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    Underscore.js is a tiny JavaScript utility library that makes working with some of the common data structures used in JavaScript much easier. Minified and GZipped it weighs in at less than 4Kb and where possible it delegates functionality to native browser implementations for performance. It has no other dependencies and so adds very little overhead to your total script assets. It can be used on the client or server with equal ease.

    Using Underscore is extremely easy; it isn’t tightly bound to the DOM, doesn’t make any assumptions about other libraries or frameworks that may be in use and doesn’t require any setup or configuration; you simply include the library file in your page and you can start calling its methods.

    We’ll be using the following JavaScript arrays in the examples shown in this tutorial:

    var myArray = ["one", "two", "three", "four", "five"];
    var myArray2 = [1, 2, 3, 4, 5];
    var myArray3 = [1, 5, 2, 2, 8, 5, 3, 8, 9, 0];

    Working with arrays

    Arrays are very common data structures in JavaScript, heavily used by many popular JavaScript libraries, for example, whenever jQuery returns an element (or elements) from the DOM, it returns it (or them) in an array. You may be using arrays and not even know it! In case anyone doesn’t know, an array is simply a collection of native values (strings, integers, objects, etc) where each value has a numerical index associated with it. Let’s look at some of the useful array methods provided by Underscore:


    JavaScript has long had the indexOf() method for finding a character or sequence of characters within a string of text. The latest version of JavaScript (or ECMA Script upon which each browser’s implementation of JavaScript is based) includes an indexOf() method that works in a similar way but with arrays. It returns either the index of the first matching item in the array or -1 if a matching item is not found.

    But guess what, only the very latest version of Internet Explorer supports this incredibly useful array method, so if older versions of IE must be supported, we need some other way of adding support. Enter the ring Underscore, which allows us to make use of it on older browsers.

    To use this method to find the index of the string three in our test array, we could use the following code:

    _.indexOf(myArray, "three")

    All Underscore methods are attached the _ object, much in the same way that jQuery methods are attached to the $ object. To use indexOf() we supply the array to search as the first argument and the term we are searching for as the second. There is also a third argument which takes a Boolean indicating whether the array is already sorted, which runs a much faster search algorithm. Underscore also ships with a lastIndexOf() method which finds the last occurrence of something within an array. Pretty useful.


    Another useful method, this time with no native JavaScript counterpart to leverage, is the union() method. This method takes any number of arrays and returns a single array containing any items that appear in one or more of the original arrays. To see the union of two of our example arrays, we could use the following code:

    _.union(myArray2, myArray3)

    The returned array will contain the items in the order in which the source arrays are passed in.


    Another exceptionally useful method, similar in some respects to the union() method is uniq(). This method accepts a single array as an argument and returns an array containing only the unique items from the source array. To filter out the duplicates in our third example array, we could pass it to uniq() like this:


    Just like the indexOf() method that we saw a moment ago, this method can accept a Boolean as the second argument which specifies whether the source array has already been sorted. If it has a much faster algorithm is used to weed out the duplicates.

    This method uses the strict equality test (===) in its comparator function to determine whether two items are identical or not. In some situations, this may be more strict than we require, for example this !== This. To help alleviate issues such as this, we can pass a transformation function to this method as the third argument. If we were trying to make the method case-insensitive, we could use this:

    _.uniq(myArray3, false, function(item) {
    return item.toLowerCase();

    The transformation function receives the ‘current’ item each time it is invoked (it will be invoked for each item in the source array) and should return the transformed item. In this case, we just convert the item to lowercase before returning it. No more casing issues.


    The zip() method takes any number of input arrays and returns a number of arrays where the same items from the same indices or the source arrays have been merged. It sounds tricky in words, but when you see what happens you’ll understand immediately. To zip up the first and second of our test arrays for example, we would use the method in this way:

    _.zip(myArray, myArray2)

    When you see the output of this expression, you’ll see that the first array returned contains the items “one” and 1, the second array contains the items “two” and 2, etc.

    In this example, both source arrays contain the same number of items, so the output is pleasingly neat. We can also pass arrays with differing numbers of items and the method will still work, but some of the arrays returned will have undefined values at some of the indices.


