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Collection Functions (Arrays or Objects)

each_.each(list, iteratee, [context])Alias:forEachsource
Iterates over alist of elements, yielding each in turn to aniteratee function. Theiteratee is bound to thecontext object, if one is passed. Each invocation ofiteratee is called with three arguments:(element, index, list). Iflist is a JavaScript object,iteratee's arguments will be(value, key, list). Returns thelist 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 asarguments,NodeList and similar. But it works by duck-typing, so avoid passing objects with a numericlength property. It's also good to note that aneach loop cannot be broken out of — to break, use_.find instead.

map_.map(list, iteratee, [context])Alias:collectsource
Produces a new array of values by mapping each value inlist through a transformation function (iteratee). The iteratee is passed three arguments: thevalue, then theindex (orkey) of the iteration, and finally a reference to the entirelist.

_.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,foldlsource
Also known asinject andfoldl, reduce boils down alist of values into a single value.Memo is the initial state of the reduction, and each successive step of it should be returned byiteratee. The iteratee is passed four arguments: thememo, then thevalue andindex (or key) of the iteration, and finally a reference to the entirelist.

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:foldrsource
The right-associative version ofreduce.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:detectsource
Looks through each value in thelist, returning the first one that passes a truth test (predicate), orundefined if no value passes the test. The function returns as soon as it finds an acceptable element, and doesn't traverse the entire list.predicate is transformed throughiteratee to facilitate shorthand syntaxes.

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

filter_.filter(list, predicate, [context])Alias:selectsource
Looks through each value in thelist, returning an array of all the values that pass a truth test (predicate).predicate is transformed throughiteratee to facilitate shorthand syntaxes.

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

findWhere_.findWhere(list, properties)source
Looks through thelist and returns thefirst value thatmatches all of the key-value pairs listed inproperties.

If no match is found, or iflist 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."}

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

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

reject_.reject(list, predicate, [context])source
Returns the values inlist without the elements that the truth test (predicate) passes. The opposite offilter.predicate is transformed throughiteratee to facilitate shorthand syntaxes.

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

every_.every(list, [predicate], [context])Alias:allsource
Returnstrue if all of the values in thelist pass thepredicate truth test. Short-circuits and stops traversing the list if a false element is found.predicate is transformed throughiteratee to facilitate shorthand syntaxes.

_.every([2, 4, 5], function(num) { return num % 2 == 0; });=> false

some_.some(list, [predicate], [context])Alias:anysource
Returnstrue if any of the values in thelist pass thepredicate truth test. Short-circuits and stops traversing the list if a true element is found.predicate is transformed throughiteratee to facilitate shorthand syntaxes.

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

contains_.contains(list, value, [fromIndex])Aliases:include,includessource
Returnstrue if thevalue is present in thelist. UsesindexOf internally, iflist is an Array. UsefromIndex to start your search at a given index.

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

invoke_.invoke(list, methodName, *arguments)source
Calls the method named bymethodName on each value in thelist. Any extra arguments passed toinvoke 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)source
A convenient version of what is perhaps the most common use-case formap: 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])source
Returns the maximum value inlist. If aniteratee function is provided, it will be used on each value to generate the criterion by which the value is ranked.-Infinity is returned iflist is empty, so anisEmpty guard may be required. This function can currently only compare numbers reliably. This function uses operator< (note).

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])source
Returns the minimum value inlist. If aniteratee function is provided, it will be used on each value to generate the criterion by which the value is ranked.Infinity is returned iflist is empty, so anisEmpty guard may be required. This function can currently only compare numbers reliably. This function uses operator< (note).

var numbers = [10, 5, 100, 2, 1000];_.min(numbers);=> 2

sortBy_.sortBy(list, iteratee, [context])source
Returns a (stably) sorted copy oflist, ranked in ascending order by the results of running each value throughiteratee. iteratee may also be the string name of the property to sort by (eg.length). This function uses operator< (note).

_.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])source
Splits a collection into sets, grouped by the result of running each value throughiteratee. Ifiteratee is a string instead of a function, groups by the property named byiteratee 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])source
Given alist, and aniteratee 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 likegroupBy, 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])source
Sorts a list into groups and returns a count for the number of objects in each group. Similar togroupBy, 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}

shuffle_.shuffle(list)source
Returns a shuffled copy of thelist, using a version of theFisher-Yates shuffle.

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

sample_.sample(list, [n])source
Produce a random sample from thelist. Pass a number to returnn 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]

toArray_.toArray(list)source
Creates a real Array from thelist (anything that can be iterated over). Useful for transmuting thearguments object.

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

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

_.size([1, 2, 3, 4, 5]);=> 5_.size({one: 1, two: 2, three: 3});=> 3

partition_.partition(list, predicate)source
Splitlist into two arrays: one whose elements all satisfypredicate and one whose elements all do not satisfypredicate.predicate is transformed throughiteratee to facilitate shorthand syntaxes.

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

compact_.compact(list)source
Returns a copy of thelist with all falsy values removed. In JavaScript,false,null,0,"",undefined andNaN are all falsy.

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

Array Functions

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

first_.first(array, [n])Aliases:head,takesource
Returns the first element of anarray. Passingn will return the firstn elements of the array.

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

initial_.initial(array, [n])source
Returns everything but the last entry of the array. Especially useful on the arguments object. Passn to exclude the lastn elements from the result.

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

last_.last(array, [n])source
Returns the last element of anarray. Passingn will return the lastn elements of the array.

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

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

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

flatten_.flatten(array, [depth])source
Flattens a nestedarray. If you passtrue or1 as thedepth, the array will only be flattened a single level. Passing a greater number will cause the flattening to descend deeper into the nesting hierarchy. Omitting thedepth argument, or passingfalse orInfinity, flattens the array all the way to the deepest nesting level.

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

without_.without(array, *values)source
Returns a copy of thearray with all instances of thevalues removed.

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

union_.union(*arrays)source
Computes the union of the passed-inarrays: the list of unique items, in order, that are present in one or more of thearrays.

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

intersection_.intersection(*arrays)source
Computes the list of values that are the intersection of all thearrays. Each value in the result is present in each of thearrays.

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

difference_.difference(array, *others)source
Similar towithout, but returns the values fromarray that are not present in theother arrays.

_.difference([1, 2, 3, 4, 5], [5, 2, 10]);=> [1, 3, 4]

uniq_.uniq(array, [isSorted], [iteratee])Alias:uniquesource
Produces a duplicate-free version of thearray, using=== to test object equality. In particular only the first occurrence of each value is kept. If you know in advance that thearray is sorted, passingtrue forisSorted will run a much faster algorithm. If you want to compute unique items based on a transformation, pass aniteratee function.

