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Underscore Reference — Smooth CoffeeScript

This reference is an adaptation of the documentation at Underscore.js. It is interactive in its HTML5 form. Edit a CoffeeScript segment to try it. You can see the generated JavaScript when you write a CoffeeScript function by typing ‘show name’ after its definition.



Underscore is a library for functional style programming. It 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.

You can find more information and updates at Underscore.js. Extensions to Underscore are listed in the Mixin Catalog Wiki. Underscore is an open-source component of DocumentCloud.


Right-click, and use “Save As”

show "Underscore version #{_.VERSION} is used in this documentation"

Collection Functions


_.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 ], (num) -> show num
_.each {one : 1, two : 2, three : 3}, (num, key) -> show num

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.

show [ 1, 2, 3 ], (num) -> num * 3
one: 1
two: 2
three: 3
, (num, key) ->
num * 3


_.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.

show sum = _.reduce [1, 2, 3], ((memo, num) -> memo + num), 0


_.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.

list = [ [ 0, 1 ], [ 2, 3 ], [ 4, 5 ] ]
flat = _.reduceRight list, (a, b) ->
a.concat b
, []
show flat


_.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.

show even = _.find [1..6], (num) -> num % 2 is 0


_.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.

show evens = _.filter [1..6], (num) -> num % 2 is 0


_.reject list, iterator, [context]

Returns the values in list without the elements that the truth test (iterator) passes. The opposite of filter.

show odds = _.reject [1..6], (num) -> num % 2 is 0


_.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.

show _.all [true, 1, null, 'yes'], _.identity


_.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.

show _.any [null, 0, 'yes', false]


_.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.

show _.include [1, 2, 3], 3


_.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.

view _.invoke [[5, 1, 7], [3, 2, 1]], 'sort'


_.pluck list, propertyName

A convenient version of what is perhaps the most common use-case for map: extracting a list of property values.

stooges = [
{name : 'moe', age : 40}
{name : 'larry', age : 50}
{name : 'curly', age : 60}
show _.pluck stooges, 'name'


_.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.

stooges = [
{name : 'moe', age : 40}
{name : 'larry', age : 50}
{name : 'curly', age : 60}
view _.max stooges, (stooge) -> stooge.age


_.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.

numbers = [10, 5, 100, 2, 1000]
show _.min numbers


_.sortBy list, iterator, [context]

Returns a sorted copy of list, ranked in ascending order by the results of running each value through iterator.

show _.sortBy [1..6], (num) -> Math.sin num


_.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.

view _.groupBy [1.3, 2.1, 2.4], (num) -> Math.floor num
view _.groupBy ['one', 'two', 'three'], 'length'


_.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.

show _.sortedIndex [10, 20, 30, 40, 50], 35


_.shuffle list

Returns a shuffled copy of the list, using a version of the Fisher-Yates shuffle.

show _.shuffle [1..6]


_.toArray list

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

(-> show _.toArray(arguments).slice(0))(1, 2, 3)


_.size list

Return the number of values in the list.

show _.size {one : 1, two : 2, three : 3}

Array Functions

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


_.first array, [n] Alias: head

Returns the first element of an array. Passing n will return the first n elements of the array.

show _.first [5, 4, 3, 2, 1]


_.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.

view _.initial [5, 4, 3, 2, 1]


_.last array, [n]

Returns the last element of an array. Passing n will return the last n elements of the array.

show _.last [5, 4, 3, 2, 1]

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.

view [5, 4, 3, 2, 1]


_.compact array

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

view _.compact [0, 1, false, 2, '', 3]


_.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.

view _.flatten [1, [2], [3, [[4]]]]
view _.flatten [1, [2], [3, [[4]]]], true


_.without array, [*values]

Returns a copy of the array with all instances of the values removed. === is used for the equality test.

view _.without [1, 2, 1, 0, 3, 1, 4], 0, 1


_.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.

view _.union [1, 2, 3], [101, 2, 1, 10], [2, 1]


_.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.

view _.intersection [1, 2, 3], [101, 2, 1, 10], [2, 1]


_.difference array, *others

Similar to without, but returns the values from array that are not present in the other arrays.

view _.difference [1, 2, 3, 4, 5], [5, 2, 10]


_.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.

view _.uniq [1, 2, 1, 3, 1, 4]

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.

view ['moe', 'larry', 'curly'], [30, 40, 50], [true, false, false]


_.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.

show _.indexOf [1, 2, 3], 2


_.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.

show _.lastIndexOf [1, 2, 3, 1, 2, 3], 2


_.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.

view _.range 10
view _.range 1, 11
view _.range 0, 30, 5
view _.range 0, -10, -1
view _.range 0

Function Functions


_.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.

func = (greeting) -> greeting + ': ' +
func = _.bind func, {name : 'moe'}, 'hi'
show func()


_.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.

buttonView = {
label : 'underscore'
onClick : -> show 'clicked: ' + this.label
onHover : -> show 'hovering: ' + this.label
_.bindAll buttonView
jQuery('#underscore_button').bind 'click', buttonView.onClick


_.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.

timeIt = (func, a...) ->
before = new Date
result = func a...
show "Elapsed: #{new Date - before}ms"

fibonacci = _.memoize (n) ->
if n < 2 then n else fibonacci(n - 1) + fibonacci(n - 2)

show timeIt fibonacci, 1000
show timeIt fibonacci, 1000


_.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.

log = _.bind show, console ? window
_.delay log, 1, 'logged later'
# See the end of this document for the output


_.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 -> show 'deferred'
# See the end of this document for the output


_.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.

updatePosition = (evt) -> show "Position #{evt}"
throttled = _.throttle updatePosition, 100
for i in [0..10]
throttled i
# $(window).scroll throttled


_.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.

calculateLayout = -> show "It's quiet now"
lazyLayout = _.debounce calculateLayout, 100
# $(window).resize lazyLayout


_.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.

createApplication = -> show "Created"
initialize = _.once createApplication
# Application is only created once.