    The range() method is interesting because although it is used with arrays, it doesn’t accept any arrays as arguments. Instead it accepts integers, and uses them to return an array containing a sequence of numbers.
    It can accept several arguments; the first argument, which is optional and defaults to 0, is the starting number. The second argument is the number to stop at, and the third argument is the step value. Again, this may be a tricky method to get your head around theoretically, but in this case, examples speak louder than words, so let’s take a look at a couple of variations.

    If a single argument is provided, it is assumed to be the number to stop at and 0 is used as the starting number. 1 is used as the step:


    The output of this would be an array containing the values 0, 1 and 2 (remember, array indices are zero-based). If two arguments are provided they are assumed to be the starting and stopping number. 1 is used as the step:


    This would give the same result as the first example. To use a step value other than 1 we muist provide all three arguments:


    This now gives the result 0,10,20. We can provide a lower ending number than the starting number if we wish; in this case, the step value provided should be negative:

    _.range(30, 0,-10)

    In this case, the output is now 30,20,10. Although possibly not used as often as other method we have looked at, range() is nevertheless a useful method when a list of numbers between a particular range is needed.


    Collection Functions (Arrays or Objects)

    each_.each(list, iteratee, [context]) Alias: forEach
    Iterates over a list of elements, yielding each in turn to an iteratee function. The iteratee is bound to the context object, if one is passed. Each invocation of iteratee is called with three arguments: (element, index, list). If list is a JavaScript object, iteratee's arguments will be (value, key, list). Returns the list for chaining.

    _.each([1, 2, 3], alert);
    => alerts each number in turn...
    _.each({one: 1, two: 2, three: 3}, alert);
    => alerts each number value in turn...

    Note: Collection functions work on arrays, objects, and array-like objects such as arguments, NodeList and similar. But it works by duck-typing, so avoid passing objects with a numeric length property. It's also good to note that an each loop cannot be broken out of — to break, use _.find instead.

    map_.map(list, iteratee, [context]) Alias: collect
    Produces a new array of values by mapping each value in list through a transformation function (iteratee). The iteratee is passed three arguments: the value, then the index (or key) of the iteration, and finally a reference to the entire 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]
    _.map([[1, 2], [3, 4]], _.first);
    => [1, 3]

    reduce_.reduce(list, iteratee, [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 iteratee. The iteratee is passed four arguments: the memo, then the value and index (or key) of the iteration, and finally a reference to the entire list.

    If no memo is passed to the initial invocation of reduce, the iteratee is not invoked on the first element of the list. The first element is instead passed as the memo in the invocation of the iteratee on the next element in the list.

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

    reduceRight_.reduceRight(list, iteratee, 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, predicate, [context]) Alias: detect
    Looks through each value in the list, returning the first one that passes a truth test (predicate), or undefined if no value passes the test. 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, predicate, [context]) Alias: select
    Looks through each value in the list, returning an array of all the values that pass a truth test (predicate).

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

    where_.where(list, properties)
    Looks through each value in the list, returning an array of all the values that contain all of the key-value pairs listed in properties.

    _.where(listOfPlays, {author: "Shakespeare", year: 1611});
    => [{title: "Cymbeline", author: "Shakespeare", year: 1611},
        {title: "The Tempest", author: "Shakespeare", year: 1611}]

    findWhere_.findWhere(list, properties)
    Looks through the list and returns the first value that matches all of the key-value pairs listed in properties.

    If no match is found, or if list is empty, undefined will be returned.

    _.findWhere(publicServicePulitzers, {newsroom: "The New York Times"});
    => {year: 1918, newsroom: "The New York Times",
      reason: "For its public service in publishing in full so many official reports,
      documents and speeches by European statesmen relating to the progress and
      conduct of the war."}

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

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

    every_.every(list, [predicate], [context]) Alias: all
    Returns true if all of the values in the list pass the predicate truth test.

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

    some_.some(list, [predicate], [context]) Alias: any
    Returns true if any of the values in the list pass the predicate truth test. Short-circuits and stops traversing the list if a true element is found.