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

zip_.zip(*arrays)source
Merges together the values of each of thearrays with the values at the corresponding position. Useful when you have separate data sources that are coordinated through matching array indexes.

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

unzip_.unzip(array)Alias:transposesource
The opposite ofzip. Given anarray 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. If you're working with a matrix of nested arrays, this can be used to transpose the matrix.

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

object_.object(list, [values])source
Converts arrays into objects. Pass either a single list of[key, value] pairs, or a list of keys, and a list of values. Passing by pairs is the reverse ofpairs. 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}

chunk_.chunk(array, length)source
Chunks anarray into multiple arrays, each containinglength or fewer items.

var partners = _.chunk(_.shuffle(kindergarten), 2);=> [["Tyrone", "Elie"], ["Aidan", "Sam"], ["Katrina", "Billie"], ["Little Timmy"]]

indexOf_.indexOf(array, value, [isSorted])source
Returns the index at whichvalue can be found in thearray, or-1 if value is not present in thearray. If you're working with a large array, and you know that the array is already sorted, passtrue forisSorted 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. IfisSorted istrue, this function uses operator< (note).

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

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

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

sortedIndex_.sortedIndex(array, value, [iteratee], [context])source
Uses a binary search to determine the smallest index at which thevalueshould be inserted into thearray in order to maintain thearray's sorted order. If aniteratee function is provided, it will be used to compute the sort ranking of each value, including thevalue you pass. The iteratee may also be the string name of the property to sort by (eg.length). This function uses operator< (note).

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

findIndex_.findIndex(array, predicate, [context])source
Similar to_.indexOf, returns the first index where thepredicate 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])source
Like_.findIndex but iterates the array in reverse, returning the index closest to the end where thepredicate 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])source
A function to create flexibly-numbered lists of integers, handy foreach andmap loops.start, if omitted, defaults to0;step defaults to1 ifstart is beforestop, otherwise-1. Returns a list of integers fromstart (inclusive) tostop (exclusive), incremented (or decremented) bystep.

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

Function (uh, ahem) Functions

bind_.bind(function, object, *arguments)source
Bind afunction to anobject, meaning that whenever the function is called, the value ofthis will be theobject. Optionally, passarguments to thefunction to pre-fill them, also known aspartial application. For partial application without context binding, usepartial.

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

bindAll_.bindAll(object, *methodNames)source
Binds a number of methods on theobject, specified bymethodNames, 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 uselessthis.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').on('click', buttonView.onClick);

partial_.partial(function, *arguments)source
Partially apply a function by filling in any number of itsarguments,without changing its dynamicthis value. A close cousin ofbind. You may pass_ in your list ofarguments to specify an argument that should not be pre-filled, but left open to supply at call-time.Note: if you need_ placeholders and athis binding at the same time, use both_.partial and_.bind.

var subtract = function(a, b) { return b - a; };sub5 = _.partial(subtract, 5);sub5(20);=> 15// Using a placeholdersubFrom20 = _.partial(subtract, _, 20);subFrom20(5);=> 15

memoize_.memoize(function, [hashFunction])source
Memoizes a givenfunction by caching the computed result. Useful for speeding up slow-running computations. If passed an optionalhashFunction, it will be used to compute the hash key for storing the result, based on the arguments to the original function. The defaulthashFunction just uses the first argument to the memoized function as the key. The cache of memoized values is available as thecache 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)source
Much likesetTimeout, invokesfunction afterwait milliseconds. If you pass the optionalarguments, they will be forwarded on to thefunction 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)source
Defers invoking thefunction until the current call stack has cleared, similar to usingsetTimeout 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 optionalarguments, they will be forwarded on to thefunction when it is invoked.

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

throttle_.throttle(function, wait, [options])source
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 everywait 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 thewait 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);$(window).scroll(throttled);

If you need to cancel a scheduled throttle, you can call.cancel() on the throttled function.

debounce_.debounce(function, wait, [immediate])source
Creates and returns a new debounced version of the passed function which will postpone its execution until afterwait milliseconds have elapsed since the last time it was invoked. Useful for implementing behavior that should only happenafter 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.

At the end of thewait interval, the function will be called with the arguments that were passedmost recently to the debounced function.

Passtrue for theimmediate argument to causedebounce to trigger the function on the leading instead of the trailing edge of thewait interval. Useful in circumstances like preventing accidental double-clicks on a "submit" button from firing a second time.

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

If you need to cancel a scheduled debounce, you can call.cancel() on the debounced function.

once_.once(function)source
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)source
Creates a wrapper offunction that does nothing at first. From thecount-th call onwards, it starts actually callingfunction. 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)source
Creates a wrapper offunction that memoizes its return value. From thecount-th call onwards, the memoized result of the last invocation is returned immediately instead of invokingfunction again. So the wrapper will invokefunction at mostcount - 1 times.

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

wrap_.wrap(function, wrapper)source
Wraps the firstfunction inside of thewrapper function, passing it as the first argument. This allows thewrapper to execute code before and after thefunction 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'

negate_.negate(predicate)source
Returns a new negated version of thepredicate function.

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

compose_.compose(*functions)source
Returns the composition of a list offunctions, where each function consumes the return value of the function that follows. In math terms, composing the functionsf(),g(), andh() producesf(g(h())).

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

restArguments_.restArguments(function, [startIndex])source
Returns a version of thefunction that, when called, receives all arguments from and beyondstartIndex collected into a single array. If you don’t pass an explicitstartIndex, it will be determined by looking at the number of arguments to thefunction itself. Similar to ES6’srest parameters syntax.

var raceResults = _.restArguments(function(gold, silver, bronze, everyoneElse) {  _.each(everyoneElse, sendConsolations);});raceResults("Dopey", "Grumpy", "Happy", "Sneezy", "Bashful", "Sleepy", "Doc");

Object Functions

keys_.keys(object)source
Retrieve all the names of theobject's own enumerable properties.

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

allKeys_.allKeys(object)source
Retrieveall the names ofobject's own and inherited properties.

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

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

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

mapObject_.mapObject(object, iteratee, [context])source
Likemap, 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}

pairs_.pairs(object)source
Convert an object into a list of[key, value] pairs. The opposite ofobject.

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

invert_.invert(object)source
Returns a copy of theobject 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)source
Creates a new object with the given prototype, optionally attachingprops asown properties. Basically,Object.create, but without all of the property descriptor jazz.

