 
    
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, and Backbone.js's suspenders.
Underscore provides 60-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 modern browsers will use the native implementations of forEach, map, reduce, filter, every, some and indexOf.
A complete Test & Benchmark Suite is included for your perusal.
You may also read through the annotated source code.
The project is hosted on GitHub. You can report bugs and discuss features on the issues page, on Freenode in the #documentcloud channel, or send tweets to @documentcloud.
Underscore is an open-source component of DocumentCloud.
| Development Version (1.3.1) | 34kb, Uncompressed with Comments | 
| Production Version (1.3.1) | < 4kb, Minified and Gzipped | 
        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])
        Alias: collect
        
        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]
      
        find_.find(list, iterator, [context])
        Alias: detect
        
        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 = _.find([1, 2, 3, 4, 5, 6], function(num){ return num % 2 == 0; });
=> 2
      
        filter_.filter(list, iterator, [context])
        Alias: select
        
        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 = _.filter([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 filter.
      
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. Delegates to the native method every, if present.
      
_.all([true, 1, null, 'yes'], _.identity); => 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)
        
        A 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 copy of list, ranked in ascending order 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]
      
        groupBy_.groupBy(list, iterator)
        
        Splits a collection into sets, grouped by the result of running each
        value through iterator. If iterator is a string instead of
        a function, groups by the property named by iterator on each of
        the values.
      
_.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"]}
      
        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
        shuffle_.shuffle(list)
        
        Returns a shuffled copy of the list, using a version of the 
        Fisher-Yates shuffle.
      
_.shuffle([1, 2, 3, 4, 5, 6]); => [4, 1, 6, 3, 5, 2]
        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
      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
        initial_.initial(array, [n])
        
        Returns everything but the last entry of the array. Especially useful on
        the arguments object. Pass n to exclude the last n elements
        from the result.
      
_.initial([5, 4, 3, 2, 1]); => [5, 4, 3, 2]
        last_.last(array, [n])
        
        Returns the last element of an array. Passing n will return
        the last n elements of the array.
      
_.last([5, 4, 3, 2, 1]); => 1
        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]
        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, [shallow])
        
        Flattens a nested array (the nesting can be to any depth). If you
        pass shallow, the array will only be flattened a single level.
      
_.flatten([1, [2], [3, [[4]]]]); => [1, 2, 3, 4]; _.flatten([1, [2], [3, [[4]]]], true); => [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]
        union_.union(*arrays)
        
        Computes the union of the passed-in arrays: the list of unique items,
        in order, that are present in one or more of the arrays.
      
_.union([1, 2, 3], [101, 2, 1, 10], [2, 1]); => [1, 2, 3, 101, 10]
        intersection_.intersection(*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.
      
_.intersection([1, 2, 3], [101, 2, 1, 10], [2, 1]); => [1, 2]
        difference_.difference(array, *others)
        
        Similar to without, but returns the values from array that
        are not present in the other arrays.
      
_.difference([1, 2, 3, 4, 5], [5, 2, 10]); => [1, 3, 4]
        uniq_.uniq(array, [isSorted], [iterator])
        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.
        If you want to compute unique items based on a transformation, pass an
        iterator function.
      
_.uniq([1, 2, 1, 3, 1, 4]); => [1, 2, 3, 4]
        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); => []
        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 partial application.
      
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 = _.memoize(function(n) {
  return n < 2 ? n : fibonacci(n - 1) + fibonacci(n - 2);
});
      
        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)
        
        Creates and returns a new, throttled version of the passed function, 
        that, when invoked repeatedly, will only actually call the original 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)
        
        Creates and returns a new debounced version of the passed function that
        will postpone its execution until after 
        wait milliseconds have elapsed since the last time it  
        was invoked. 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, and so on.
      
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!'
      
        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, so 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.
      
_.chain([1,2,3,200])
  .filter(function(num) { return num % 2 == 0; })
  .tap(console.log)
  .map(function(num) { return num * num })
  .value();
=> [2, 200]
=> [4, 40000]
      
        has_.has(object, key)
        
        Does the object contain the given key? Identical to 
        object.hasOwnProperty(key), but uses a safe reference to the
        hasOwnProperty function, in case it's been 
        overridden accidentally.
      
_.has({a: 1, b: 2, c: 3}, "b");
=> true
      
        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 (including NaN).
      
_.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
        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'
      
        escape_.escape(string)
        
        Escapes a string for insertion into HTML, replacing 
        &, <, >, ", ', and / characters.
      
_.escape('Curly, Larry & Moe');
=> "Curly, Larry & Moe"
      
        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
        <% … %>. If you wish to interpolate a value, and have
        it be HTML-escaped, use <%- … %> 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>"
var template = _.template("<b><%- value %></b>");
template({value : '<script>'});
=> "<b><script></b>"
      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 to match expressions that should be interpolated verbatim, an escape regex to match expressions that should be inserted after being HTML escaped, and an evaluate regex to match expressions that should be evaluated without insertion into the resulting string. You may define or omit any combination of the three. For example, to perform Mustache.js style templating:
_.templateSettings = {
  interpolate : /\{\{(.+?)\}\}/g
};
var template = _.template("Hello {{ name }}!");
template({name : "Mustache"});
=> "Hello Mustache!"
      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"}
];
_.chain(lyrics)
  .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.
        chain_.chain(obj)
        
        Returns a wrapped object. Calling methods on this object will continue
        to return wrapped objects until value is used.
      
var stooges = [{name : 'curly', age : 25}, {name : 'moe', age : 21}, {name : 'larry', age : 23}];
var youngest = _.chain(stooges)
  .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]
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.php, a PHP port of the functions that are applicable in both languages. Includes OOP-wrapping and chaining. The source is available on GitHub.
Underscore-perl, a Perl port of many of the Underscore.js functions, aimed at on Perl hashes and arrays, also 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.
Michael Aufreiter's Data.js, a data manipulation + persistence library for JavaScript.
Python's itertools.
        1.3.1 — Jan. 23, 2012
        
        1.3.0 — Jan. 11, 2012
        
        1.2.4 — Jan. 4, 2012
        
        1.2.3 — Dec. 7, 2011
        
        1.2.2 — Nov. 14, 2011
        
        1.2.1 — Oct. 24, 2011
        
        1.2.0 — Oct. 5, 2011
        
        1.1.7 — July 13, 2011
        Added _.groupBy, which aggregates a collection into groups of like items.
        Added _.union and _.difference, to complement the 
        (re-named) _.intersection.
        Various improvements for support of sparse arrays.
        _.toArray now returns a clone, if directly passed an array.
        _.functions now also returns the names of functions that are present
        in the prototype chain.
      
        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.