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

    contains_.contains(list, value, [fromIndex]) Alias: includes
    Returns true if the value is present in the list. Uses indexOf internally, if list is an Array. Use fromIndex to start your search at a given index.

    _.contains([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, [iteratee], [context])
    Returns the maximum value in list. If an iteratee function is provided, it will be used on each value to generate the criterion by which the value is ranked. -Infinity is returned if list is empty, so an isEmpty guard may be required.

    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, [iteratee], [context])
    Returns the minimum value in list. If an iteratee function is provided, it will be used on each value to generate the criterion by which the value is ranked. Infinity is returned if list is empty, so an isEmpty guard may be required.

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

    sortBy_.sortBy(list, iteratee, [context])
    Returns a (stably) sorted copy of list, ranked in ascending order by the results of running each value through iteratee. iteratee may also be the string name of the property to sort by (eg. length).

    _.sortBy([1, 2, 3, 4, 5, 6], function(num){ return Math.sin(num); });
    => [5, 4, 6, 3, 1, 2]
    var stooges = [{name: 'moe', age: 40}, {name: 'larry', age: 50}, {name: 'curly', age: 60}];
    _.sortBy(stooges, 'name');
    => [{name: 'curly', age: 60}, {name: 'larry', age: 50}, {name: 'moe', age: 40}];

    groupBy_.groupBy(list, iteratee, [context])
    Splits a collection into sets, grouped by the result of running each value through iteratee. If iteratee is a string instead of a function, groups by the property named by iteratee 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"]}

    indexBy_.indexBy(list, iteratee, [context])
    Given a list, and an iteratee function that returns a key for each element in the list (or a property name), returns an object with an index of each item. Just like groupBy, but for when you know your keys are unique.

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

    countBy_.countBy(list, iteratee, [context])
    Sorts a list into groups and returns a count for the number of objects in each group. Similar to groupBy, but instead of returning a list of values, returns a count for the number of values in that group.

    _.countBy([1, 2, 3, 4, 5], function(num) {
      return num % 2 == 0 ? 'even': 'odd';
    => {odd: 3, even: 2}

    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]

    sample_.sample(list, [n])
    Produce a random sample from the list. Pass a number to return n random elements from the list. Otherwise a single random item will be returned.

    _.sample([1, 2, 3, 4, 5, 6]);
    => 4
    _.sample([1, 2, 3, 4, 5, 6], 3);
    => [1, 6, 2]

    Creates a real Array from the list (anything that can be iterated over). Useful for transmuting the arguments object.

    (function(){ return _.toArray(arguments).slice(1); })(1, 2, 3, 4);
    => [2, 3, 4]

    Return the number of values in the list.

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

    partition_.partition(array, predicate)
    Split array into two arrays: one whose elements all satisfy predicate and one whose elements all do not satisfy predicate.

    _.partition([0, 1, 2, 3, 4, 5], isOdd);
    => [[1, 3, 5], [0, 2, 4]]

    Array Functions

    Note: All array functions will also work on the arguments object. However, Underscore functions are not designed to work on "sparse" arrays.

    first_.first(array, [n]) Alias: head, take
    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, drop
    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]

    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.

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

    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]

    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], [iteratee]) Alias: unique
    Produces a duplicate-free version of the array, using === to test object equality. In particular only the first occurence of each value is kept. 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 iteratee function.

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

    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]]

    The opposite of zip. Given a number of arrays, returns a series of new arrays, the first of which contains all of the first elements in the input arrays, the second of which contains all of the second elements, and so on. Use with apply to pass in an array of arrays.

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

    object_.object(list, [values])
    Converts arrays into objects. Pass either a single list of [key, value] pairs, or a list of keys, and a list of values. If duplicate keys exist, the last value wins.

    _.object(['moe', 'larry', 'curly'], [30, 40, 50]);
    => {moe: 30, larry: 40, curly: 50}
    _.object([['moe', 30], ['larry', 40], ['curly', 50]]);
    => {moe: 30, larry: 40, curly: 50}

    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. 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 ... or, pass a number as the third argument in order to look for the first matching value in the array after the given index.