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

functions_.functions(object)Alias:methodssource
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" ...

findKey_.findKey(object, predicate, [context])source
Similar to_.findIndex but for keys in objects. Returns thekey where thepredicate truth test passes orundefined.predicate is transformed throughiteratee to facilitate shorthand syntaxes.

extend_.extend(destination, *sources)source
Shallowly copy all of the propertiesin thesource objects over to thedestination object, and return thedestination object. Any nested objects or arrays will be copied by reference, not duplicated. 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:assignsource
Likeextend, but only copiesown properties over to the destination object.

pick_.pick(object, *keys)source
Return a copy of theobject, filtered to only have values for the allowedkeys (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)source
Return a copy of theobject, filtered to omit the disallowedkeys (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)source
Returnsobject after filling in itsundefined properties with the first value present in the following list ofdefaults objects.

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

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

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

tap_.tap(object, interceptor)source
Invokesinterceptor with theobject, and then returnsobject. 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(alert)  .map(function(num) { return num * num })  .value();=> // [2, 200] (alerted)=> [4, 40000]

toPath_.toPath(path)source
Ensures thatpath is an array. Ifpath is a string, it is wrapped in a single-element array; if it is an array already, it is returned unmodified.

_.toPath('key');=> ['key']_.toPath(['a', 0, 'b']);=> ['a', 0, 'b'] // (same array)

_.toPath is used internally inhas,get,invoke,property,propertyOf andresult, as well as initeratee and all functions that depend on it, in order to normalize deep property paths. You can override_.toPath if you want to customize this behavior, for example to enable Lodash-like string path shorthands. Be advised that altering_.toPath will unavoidably cause some keys to become unreachable; override at your own risk.

// Support dotted path shorthands.var originalToPath = _.toPath;_.mixin({  toPath: function(path) {    return _.isString(path) ? path.split('.') : originalToPath(path);  }});_.get({a: [{b: 5}]}, 'a.0.b');=> 5

get_.get(object, path, [default])source
Returns the specified property ofobject.path may be specified as a simple key, or as an array of object keys or array indexes, for deep property fetching. If the property does not exist or isundefined, the optionaldefault is returned.

_.get({a: 10}, 'a');=> 10_.get({a: [{b: 2}]}, ['a', 0, 'b']);=> 2_.get({a: 10}, 'b', 100);=> 100

has_.has(object, key)source
Does the object contain the given key? Identical toobject.hasOwnProperty(key), but uses a safe reference to thehasOwnProperty function, in case it's beenoverridden accidentally.

_.has({a: 1, b: 2, c: 3}, "b");=> true

property_.property(path)source
Returns a function that will return the specified property of any passed-in object.path may be specified as a simple key, or as an array of object keys or array indexes, for deep property fetching.

var stooge = {name: 'moe'};'moe' === _.property('name')(stooge);=> truevar stooges = {moe: {fears: {worst: 'Spiders'}}, curly: {fears: {worst: 'Moe'}}};var curlysWorstFear = _.property(['curly', 'fears', 'worst']);curlysWorstFear(stooges);=> 'Moe'

propertyOf_.propertyOf(object)source
Inverse of_.property. Takes an object and returns a function which will return the value of a provided property.

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

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

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

isEqual_.isEqual(object, other)source
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)source
Tells you if the keys and values inproperties are contained inobject.

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

isEmpty_.isEmpty(collection)source
Returnstrue ifcollection has no elements. For strings and array-like objects_.isEmpty checks if the length property is 0. For other objects, it returnstrue if the object has no enumerable own-properties. Note that primitive numbers, booleans and symbols are always empty by this definition.

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

isElement_.isElement(object)source
Returnstrue ifobject is a DOM element.

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

isArray_.isArray(object)source
Returnstrue ifobject is an Array.

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

isObject_.isObject(value)source
Returnstrue ifvalue is an Object. Note that JavaScript arrays and functions are objects, while (normal) strings and numbers are not.

_.isObject({});=> true_.isObject(1);=> false

isArguments_.isArguments(object)source
Returnstrue ifobject is an Arguments object.

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

isFunction_.isFunction(object)source
Returnstrue ifobject is a Function.

_.isFunction(alert);=> true

isString_.isString(object)source
Returnstrue ifobject is a String.

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

isNumber_.isNumber(object)source
Returnstrue ifobject is a Number (includingNaN).

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

isFinite_.isFinite(object)source
Returnstrue ifobject is a finite Number.

_.isFinite(-101);=> true_.isFinite(-Infinity);=> false

isBoolean_.isBoolean(object)source
Returnstrue ifobject is eithertrue orfalse.

_.isBoolean(null);=> false

isDate_.isDate(object)source
Returnstrue ifobject is a Date.

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

isRegExp_.isRegExp(object)source
Returnstrue ifobject is a RegExp.

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

isError_.isError(object)source
Returnstrue ifobject inherits from an Error.

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

isSymbol_.isSymbol(object)source
Returnstrue ifobject is aSymbol.

_.isSymbol(Symbol());=> true

isMap_.isMap(object)source
Returnstrue ifobject is aMap.

_.isMap(new Map());=> true

isWeakMap_.isWeakMap(object)source
Returnstrue ifobject is aWeakMap.

_.isWeakMap(new WeakMap());=> true

isSet_.isSet(object)source
Returnstrue ifobject is aSet.

_.isSet(new Set());=> true

isWeakSet_.isWeakSet(object)source
Returnstrue ifobject is aWeakSet.

_.isWeakSet(WeakSet());=> true

isArrayBuffer_.isArrayBuffer(object)source
Returnstrue ifobject is anArrayBuffer.

_.isArrayBuffer(new ArrayBuffer(8));=> true

isDataView_.isDataView(object)source
Returnstrue ifobject is aDataView.

_.isDataView(new DataView(new ArrayBuffer(8)));=> true

isTypedArray_.isTypedArray(object)source
Returnstrue ifobject is aTypedArray.

_.isTypedArray(new Int8Array(8));=> true

isNaN_.isNaN(object)source
Returnstrue ifobject isNaN.
Note: this is not the same as the nativeisNaN function, which will also return true for many other not-number values, such asundefined.

_.isNaN(NaN);=> trueisNaN(undefined);=> true_.isNaN(undefined);=> false

isNull_.isNull(object)source
Returnstrue if the value ofobject isnull.

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

isUndefined_.isUndefined(value)source
Returnstrue ifvalue isundefined.

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

Utility Functions

noConflict_.noConflict()source
Give control of the global_ variable back to its previous owner. Returns a reference to theUnderscore object.

var underscore = _.noConflict();

The_.noConflict function is not present if you use the EcmaScript 6, AMD or CommonJS module system to import Underscore.

identity_.identity(value)source
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

constant_.constant(value)source
Creates a function that returns the same value that is used as the argument of_.constant.

