you to extend Underscore with your own utility functions. Pass a hash of {name: function} definitions to have your functions added to the Underscore object, as well as the OOP wrapper.

_.mixin({
  capitalize : function(string) {
    return string.charAt(0).toUpperCase() + string.substring(1).toLowerCase();
  }
});
_("fabio").capitalize();
=> "Fabio"

uniqueId_.uniqueId([prefix])
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.

_.uniqueId('contact_');
=> '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>"

You can also use print from within JavaScript code. This is sometimes more convenient than using <%= ... %>.

var compiled = _.template("<% print('Hello ' + epithet); %>");
compiled({epithet: "stooge"});
=> "Hello stooge."

If ERB-style delimiters aren't your cup of tea, you can change Underscore's template settings to use different symbols to set off interpolated code. Define an interpolate regex, and an (optional) evaluate regex to match expressions that should be inserted and evaluated, respectively. If no evaluate regex is provided, your templates will only be capable of interpolating values. For example, to perform Mustache.js style templating:

_.templateSettings = {
  interpolate : /\{\{(.+?)\}\}/g
};

var template = _.template("Hello {{ name }}!");
template({name : "Mustache"});
=> "Hello Mustache!"

Chaining

chain_(obj).chain()
Returns a wrapped object. Calling methods on this object will continue to return wrapped objects until value is used. ( A more realistic example.)

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();
=> "moe is 21"

value_(obj).value()
Extracts the value of a wrapped object.

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

Duck Typing

The isType (isArray, isFunction, isString ...) family of type-checking functions use property detection to do their work, which, although orders of magnitude faster than the alternative, isn't entirely safe when dealing with objects that are used as hashes, where arbitrary strings are being set for the keys. It's entirely possible for an object to masquerade as another type, if you're setting properties with names like "concat" and "charCodeAt". So be aware.

Links & Suggested Reading

Underscore.lua, a Lua port of the functions that are applicable in both languages. Includes OOP-wrapping and chaining. The source is available on GitHub.

Underscore.string, an Underscore extension that adds functions for string-manipulation: trim, startsWith, contains, capitalize, reverse, sprintf, and more.

Ruby's Enumerable module.

Prototype.js, which provides JavaScript with collection functions in the manner closest to Ruby's Enumerable.

Oliver Steele's Functional JavaScript, which includes comprehensive higher-order function support as well as string lambdas.

Python's itertools.

Change Log

1.1.6 — April 18, 2011
Added _.after, which will return a function that only runs after first being called a specified number of times. _.invoke can now take a direct function reference. _.every now requires an iterator function to be passed, which mirrors the ECMA5 API. _.extend no longer copies keys when the value is undefined. _.bind now errors when trying to bind an undefined value.

1.1.5 — Mar 20, 2011
Added an _.defaults function, for use merging together JS objects representing default options. Added an _.once function, for manufacturing functions that should only ever execute a single time. _.bind now delegates to the native ECMAScript 5 version, where available. _.keys now throws an error when used on non-Object values, as in ECMAScript 5. Fixed a bug with _.keys when used over sparse arrays.

1.1.4 — Jan 9, 2011
Improved compliance with ES5's Array methods when passing null as a value. _.wrap now correctly sets this for the wrapped function. _.indexOf now takes an optional flag for finding the insertion index in an array that is guaranteed to already be sorted. Avoiding the use of .callee, to allow _.isArray to work properly in ES5's strict mode.

1.1.3 — Dec 1, 2010
In CommonJS, Underscore may now be required with just:
var _ = require("underscore"). Added _.throttle and _.debounce functions. Removed _.breakLoop, in favor of an ECMA5-style un-break-able each implementation — this removes the try/catch, and you'll now have better stack traces for exceptions that are thrown within an Underscore iterator. Improved the isType family of functions for better interoperability with Internet Explorer host objects. _.template now correctly escapes backslashes in templates. Improved _.reduce compatibility with the ECMA5 version: if you don't pass an initial value, the first item in the collection is used. _.each no longer returns the iterated collection, for improved consistency with ES5's forEach.

1.1.2
Fixed _.contains, which was mistakenly pointing at _.intersect instead of _.include, like it should have been. Added _.unique as an alias for _.uniq.

1.1.1
Improved the speed of _.template, and its handling of multiline interpolations. Ryan Tenney contributed optimizations to many Underscore functions. An annotated version of the source code is now available.

1.1.0
The method signature of _.reduce has been changed to match the ECMAScript 5 signature, instead of the Ruby/Prototype.js version. This is a backwards-incompatible change. _.template may now be called with no arguments, and preserves whitespace. _.contains is a new alias for _.include.