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

    lastIndexOf_.lastIndexOf(array, value, [fromIndex])
    Returns the index of the last occurrence of value in the array, or -1 if value is not present. Pass fromIndex to start your search at a given index.

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

    sortedIndex_.sortedIndex(list, value, [iteratee], [context])
    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 iteratee function is provided, it will be used to compute the sort ranking of each value, including the value you pass. The iteratee may also be the string name of the property to sort by (eg. length).

    _.sortedIndex([10, 20, 30, 40, 50], 35);
    => 3
    var stooges = [{name: 'moe', age: 40}, {name: 'curly', age: 60}];
    _.sortedIndex(stooges, {name: 'larry', age: 50}, 'age');
    => 1

    findIndex_.findIndex(array, predicate, [context])
    Similar to _.indexOf, returns the first index where the predicate truth test passes; otherwise returns -1.

    _.findIndex([4, 6, 8, 12], isPrime);
    => -1 // not found
    _.findIndex([4, 6, 7, 12], isPrime);
    => 2

    findLastIndex_.findLastIndex(array, predicate, [context])
    Like _.findIndex but iterates the array in reverse, returning the index closest to the end where the predicate truth test passes.

    var users = [{'id': 1, 'name': 'Bob', 'last': 'Brown'},
                 {'id': 2, 'name': 'Ted', 'last': 'White'},
                 {'id': 3, 'name': 'Frank', 'last': 'James'},
                 {'id': 4, 'name': 'Ted', 'last': 'Jones'}];
    _.findLastIndex(users, {
      name: 'Ted'
    => 3

    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 (inclusive) to stop (exclusive), incremented (or decremented) by step, exclusive. Note that ranges that stop before they start are considered to be zero-length instead of negative — if you'd like a negative range, use a negative step.

    => [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]
    => []

    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, pass arguments to the function to pre-fill them, also known as partial application. For partial application without context binding, use partial.

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

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

    partial_.partial(function, *arguments)
    Partially apply a function by filling in any number of its arguments, without changing its dynamic this value. A close cousin of bind. You may pass _ in your list of arguments to specify an argument that should not be pre-filled, but left open to supply at call-time.

    var subtract = function(a, b) { return b - a; };
    sub5 = _.partial(subtract, 5);
    => 15
    // Using a placeholder
    subFrom20 = _.partial(subtract, _, 20);
    => 15

    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. The cache of memoized values is available as the cache property on the returned function.

    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, *arguments)
    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. If you pass the optional arguments, they will be forwarded on to the function when it is invoked.

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

    throttle_.throttle(function, wait, [options])
    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.

    By default, throttle will execute the function as soon as you call it for the first time, and, if you call it again any number of times during the wait period, as soon as that period is over. If you'd like to disable the leading-edge call, pass {leading: false}, and if you'd like to disable the execution on the trailing-edge, pass
    {trailing: false}.

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

    debounce_.debounce(function, wait, [immediate])
    Creates and returns a new debounced version of the passed function which 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.

    Pass true for the immediate argument to cause debounce to trigger the function on the leading instead of the trailing edge of the wait interval. Useful in circumstances like preventing accidental double-clicks on a "submit" button from firing a second time.

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

    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);
    // 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.

    before_.before(count, function)
    Creates a version of the function that can be called no more than count times. The result of the last function call is memoized and returned when count has been reached.

    var monthlyMeeting = _.before(3, askForRaise);
    // the result of any subsequent calls is the same as the second call

    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 a new negated version of the predicate function.

    var isFalsy = _.negate(Boolean);
    _.find([-2, -1, 0, 1, 2], isFalsy);
    => 0

    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.toUpperCase() + "!"; };
    var welcome = _.compose(greet, exclaim);
    => 'hi: MOE!'

    Object Functions

    Retrieve all the names of the object's own enumerable properties.

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

    Retrieve all the names of object's own and inherited properties.

    function Stooge(name) {
      this.name = name;
    Stooge.prototype.silly = true;
    _.allKeys(new Stooge("Moe"));
    => ["name", "silly"]

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

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

    mapObject_.mapObject(object, iteratee, [context])
    Like map, but for objects. Transform the value of each property in turn.