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

noop_.noop()source
Returnsundefined irrespective of the arguments passed to it. Useful as the default for optional callback arguments.

obj.initialize = _.noop;

times_.times(n, iteratee, [context])source
Invokes the given iteratee functionn times. Each invocation ofiteratee is called with anindex argument. Produces an array of the returned values.

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

random_.random(min, max)source
Returns a random integer betweenmin andmax, inclusive. If you only pass one argument, it will return a number between0 and that number.

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

mixin_.mixin(object)source
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. Returns the Underscore object to facilitate chaining.

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

iteratee_.iteratee(value, [context])source
Generates a callback that can be applied to each element in a collection._.iteratee supports a number of shorthand syntaxes for common callback use cases. Depending uponvalue's type,_.iteratee will return:

// No value_.iteratee();=> _.identity()// Function_.iteratee(function(n) { return n * 2; });=> function(n) { return n * 2; }// Object_.iteratee({firstName: 'Chelsea'});=> _.matcher({firstName: 'Chelsea'});// Anything else_.iteratee('firstName');=> _.property('firstName');

The following Underscore methods transform their predicates through_.iteratee:countBy,every,filter,find,findIndex,findKey,findLastIndex,groupBy,indexBy,map,mapObject,max,min,partition,reject,some,sortBy,sortedIndex, anduniq

You may overwrite_.iteratee with your own custom function, if you want additional or different shorthand syntaxes:

// Support `RegExp` predicate shorthand.var builtinIteratee = _.iteratee;_.iteratee = function(value, context) {  if (_.isRegExp(value)) return function(obj) { return value.test(obj) };  return builtinIteratee(value, context);};

uniqueId_.uniqueId([prefix])source
Generate a globally-unique id for client-side models or DOM elements that need one. Ifprefix is passed, the id will be appended to it.

_.uniqueId('contact_');=> 'contact_104'

escape_.escape(string)source
Escapes a string for insertion into HTML, replacing&,<,>,",`, and' characters.

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

unescape_.unescape(string)source
The opposite ofescape, replaces&amp;,&lt;,&gt;,&quot;,&#x60; and&#x27; with their unescaped counterparts.

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

result_.result(object, property, [defaultValue])source
If the value of the namedproperty is a function then invoke it with theobject 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. IfdefaultValue 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"

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

_.now();=> 1392066795351

template_.template(templateString, [settings])source
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 adata object that has properties corresponding to the template's free variables. Thesettings 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 useprint 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 aninterpolate regex to match expressions that should be interpolated verbatim, anescape regex to match expressions that should be inserted after being HTML-escaped, and anevaluate regex to match expressions that should be evaluated without insertion into the resulting string. Note that if part of your template matches more than one of these regexes, the first will be applied by the following order of priority: (1)escape, (2)interpolate, (3)evaluate. You may define or omit any combination of the three. For example, to performMustache.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 thewith statement. However, you can specify a single variable name with thevariable 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. Thesource property is available on the compiled template function for easy precompilation.

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

VERSION
It is possible to get the current Underscore version via_.VERSION .

_.VERSION => 1.13.7

Object-Oriented Style

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.source,source

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

Chaining

Callingchain will cause all future method calls to return wrapped objects. When you've finished the computation, callvalue to retrieve the final value. Here's an example of chaining together amap/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, theArray prototype's methods are proxied through the chained Underscore object, so you can slip areverse or apush into your chain, and continue to modify the array.

chain_.chain(obj)source
Returns a wrapped object. Calling methods on this object will continue to return wrapped objects untilvalue 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; })  .first()  .value();=> "moe is 21"

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

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

Links & Suggested Reading

Underscore.lua, a Lua port of the functions that are applicable in both languages. Includes OOP-wrapping and chaining. (source)

Dollar.swift, a Swift port of many of the Underscore.js functions and more. (source)

Underscore.m, an Objective-C port of many of the Underscore.js functions, using a syntax that encourages chaining. (source)

_.m, an alternative Objective-C port that tries to stick a little closer to the original Underscore.js API. (source)

Underscore.php, a PHP port of the functions that are applicable in both languages. Tailored for PHP 5.4 and made with data-type tolerance in mind. (source)

Underscore-perl, a Perl port of many of the Underscore.js functions, aimed at on Perl hashes and arrays. (source)

Underscore.cfc, a Coldfusion port of many of the Underscore.js functions. (source)

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

Underscore-java, a java port of the functions that are applicable in both languages. Includes OOP-wrapping and chaining. (source)

Ruby'sEnumerable module.

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

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

Michael Aufreiter'sData.js, a data manipulation + persistence library for JavaScript.

Python'sitertools.

PyToolz, a Python port that extends itertools and functools to include much of the Underscore API.

Funcy, a practical collection of functional helpers for Python, partially inspired by Underscore.

Notes

On the use of< in Underscore
Underscore functions that depend on ordering, such as_.sortBy and_.sortedIndex, use JavaScript’s built-inrelational operators, specifically the “less than” operator<. It is important to understand that these operators are only meaningful for numbers and strings. You can throw any value to them, but JavaScript will convert the operands to string or number first before performing the actual comparison. If you pass an operand that cannot be meaningfully converted to string or number, it ends up beingNaN by default. This value is unsortable.

Ideally, the values that you are sorting should either be all (meaningfully convertible to) strings or all (meaningfully convertible to) numbers. If this is not the case, you have two options:

• _.filter out all unsortable values first.
• Pick a target type, i.e., either string or number, and pass aniteratee to your Underscore function that will convert its argument to a sensible instance of the target type. For example, if you have an array of numbers that you want to sort and that may occasionally containnull orundefined, you can control whether you want to sort these before or after all numbers by passing aniteratee to_.sortBy that returns-Infinity or+Infinity for such values, respectively. Or maybe you want to treat them as zeros; it is up to you. The sameiteratee can also be passed to other Underscore functions to ensure that the behavior is consistent.

Change Log

1.13.7 —July 24, 2024 —Diff —Docs

• Fixes a bug where Underscore might throw an error on load if the executing environment overrides the nativeDataView.
• Adds a direct link to the corresponding source code for each function in the documentation.
• Clarifies the documentation for thestep argument of therange function.
• Adds dark mode support to the home page.
• Other infrastructural improvements to funding, testing and building.

1.13.6 —September 24, 2022 —Diff —Docs
Hotfix for version 1.13.5 to remove apostinstall script from the package.json, which unexpectedly broke many people's builds.

1.13.5 —September 23, 2022 —Diff —Docs

• Adds amodule sub-entry to the package.json’sexports.require condition. When a bundling tool, such as Rollup with recent versions of@rollup/plugin-node-resolve, takes the exports map very literally, this should prevent situations in which the final bundle includes multiple copies of Underscore in different module formats.
• Updates to the testing infrastructure and development dependencies.
• No code changes.