1.0.4
Andri Möll contributed the _.memoize function, which can be used to speed up expensive repeated computations by caching the results.

1.0.3
Patch that makes _.isEqual return false if any property of the compared object has a NaN value. Technically the correct thing to do, but of questionable semantics. Watch out for NaN comparisons.

1.0.2
Fixes _.isArguments in recent versions of Opera, which have arguments objects as real Arrays.

1.0.1
Bugfix for _.isEqual, when comparing two objects with the same number of undefined keys, but with different names.

1.0.0
Things have been stable for many months now, so Underscore is now considered to be out of beta, at 1.0. Improvements since 0.6 include _.isBoolean, and the ability to have _.extend take multiple source objects.

0.6.0
Major release. Incorporates a number of Mile Frawley's refactors for safer duck-typing on collection functions, and cleaner internals. A new _.mixin method that allows you to extend Underscore with utility functions of your own. Added _.times, which works the same as in Ruby or Prototype.js. Native support for ECMAScript 5's Array.isArray, and Object.keys.

0.5.8
Fixed Underscore's collection functions to work on NodeLists and HTMLCollections once more, thanks to Justin Tulloss.

0.5.7
A safer implementation of _.isArguments, and a faster _.isNumber,
thanks to Jed Schmidt.

0.5.6
Customizable delimiters for _.template, contributed by Noah Sloan.

0.5.5
Fix for a bug in MobileSafari's OOP-wrapper, with the arguments object.

0.5.4
Fix for multiple single quotes within a template string for _.template. See: Rick Strahl's blog post.

0.5.2
New implementations of isArray, isDate, isFunction, isNumber, isRegExp, and isString, thanks to a suggestion from Robert Kieffer. Instead of doing Object#toString comparisons, they now check for expected properties, which is less safe, but more than an order of magnitude faster. Most other Underscore functions saw minor speed improvements as a result. Evgeniy Dolzhenko contributed _.tap, similar to Ruby 1.9's, which is handy for injecting side effects (like logging) into chained calls.

0.5.1
Added an _.isArguments function. Lots of little safety checks and optimizations contributed by Noah Sloan and Andri Möll.

0.5.0
[API Changes] _.bindAll 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. _.functions now takes a single argument and returns the names of its Function properties. Calling _.functions(_) will get you the previous behavior. Added _.isRegExp so that isEqual can now test for RegExp equality. All of the "is" functions have been shrunk down into a single definition. Karl Guertin contributed patches.

0.4.7
Added isDate, isNaN, and isNull, for completeness. Optimizations for isEqual when checking equality between Arrays or Dates. _.keys is now 25%–2X faster (depending on your browser) which speeds up the functions that rely on it, such as _.each.

0.4.6
Added the range function, a port of the Python function of the same name, for generating flexibly-numbered lists of integers. Original patch contributed by Kirill Ishanov.

0.4.5
Added rest for Arrays and arguments objects, and aliased first as head, and rest as tail, thanks to Luke Sutton's patches. Added tests ensuring that all Underscore Array functions also work on arguments objects.

0.4.4
Added isString, and isNumber, for consistency. Fixed _.isEqual(NaN, NaN) to return true (which is debatable).

0.4.3
Started using the native StopIteration object in browsers that support it. Fixed Underscore setup for CommonJS environments.

0.4.2
Renamed the unwrapping function to value, for clarity.

0.4.1
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 breakLoop method to break in the middle of any Underscore iteration. Added an isEmpty function that works on arrays and objects.

0.4.0
All Underscore functions can now be called in an object-oriented style, like so: _([1, 2, 3]).map(...);. Original patch provided by Marc-André Cournoyer. Wrapped objects can be chained through multiple method invocations. A functions method was added, providing a sorted list of all the functions in Underscore.

0.3.3
Added the JavaScript 1.8 function reduceRight. Aliased it as foldr, and aliased reduce as foldl.

0.3.2
Now runs on stock Rhino interpreters with: load("underscore.js"). Added identity as a utility function.

0.3.1
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.

0.3.0
Added Dmitry Baranovskiy's comprehensive optimizations, merged in Kris Kowal's patches to make Underscore CommonJS and Narwhal compliant.

0.2.0
Added compose and lastIndexOf, renamed inject to reduce, added aliases for inject, filter, every, some, and forEach.

0.1.1
Added noConflict, so that the "Underscore" object can be assigned to other variables.

0.1.0
Initial release of Underscore.js.