    _.mapObject({start: 5, end: 12}, function(val, key) {
      return val + 5;
    => {start: 10, end: 17}

    Convert an object into a list of [key, value] pairs.

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

    Returns a copy of the object where the keys have become the values and the values the keys. For this to work, all of your object's values should be unique and string serializable.

    _.invert({Moe: "Moses", Larry: "Louis", Curly: "Jerome"});
    => {Moses: "Moe", Louis: "Larry", Jerome: "Curly"};

    create_.create(prototype, props)
    Creates a new object with the given prototype, optionally attaching props as own properties. Basically, Object.create, but without all of the property descriptor jazz.

    var moe = _.create(Stooge.prototype, {name: "Moe"});

    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.

    => ["all", "any", "bind", "bindAll", "clone", "compact", "compose" ...

    findKey_.findKey(object, predicate, [context])
    Similar to _.findIndex but for keys in objects. Returns the key where the predicate truth test passes or undefined.

    extend_.extend(destination, *sources)
    Copy all of the properties in the source objects over to the destination object, and return 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}

    extendOwn_.extendOwn(destination, *sources) Alias: assign
    Like extend, but only copies own properties over to the destination object.

    pick_.pick(object, *keys)
    Return a copy of the object, filtered to only have values for the whitelisted keys (or array of valid keys). Alternatively accepts a predicate indicating which keys to pick.

    _.pick({name: 'moe', age: 50, userid: 'moe1'}, 'name', 'age');
    => {name: 'moe', age: 50}
    _.pick({name: 'moe', age: 50, userid: 'moe1'}, function(value, key, object) {
      return _.isNumber(value);
    => {age: 50}

    omit_.omit(object, *keys)
    Return a copy of the object, filtered to omit the blacklisted keys (or array of keys). Alternatively accepts a predicate indicating which keys to omit.

    _.omit({name: 'moe', age: 50, userid: 'moe1'}, 'userid');
    => {name: 'moe', age: 50}
    _.omit({name: 'moe', age: 50, userid: 'moe1'}, function(value, key, object) {
      return _.isNumber(value);
    => {name: 'moe', userid: 'moe1'}

    defaults_.defaults(object, *defaults)
    Fill in undefined properties in object with the first value present in the following list of defaults objects.

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

    Create a shallow-copied clone of the provided plain 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.

      .filter(function(num) { return num % 2 == 0; })
      .map(function(num) { return num * num })
    => // [2, 200] (alerted)
    => [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

    Returns a function that will itself return the key property of any passed-in object.

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

    Inverse of _.property. Takes an object and returns a function which will return the value of a provided property.

    var stooge = {name: 'moe'};
    => 'moe'

    matcher_.matcher(attrs) Alias: matches
    Returns a predicate function that will tell you if a passed in object contains all of the key/value properties present in attrs.

    var ready = _.matcher({selected: true, visible: true});
    var readyToGoList = _.filter(list, ready);

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

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

    isMatch_.isMatch(object, properties)
    Tells you if the keys and values in properties are contained in object.

    var stooge = {name: 'moe', age: 32};
    _.isMatch(stooge, {age: 32});
    => true

    Returns true if an enumerable object contains no values (no enumerable own-properties). For strings and array-like objects _.isEmpty checks if the length property is 0.

    _.isEmpty([1, 2, 3]);
    => false
    => 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 value is an Object. Note that JavaScript arrays and functions are objects, while (normal) strings and numbers are not.

    => true
    => false

    Returns true if object is an Arguments object.

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

    Returns true if object is a Function.

    => true

    Returns true if object is a String.

    => true

    Returns true if object is a Number (including NaN).

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

    Returns true if object is a finite Number.

    => true
    => false

    Returns true if object is either true or false.

    => false

    Returns true if object is a Date.

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

    Returns true if object is a RegExp.