1.13.4 —June 2, 2022 —Diff —Docs

• Fixes a compatibility issue with WebPack module federation.
• Documentation improvements.

1.13.3 —April 23, 2022 —Diff —Docs

• Fixes a compatibility issue with ExtendScript.
• Various improvements to testing and continuous integration, including the addition of security scanning and a reduced carbon footprint.

1.13.2 —December 16, 2021 —Diff —Docs

• Fixes a regression introduced in 1.9.0 that caused_.sample and_.shuffle to no longer work on strings.
• Fixes an issue in IE8 compatibility code.
• Makes the website mobile-friendly.
• Various other minor documentation enhancements and a new test.

1.13.1 —April 15, 2021 —Diff —Docs

• Restores theunderscore.js alias committed to the GitHub repository.
• Adds some build clarifications to the documentation.
• No code changes.

1.13.0 —April 9, 2021 —Diff —Docs

• Merges the changes from the 1.13.0-0 through 1.13.0-3 preview releases into the main release stream following version 1.12.1. As of this release, ESM support is 100%.
• Adds asecurity policy to the documentation.
• Adds funding information to the documentation.

1.13.0-3 —March 31, 2021 —Diff —Docs

• Adds a"module" exports condition to thepackage.json, which should theoretically help to avoid duplicate code bundling withexports-aware build tools.
• Re-synchronizes some comments and documentation text with the 1.12.x branch.

1.13.0-2 —March 15, 2021 —Diff —Docs

• Fixes the same security issue in_.template as the parallel 1.12.1 release.

1.12.1 —March 15, 2021 —Diff —Docs

• Fixes a security issue in_.template that could enable a third party to inject code in compiled templates. This issue affects all versions of Underscore between 1.3.2 and 1.12.0, inclusive, as well as preview releases 1.13.0-0 and 1.13.0-1. The fix in this release is also included in the parallel preview release 1.13.0-2.CVE-2021-23358
• Restores an optimization in_.debounce that was unintentionally lost in version 1.9.0 (same as in parallel preview release 1.13.0-0).
• Various test and documentation enhancements (same as in parallel preview releases 1.13.0-0 and 1.13.0-1).

1.13.0-1 —March 11, 2021 —Diff —Docs

• Fixes an issue that caused aliases to be absent among the named exports in the new native ESM entry point for Node.js 12+.
• More test and documentation fixes and enhancements.

1.13.0-0 —March 9, 2021 —Diff —Docs

• Adds experimental support for native ESM imports in Node.js. You can now also do named imports or even deep module imports directly from a Node.js process in Node.js version 12 and later. Monolithic imports are recommended for use in production. State (such as mixed-in functions) is shared between CommonJS and ESM consumers.
• Renames the UMD bundle tounderscore-umd.js for consistency with the other bundle names. An alias namedunderscore.js is retained for backwards compatibility.
• Restores an optimization in_.debounce that was unintentionally lost in version 1.9.0.
• Various test and documentation enhancements.

1.12.0 —November 24, 2020 —Diff —Docs

• Adds the_.get and_.toPath functions. The latter can be overridden in order to customize the interpretation of deep property paths throughout Underscore. A future version of Underscore-contrib will be providing a ready-made function for this purpose; users will be able to opt in to string-based path shorthands such as'a.0.b' and'a[0]["b"]' by using that function from Underscore-contrib to override_.toPath.
• Fixes a bug in_.isEqual that caused typed arrays to compare equal when viewing different segments of the same underlyingArrayBuffer.
• Improves the compatibility of_.isEqual,_.isDataView,_.isMap,_.isWeakMap and_.isSet with some older browsers, especially IE 11.
• Significantly enhances the performance of_.isEmpty and several members of the isType family of functions.
• Speeds up_.isEqual comparison of typed arrays andDataViews with identialbuffer,byteOffset andbyteLength.
• Restores cross-browser testing during continuous integration to its former glory and adds documentation about engine compatibility.
• Slims down the development dependencies for testing.

1.11.0 —August 28, 2020 —Diff —Docs —Article

• Puts the source of every function in a separate module, following up on the move to EcmaScript 6export notation inversion 1.10.0. AMD and CommonJS versions of the function modules are provided as well. This brings perfect treeshaking to all users and unlocks the possibility to create arbitrary custom Underscore builds without code size overhead.modules/index.js is still present and the UMD bundle is still recommended for most users.
  Since the modularization obfuscates the diff, piecewise diffs are provided below.
  • Changes before modularization
  • Modularization itself
  • Changes after modularization
• Adds a monolithic bundle in EcmaScript 6 module format,underscore-esm.js, as a modern alternative to the monolithic UMD bundle. Users who want to use ES module imports in the browser are advised to use this new bundle instead ofmodules/index.js, becauseunderscore-esm.js provides the complete Underscore interface in a single download.
• Adds a modular version of the annotated source, reflecting the full internal structure of the primary source code.
• Adds_.isArrayBuffer,_.isDataView and_.isTypedArray functions, as well as support for the corresponding value types to_.isEqual.
• Adds the option to flatten arrays to a specific depth:_.flatten(anArray, 3).
• Adds_.transpose as an alias to_.unzip.
• Fixes an inconsistency whereArray.prototype methods on the Underscore wrapper would error when the wrapped value isnull orundefined. These methods now perform a no-op on null values like the other Underscore functions.
• Fixes a bug that caused_.first and_.last to return[] instead ofundefined for empty arrays when used as an iteratee.
• Fixes a regression introduced in version 1.9.0 that caused_.bindAll to returnundefined instead of the bound object.
• Restores continuous integration testing with Travis CI.
• Replaces stigmatizing “whitelist”/“blacklist” terminology in comments and documentation by neutral “allowed”/“disallowed” terminology.
• Various clarifications and minor enhancements and fixes to the documentation, source comments and a test.

1.10.2 —March 30, 2020 —Diff —Docs

• Fixes a bug introduced with1.10.0, while using the legacy Node.js require API:var _ = require("underscore")._

1.10.1 —March 30, 2020 —Diff —Docs

• Fixed relative links among the ES Modules to include the file extension, for web browser support.

1.10.0 —March 30, 2020 —Diff —Docs

• Reformats the source code to use EcmaScript 6export notation. Theunderscore.js UMD bundle is nowcompiled from underlying source modules instead ofbeing the source. From now on, Rollup users have the option to import from the underlying source module in order to enable treeshaking.
• Explicitly states in the documentation, and verifies in the unittests, that_.sortedIndex(array, value) always returns the lower bound, i.e., the smallest index, at whichvalue may be inserted inarray.
• Makes the notation of the_.max unittest consistent with other unittests.
• Fixes a bug that would cause infinite recursion if an overridden implementation of_.iteratee attempted to fall back to the original implementation.
• Restores compatibility with EcmaScript 3 and ExtendScript.