_.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.

skipFirst = _.after 3, show
for i in [0..3]
skipFirst i
# renderNotes is run once, after all notes have saved.
renderNotes = _.after notes.length, render
_.each notes, (note) ->
note.asyncSave {success: renderNotes}


_.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.

hello = (name) -> "hello: " + name
hello = _.wrap hello, (func) ->
"before, #{func "moe"}, after"
show hello()


_.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())).

greet    = (name) -> "hi: " + name
exclaim = (statement) -> statement + "!"
welcome = _.compose exclaim, greet
show welcome 'moe'

Object Functions


_.keys object

Retrieve all the names of the object’s properties.

show _.keys {one : 1, two : 2, three : 3}


_.values object

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

show _.values {one : 1, two : 2, three : 3}


_.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.

show _.functions _


_.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.

view _.extend {name : 'moe'}, {age : 50}


_.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.

iceCream = {flavor : "chocolate"}
view _.defaults iceCream, {flavor : "vanilla", sprinkles : "lots"}


_.clone object

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

view _.clone {name : 'moe'}


_.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.

show _.chain([1,2,3,200])
.filter((num) -> num % 2 is 0)
.map((num) -> num * num)


_.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.

show _.has a: 1, b: 2, c: 3, 'b'


_.isEqual object, other

Performs an optimized deep comparison between the two objects, to determine if they should be considered equal.

moe   = {name : 'moe', luckyNumbers : [13, 27, 34]}
clone = {name : 'moe', luckyNumbers : [13, 27, 34]}
moe is clone
show _.isEqual(moe, clone)


_.isEmpty object

Returns true if object contains no values.

show _.isEmpty([1, 2, 3])
show _.isEmpty({})


_.isElement object

Returns true if object is a DOM element.

show _.isElement document?.getElementById 'page'


_.isArray object

Returns true if object is an Array.

show (-> _.isArray arguments)()
show _.isArray [1,2,3]


_.isArguments object

Returns true if object is an Arguments object.

show (-> _.isArguments arguments)(1, 2, 3)
show _.isArguments [1,2,3]


_.isFunction object

Returns true if object is a Function.

show _.isFunction console?.debug


_.isString object

Returns true if object is a String.

show _.isString "moe"


_.isNumber object

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

show _.isNumber 8.4 * 5


_.isBoolean object

Returns true if object is either true or false.

show _.isBoolean null


_.isDate object

Returns true if object is a Date.

show _.isDate new Date()


_.isRegExp object

Returns true if object is a RegExp.

show _.isRegExp /moe/


_.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.

show _.isNaN NaN
show isNaN undefined
show _.isNaN undefined


_.isNull object

Returns true if the value of object is null.

show _.isNull null
show _.isNull undefined


_.isUndefined variable

Returns true if variable is undefined.

show _.isUndefined window?.missingVariable

Utility Functions



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

# The examples will stop working if this is enabled
# underscore = _.noConflict()


_.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.

moe = {name : 'moe'}
show moe is _.identity(moe)


_.times n, iterator

Invokes the given iterator function n times.

(genie = {}).grantWish = -> show 'Served'
_(3).times -> genie.grantWish()


_.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.

capitalize : (string) ->
string.charAt(0).toUpperCase() +
show _("fabio").capitalize()


_.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.

show _.uniqueId 'contact_'
show _.uniqueId 'contact_'


_.escape string

Escapes a string for insertion into HTML, replacing &, <, >, ", ', and / characters.

show _.escape 'Curly, Larry & Moe'


_.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.

compiled = _.template "hello: <%= name %>"
show compiled name : 'moe'
list = "<% _.each(people, function(name) { %> <li><%= name %></li> <% }); %>"
show _.escape _.template list, people : ['moe', 'curly', 'larry']
template = _.template "<b><%- value %></b>"
show _.escape template value : '<script>'

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

compiled = _.template "<% print('Hello ' + epithet) %>"
show compiled {epithet: "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:

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

template = _.template "Hello {{ name }}!"
show template name : "Mustache"

_.templateSettings = saveSettings


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.

show [ 1, 2, 3 ], (n) -> n * 2
show _([ 1, 2, 3 ]).map (n) -> 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.

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"}
view _.chain(lyrics)
.map((line) -> line.words.split " ")
.reduce(((counts, word) ->
counts[word] = (counts[word] or 0) + 1
counts), {}).value()

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 used.

stooges = [
{name : 'curly', age : 25}
{name : 'moe', age : 21}
{name : 'larry', age : 23}
youngest = _.chain(stooges)
.sortBy((stooge) -> stooge.age)
.map((stooge) -> + ' is ' + stooge.age)
show youngest



Extracts the value of a wrapped object.

show _([1, 2, 3]).value()

The end

show 'Delayed output will show up here'

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Underscore is under an MIT license © 2011