    => true

    Returns true if object inherrits from an Error.

    try {
      throw new TypeError("Example");
    } catch (o_O) {
    => true

    Returns true if object is NaN.
    Note: this is not the same as the native isNaN function, which will also return true for many other not-number values, such as undefined.

    => true
    => true
    => false

    Returns true if the value of object is null.

    => true
    => false

    Returns true if value 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();

    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 iteratee.

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

    Creates a function that returns the same value that is used as the argument of _.constant.

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

    Returns undefined irrespective of the arguments passed to it. Useful as the default for optional callback arguments.

    obj.initialize = _.noop;

    times_.times(n, iteratee, [context])
    Invokes the given iteratee function n times. Each invocation of iteratee is called with an index argument. Produces an array of the returned values.
    Note: this example uses the chaining syntax.

    _(3).times(function(n){ genie.grantWishNumber(n); });

    random_.random(min, max)
    Returns a random integer between min and max, inclusive. If you only pass one argument, it will return a number between 0 and that number.

    _.random(0, 100);
    => 42

    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.

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

    iteratee_.iteratee(value, [context])
    A mostly-internal function to generate callbacks that can be applied to each element in a collection, returning the desired result — either identity, an arbitrary callback, a property matcher, or a property accessor.
    The full list of Underscore methods that transform predicates through _.iteratee is map, find, filter, reject, every, some, max, min, sortBy, groupBy, indexBy, countBy, sortedIndex, partition, and unique.

    var stooges = [{name: 'curly', age: 25}, {name: 'moe', age: 21}, {name: 'larry', age: 23}];
    _.map(stooges, _.iteratee('age'));
    => [25, 21, 23];

    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'

    Escapes a string for insertion into HTML, replacing &, <, &ggt;, ", `, and ' characters.

    _.escape('Curly, Larry & Moe');
    => "Curly, Larry &amp; Moe"

    The opposite of escape, replaces &amp;, &lt;, &gt;, &quot;, &#96; and &#x27; with their unescaped counterparts.

    _.unescape('Curly, Larry &amp; Moe');
    => "Curly, Larry & Moe"

    result_.result(object, property, [defaultValue])
    If the value of the named property is a function then invoke it with the object as context; otherwise, return it. If a default value is provided and the property doesn't exist or is undefined then the default will be returned. If defaultValue is a function its result will be returned.

    var object = {cheese: 'crumpets', stuff: function(){ return 'nonsense'; }};
    _.result(object, 'cheese');
    => "crumpets"
    _.result(object, 'stuff');
    => "nonsense"
    _.result(object, 'meat', 'ham');
    => "ham"

    Returns an integer timestamp for the current time, using the fastest method available in the runtime. Useful for implementing timing/animation functions.

    => 1392066795351

    template_.template(templateString, [settings])
    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 values, 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 data object that has properties corresponding to the template's free variables. The settings argument should be a hash containing any _.templateSettings that should be overridden.

    var compiled = _.template("hello: <%= name %>");
    compiled({name: 'moe'});
    => "hello: moe"
    var template = _.template("<b><%- value %></b>");
    template({value: '<script>'});
    => "<b>&lt;script&gt;</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!"

    By default, template places the values from your data in the local scope via the with statement. However, you can specify a single variable name with the variable setting. This can significantly improve the speed at which a template is able to render.

    _.template("Using 'with': <%= data.answer %>", {variable: 'data'})({answer: 'no'});
    => "Using 'with': no"

    Precompiling your templates can be a big help when debugging errors you can't reproduce. This is because precompiled templates can provide line numbers and a stack trace, something that is not possible when compiling templates on the client. The source property is available on the compiled template function for easy precompilation.

      JST.project = <%= _.template(jstText).source %>;


    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; });

    Calling chain will cause all future method calls to return wrapped objects. When you've finished the computation, call 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"}
      .map(function(line) { return line.words.split(' '); })
      .reduce(function(counts, word) {
        counts[word] = (counts[word] || 0) + 1;
        return counts;
      }, {})
    => {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.

    Returns a wrapped object. Calling methods on this object will continue to return wrapped objects until value is called.

    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; })
    => "moe is 21"

    Extracts the value of a wrapped object.

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