1.9.2 —Jan 6, 2020 —Diff —Docs

• No code changes. Updated a test to help outCITGM.

1.9.1 —May 31, 2018 —Diff —Docs

• Fixes edge-case regressions from 1.9.0, including certain forms of calling_.first and_.last on an empty array, and passing arrays as keys to_.countBy and_.groupBy.

1.9.0 —April 18, 2018 —Diff —Docs

• Adds the_.restArguments function for variadic function handling.
• Adds the_.chunk function for chunking up an array.
• Adds a_.isSymbol,_.isMap,_.isWeakMap,_.isSet and_.isWeakSet functions.
• _.throttle and_.debounce return functions that now have a.cancel() method, which can be used to cancel any scheduled calls.
• _.property now accepts arrays of keys and indexes as path specifiers, for looking up a deep properties of a value.
• _.range now accepts negative ranges to generate descending arrays.
• Adds support for several environments including: WebWorkers, browserify and ES6 imports.
• Removes thecomponent.json as the Component package management system is discontinued.
• The placeholder used for partial is now configurable by setting_.partial.placeholder.
• _.bindAll now accepts arrays or arguments for keys.
• Three years of performance improvements.

1.8.3 —April 2, 2015 —Diff —Docs

• Adds an_.create method, as a slimmed down version ofObject.create.
• Works around an iOS bug that can improperly causeisArrayLike to be JIT-ed. Also fixes a bug when passing0 toisArrayLike.

1.8.2 —Feb. 22, 2015 —Diff —Docs

• Restores the previous old-Internet-Explorer edge cases changed in 1.8.1.
• Adds afromIndex argument to_.contains.

1.8.1 —Feb. 19, 2015 —Diff —Docs

• Fixes/changes some old-Internet Explorer and related edge case behavior. Test your app with Underscore 1.8.1 in an old IE and let us know how it's doing...

1.8.0 —Feb. 19, 2015 —Diff —Docs

• Added_.mapObject, which is similar to_.map, but just for the values in your object. (A real crowd pleaser.)
• Added_.allKeys which returnsall the enumerable property names on an object.
• Reverted a 1.7.0 change where_.extend only copied "own" properties. Hopefully this will un-break you — if it breaks you again, I apologize.
• Added_.extendOwn — a less-useful form of_.extend that only copies over "own" properties.
• Added_.findIndex and_.findLastIndex functions, which nicely complement their twin-twins_.indexOf and_.lastIndexOf.
• Added an_.isMatch predicate function that tells you if an object matches key-value properties. A kissing cousin of_.isEqual and_.matcher.
• Added an_.isError function.
• Restored the_.unzip function as the inverse ofzip. Flip-flopping. I know.
• _.result now takes an optional fallback value (or function that provides the fallback value).
• Added the_.propertyOf function generator as a mirror-world version of_.property.
• Deprecated_.matches. It's now known by a more harmonious name —_.matcher.
• Various and diverse code simplifications, changes for improved cross-platform compatibility, and edge case bug fixes.

1.7.0 —August 26, 2014 —Diff —Docs

• For consistency and speed across browsers, Underscore now ignores native array methods forforEach,map,reduce,reduceRight,filter,every,some,indexOf, andlastIndexOf. "Sparse" arrays are officially dead in Underscore.
• Added_.iteratee to customize the iterators used by collection functions. Many Underscore methods will take a string argument for easier_.property-style lookups, an object for_.where-style filtering, or a function as a custom callback.
• Added_.before as a counterpart to_.after.
• Added_.negate to invert the truth value of a passed-in predicate.
• Added_.noop as a handy empty placeholder function.
• _.isEmpty now works witharguments objects.
• _.has now guards against nullish objects.
• _.omit can now take an iteratee function.
• _.partition is now called withindex andobject.
• _.matches creates a shallow clone of your object and only iterates over own properties.
• Aligning better with the forthcoming ECMA6Object.assign,_.extend only iterates over the object's own properties.
• Falsy guards are no longer needed in_.extend and_.defaults—if the passed in argument isn't a JavaScript object it's just returned.
• Fixed a few edge cases in_.max and_.min to handle arrays containingNaN (like strings or other objects) andInfinity and-Infinity.
• Override base methods likeeach andsome and they'll be used internally by other Underscore functions too.
• The escape functions handle backticks (`), to deal with an IE ≤ 8 bug.
• For consistency,_.union and_.difference now only work with arrays and not variadic args.
• _.memoize exposes the cache of memoized values as a property on the returned function.
• _.pick acceptsiteratee andcontext arguments for a more advanced callback.
• Underscore templates no longer accept an initialdata object._.template always returns a function now.
• Optimizations and code cleanup aplenty.

1.6.0 —February 10, 2014 —Diff —Docs

• Underscore now registers itself for AMD (Require.js), Bower and Component, as well as being a CommonJS module and a regular (Java)Script. An ugliness, but perhaps a necessary one.
• Added_.partition, a way to split a collection into two lists of results — those that pass and those that fail a particular predicate.
• Added_.property, for easy creation of iterators that pull specific properties from objects. Useful in conjunction with other Underscore collection functions.
• Added_.matches, a function that will give you a predicate that can be used to tell if a given object matches a list of specified key/value properties.
• Added_.constant, as a higher-order_.identity.
• Added_.now, an optimized way to get a timestamp — used internally to speed updebounce andthrottle.
• The_.partial function may now be used to partially apply any of its arguments, by passing_ wherever you'd like a placeholder variable, to be filled-in later.
• The_.each function now returns a reference to the list for chaining.
• The_.keys function now returns an empty array for non-objects instead of throwing.
• … and more miscellaneous refactoring.

1.5.2 —September 7, 2013 —Diff —Docs

• Added anindexBy function, which fits in alongside its cousins,countBy andgroupBy.
• Added asample function, for sampling random elements from arrays.
• Some optimizations relating to functions that can be implemented in terms of_.keys (which includes, significantly,each on objects). Also fordebounce in a tight loop.
• The_.escape function no longer escapes '/'.

1.5.1 —July 8, 2013 —Diff —Docs

• Removedunzip, as it's simply the application ofzip to an array of arguments. Use_.zip.apply(_, list) to transpose instead.

1.5.0 —July 6, 2013 —Diff —Docs

• Added a newunzip function, as the inverse of_.zip.
• Thethrottle function now takes anoptions argument, allowing you to disable execution of the throttled function on either theleading ortrailing edge.
• A source map is now supplied for easier debugging of the minified production build of Underscore.
• Thedefaults function now only overridesundefined values, notnull ones.
• Removed the ability to call_.bindAll with no method name arguments. It's pretty much always wiser to allow the names of the methods you'd like to bind.
• Removed the ability to call_.after with an invocation count of zero. The minimum number of calls is (naturally) now1.

1.4.4 —January 30, 2013 —Diff —Docs

• Added_.findWhere, for finding the first element in a list that matches a particular set of keys and values.
• Added_.partial, for partially applying a functionwithout changing its dynamic reference tothis.
• Simplifiedbind by removing some edge cases involving constructor functions. In short: don't_.bind your constructors.
• A minor optimization toinvoke.
• Fix bug in the minified version due to the minifier incorrectly optimizing-awayisFunction.

1.4.3 —December 4, 2012 —Diff —Docs

• Improved Underscore compatibility with Adobe's JS engine that can be used to script Illustrator, Photoshop, and friends.
• Added a default_.identity iterator tocountBy andgroupBy.
• Theuniq function can now takearray, iterator, context as the argument list.
• Thetimes function now returns the mapped array of iterator results.
• Simplified and fixed bugs inthrottle.

1.4.2 —October 6, 2012 —Diff —Docs

• For backwards compatibility, returned to pre-1.4.0 behavior when passingnull to iteration functions. They now become no-ops again.

1.4.1 —October 1, 2012 —Diff —Docs

• Fixed a 1.4.0 regression in thelastIndexOf function.

1.4.0 —September 27, 2012 —Diff —Docs

• Added apairs function, for turning a JavaScript object into[key, value] pairs ... as well as anobject function, for converting an array of[key, value] pairs into an object.
• Added acountBy function, for counting the number of objects in a list that match a certain criteria.
• Added aninvert function, for performing a simple inversion of the keys and values in an object.
• Added awhere function, for easy cases of filtering a list for objects with specific values.
• Added anomit function, for filtering an object to remove certain keys.
• Added arandom function, to return a random number in a given range.
• _.debounce'd functions now return their last updated value, just like_.throttle'd functions do.
• ThesortBy function now runs a stable sort algorithm.
• Added the optionalfromIndex option toindexOf andlastIndexOf.
• "Sparse" arrays are no longer supported in Underscore iteration functions. Use afor loop instead (or better yet, an object).
• Themin andmax functions may now be called onvery large arrays.
• Interpolation in templates now representsnull andundefined as the empty string.
• Underscore iteration functions no longer acceptnull values as a no-op argument. You'll get an early error instead.
• A number of edge-cases fixes and tweaks, which you can spot in thediff. Depending on how you're using Underscore,1.4.0 may be more backwards-incompatible than usual — please test when you upgrade.

1.3.3 —April 10, 2012 —Diff —Docs

• Many improvements to_.template, which now provides thesource of the template function as a property, for potentially even more efficient pre-compilation on the server-side. You may now also set thevariable option when creating a template, which will cause your passed-in data to be made available under the variable you named, instead of using awith statement — significantly improving the speed of rendering the template.
• Added thepick function, which allows you to filter an object literal with a list of allowed property names.
• Added theresult function, for convenience when working with APIs that allow either functions or raw properties.
• Added theisFinite function, because sometimes knowing that a value is a number just ain't quite enough.
• ThesortBy function may now also be passed the string name of a property to use as the sort order on each object.
• Fixeduniq to work with sparse arrays.
• Thedifference function now performs a shallow flatten instead of a deep one when computing array differences.
• Thedebounce function now takes animmediate parameter, which will cause the callback to fire on the leading instead of the trailing edge.

1.3.1 —January 23, 2012 —Diff —Docs

• Added an_.has function, as a safer way to usehasOwnProperty.
• Added_.collect as an alias for_.map. Smalltalkers, rejoice.
• Reverted an old change so that_.extend will correctly copy over keys with undefined values again.
• Bugfix to stop escaping slashes within interpolations in_.template.

1.3.0 —January 11, 2012 —Diff —Docs

• Removed AMD (RequireJS) support from Underscore. If you'd like to use Underscore with RequireJS, you can load it as a normal script, wrap or patch your copy, or download a forked version.

1.2.4 —January 4, 2012 —Diff —Docs

• You now can (and probably should, as it's simpler) write_.chain(list) instead of_(list).chain().
• Fix for escaped characters in Underscore templates, and for supporting customizations of_.templateSettings that only define one or two of the required regexes.
• Fix for passing an array as the first argument to an_.wrap'd function.
• Improved compatibility with ClojureScript, which adds acall function toString.prototype.

1.2.3 —December 7, 2011 —Diff —Docs

• Dynamic scope is now preserved for compiled_.template functions, so you can use the value ofthis if you like.
• Sparse array support of_.indexOf,_.lastIndexOf.
• Both_.reduce and_.reduceRight can now be passed an explicitlyundefined value. (There's no reason why you'd want to do this.)

1.2.2 —November 14, 2011 —Diff —Docs

• Continued tweaks to_.isEqual semantics. Now JS primitives are considered equivalent to their wrapped versions, and arrays are compared by their numeric properties only(#351).
• _.escape no longer tries to be smart about not double-escaping already-escaped HTML entities. Now it just escapes regardless(#350).
• In_.template, you may now leave semicolons out of evaluated statements if you wish:<% }) %>(#369).
• _.after(callback, 0) will now trigger the callback immediately, making "after" easier to use with asynchronous APIs(#366).

1.2.1 —October 24, 2011 —Diff —Docs

• Several important bug fixes for_.isEqual, which should now do better on mutated Arrays, and on non-Array objects withlength properties.(#329)
• James Burke contributed Underscore exporting for AMD module loaders, andTony Lukasavage for Appcelerator Titanium.(#335, #338)
• You can now_.groupBy(list, 'property') as a shortcut for grouping values by a particular common property.
• _.throttle'd functions now fire immediately upon invocation, and are rate-limited thereafter(#170, #266).
• Most of the_.is[Type] checks no longer ducktype.
• The_.bind function now also works on constructors, a-la ES5 ... but you would never want to use_.bind on a constructor function.
• _.clone no longer wraps non-object types in Objects.
• _.find and_.filter are now the preferred names for_.detect and_.select.

1.2.0 —October 5, 2011 —Diff —Docs

• The_.isEqual function now supports true deep equality comparisons, with checks for cyclic structures, thanks to Kit Cambridge.
• Underscore templates now support HTML escaping interpolations, using<%- ... %> syntax.
• Ryan Tenney contributed_.shuffle, which uses a modified Fisher-Yates to give you a shuffled copy of an array.
• _.uniq can now be passed an optional iterator, to determine by what criteria an object should be considered unique.
• _.last now takes an optional argument which will return the last N elements of the list.
• A new_.initial function was added, as a mirror of_.rest, which returns all the initial values of a list (except the last N).

1.1.7 —July 13, 2011 —Diff —Docs
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 —Diff —Docs
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 ES5 API._.extend no longer copies keys when the value is undefined._.bind now errors when trying to bind an undefined value.

1.1.5 —March 20, 2011 —Diff —Docs
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 ES5 version, where available._.keys now throws an error when used on non-Object values, as in ES5. Fixed a bug with_.keys when used over sparse arrays.

1.1.4 —January 9, 2011 —Diff —Docs
Improved compliance with ES5's Array methods when passingnull as a value._.wrap now correctly setsthis 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 —December 1, 2010 —Diff —Docs
In CommonJS, Underscore may now be required with just:
var _ = require("underscore"). Added_.throttle and_.debounce functions. Removed_.breakLoop, in favor of an ES5-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 theisType family of functions for better interoperability with Internet Explorer host objects._.template now correctly escapes backslashes in templates. Improved_.reduce compatibility with the ES5 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'sforEach.

1.1.2 —October 15, 2010 —Diff —Docs
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 —October 5, 2010 —Diff —Docs
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 —August 18, 2010 —Diff —Docs
The method signature of_.reduce has been changed to match the ES5 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 —June 22, 2010 —Diff —Docs
Andri Möll contributed the_.memoize function, which can be used to speed up expensive repeated computations by caching the results.

1.0.3 —June 14, 2010 —Diff —Docs
Patch that makes_.isEqual returnfalse if any property of the compared object has aNaN value. Technically the correct thing to do, but of questionable semantics. Watch out for NaN comparisons.

1.0.2 —March 23, 2010 —Diff —Docs
Fixes_.isArguments in recent versions of Opera, which have arguments objects as real Arrays.

1.0.1 —March 19, 2010 —Diff —Docs
Bugfix for_.isEqual, when comparing two objects with the same number of undefined keys, but with different names.

1.0.0 —March 18, 2010 —Diff —Docs
Things have been stable for many months now, so Underscore is now considered to be out of beta, at1.0. Improvements since0.6 include_.isBoolean, and the ability to have_.extend take multiple source objects.

0.6.0 —February 24, 2010 —Diff —Docs
Major release. Incorporates a number ofMile 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 ES5'sArray.isArray, andObject.keys.

0.5.8 —January 28, 2010 —Diff —Docs
Fixed Underscore's collection functions to work onNodeLists andHTMLCollections once more, thanks toJustin Tulloss.

0.5.7 —January 20, 2010 —Diff —Docs
A safer implementation of_.isArguments, and a faster_.isNumber,
thanks toJed Schmidt.

0.5.6 —January 18, 2010 —Diff —Docs
Customizable delimiters for_.template, contributed byNoah Sloan.

0.5.5 —January 9, 2010 —Diff —Docs
Fix for a bug in MobileSafari's OOP-wrapper, with the arguments object.

0.5.4 —January 5, 2010 —Diff —Docs
Fix for multiple single quotes within a template string for_.template. See:Rick Strahl's blog post.

0.5.2 —January 1, 2010 —Diff —Docs
New implementations ofisArray,isDate,isFunction,isNumber,isRegExp, andisString, thanks to a suggestion fromRobert Kieffer. Instead of doingObject#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 —December 9, 2009 —Diff —Docs
Added an_.isArguments function. Lots of little safety checks and optimizations contributed byNoah Sloan andAndri Möll.

0.5.0 —December 7, 2009 —Diff —Docs
[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 thatisEqual 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 —December 6, 2009 —Diff —Docs
AddedisDate,isNaN, andisNull, for completeness. Optimizations forisEqual when checking equality between Arrays or Dates._.keys is now25%–2X faster (depending on your browser) which speeds up the functions that rely on it, such as_.each.

0.4.6 —November 30, 2009 —Diff —Docs
Added therange function, a port of thePython function of the same name, for generating flexibly-numbered lists of integers. Original patch contributed byKirill Ishanov.

0.4.5 —November 19, 2009 —Diff —Docs
Addedrest for Arrays and arguments objects, and aliasedfirst ashead, andrest astail, thanks toLuke Sutton's patches. Added tests ensuring that all Underscore Array functions also work onarguments objects.

0.4.4 —November 18, 2009 —Diff —Docs
AddedisString, andisNumber, for consistency. Fixed_.isEqual(NaN, NaN) to returntrue (which is debatable).

0.4.3 —November 9, 2009 —Diff —Docs
Started using the nativeStopIteration object in browsers that support it. Fixed Underscore setup for CommonJS environments.

0.4.2 —November 9, 2009 —Diff —Docs
Renamed the unwrapping function tovalue, for clarity.

0.4.1 —November 8, 2009 —Diff —Docs
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 abreakLoop method tobreak in the middle of any Underscore iteration. Added anisEmpty function that works on arrays and objects.

0.4.0 —November 7, 2009 —Diff —Docs
All Underscore functions can now be called in an object-oriented style, like so:_([1, 2, 3]).map(...);. Original patch provided byMarc-André Cournoyer. Wrapped objects can be chained through multiple method invocations. Afunctions method was added, providing a sorted list of all the functions in Underscore.

0.3.3 —October 31, 2009 —Diff —Docs
Added the JavaScript 1.8 functionreduceRight. Aliased it asfoldr, and aliasedreduce asfoldl.

0.3.2 —October 29, 2009 —Diff —Docs
Now runs on stockRhino interpreters with:load("underscore.js"). Addedidentity as a utility function.

0.3.1 —October 29, 2009 —Diff —Docs
All iterators are now passed in the original collection as their third argument, the same as JavaScript 1.6'sforEach. Iterating over objects is now called with(value, key, collection), for details see_.each.

0.3.0 —October 29, 2009 —Diff —Docs
AddedDmitry Baranovskiy's comprehensive optimizations, merged inKris Kowal's patches to make UnderscoreCommonJS andNarwhal compliant.

0.2.0 —October 28, 2009 —Diff —Docs
Addedcompose andlastIndexOf, renamedinject toreduce, added aliases forinject,filter,every,some, andforEach.

0.1.1 —October 28, 2009 —Diff —Docs
AddednoConflict, so that the "Underscore" object can be assigned to other variables.

0.1.0 —October 28, 2009 —Docs
Initial release of Underscore.js.