Loop through array to download files with node js

Collection Functions (Arrays or Objects)

eachAlias: forEach
Iterates over a list of elements, yielding each in turn to an iteratee function. The iteratee is bound to the context object, if one is passed. Each invocation of iteratee is called with three arguments: . If list is a JavaScript object, iteratee's arguments will be . Returns the list for chaining.

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

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

mapAlias: collect
Produces a new array of values by mapping each value in list through a transformation function (iteratee). The iteratee is passed three arguments: the , then the (or ) of the iteration, and finally a reference to the entire .

_.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]

reduceAliases: inject, foldl
Also known as inject and foldl, reduce boils down a list of values into a single value. Memo is the initial state of the reduction, and each successive step of it should be returned by iteratee. The iteratee is passed four arguments: the , then the and (or key) of the iteration, and finally a reference to the entire .

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

reduceRightAlias: foldr
The right-associative version of reduce. 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]

findAlias: detect
Looks through each value in the list, returning the first one that passes a truth test (predicate), or 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 through iteratee to facilitate shorthand syntaxes.

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

filterAlias: select
Looks through each value in the list, returning an array of all the values that pass a truth test (predicate). predicate is transformed through iteratee to facilitate shorthand syntaxes.

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

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

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

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

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

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

reject
Returns the values in list without the elements that the truth test (predicate) passes. The opposite of filter. predicate is transformed through iteratee to facilitate shorthand syntaxes.

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

everyAlias: all
Returns true if all of the values in the list pass the predicate truth test. Short-circuits and stops traversing the list if a false element is found. predicate is transformed through iteratee to facilitate shorthand syntaxes.

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

someAlias: any
Returns true if any of the values in the list pass the predicate truth test. Short-circuits and stops traversing the list if a true element is found. predicate is transformed through iteratee to facilitate shorthand syntaxes.

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

containsAliases: include, includes
Returns true if the value is present in the list. Uses indexOf internally, if list is an Array. Use fromIndex to start your search at a given index.

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

invoke
Calls the method named by methodName on each value in the list. Any extra arguments passed to invoke will be forwarded on to the method invocation.

_.invoke([[5, 1, 7], [3, 2, 1]], 'sort'); => [[1, 5, 7], [1, 2, 3]]

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

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

max
Returns the maximum value in list. If an iteratee function is provided, it will be used on each value to generate the criterion by which the value is ranked. -Infinity is returned if list is empty, so an isEmpty guard may be required. Non-numerical values in list will be ignored. 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
Returns the minimum value in list. If an iteratee function is provided, it will be used on each value to generate the criterion by which the value is ranked. Infinity is returned if list is empty, so an isEmpty guard may be required. Non-numerical values in list will be ignored. This function uses operator (note).

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

sortBy
Returns a (stably) sorted copy of list, ranked in ascending order by the results of running each value through iteratee. iteratee may also be the string name of the property to sort by (eg. ). 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
Splits a collection into sets, grouped by the result of running each value through iteratee. If iteratee is a string instead of a function, groups by the property named by iteratee on each of the values.

_.groupBy([1.3, 2.1, 2.4], function(num){ return Math.floor(num); }); => {1: [1.3], 2: [2.1, 2.4]} _.groupBy(['one', 'two', 'three'], 'length'); => {3: ["one", "two"], 5: ["three"]}

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

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

countBy
Sorts a list into groups and returns a count for the number of objects in each group. Similar to , 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
Returns a shuffled copy of the list, using a version of the Fisher-Yates shuffle.

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

sample
Produce a random sample from the list. Pass a number to return n random elements from the list. Otherwise a single random item will be returned.

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

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

size
Return the number of values in the list.

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

partition
Split list into two arrays: one whose elements all satisfy predicate and one whose elements all do not satisfy predicate. predicate is transformed through iteratee to facilitate shorthand syntaxes.

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

compact
Returns a copy of the list with all falsy values removed. In JavaScript, false, null, 0, "", undefined and NaN are all falsy.

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

Array Functions

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

firstAliases: head, take
Returns the first element of an array. Passing n will return the first n elements of the array.

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

initial
Returns everything but the last entry of the array. Especially useful on the arguments object. Pass n to exclude the last n elements from the result.

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

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

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

restAliases: tail, drop
Returns the rest of the elements in an array. Pass an index to return the values of the array from that index onward.

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

flatten
Flattens a nested array. If you pass or as the depth, 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 the depth argument, or passing or , 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
Returns a copy of the array with all instances of the values removed.

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

union
Computes the union of the passed-in arrays: the list of unique items, in order, that are present in one or more of the arrays.

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

intersection
Computes the list of values that are the intersection of all the arrays. Each value in the result is present in each of the arrays.

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

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

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

uniqAlias: unique
Produces a duplicate-free version of the array, using === to test object equality. In particular only the first occurrence of each value is kept. If you know in advance that the array is sorted, passing true for isSorted will run a much faster algorithm. If you want to compute unique items based on a transformation, pass an iteratee function.

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

zip
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.

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

unzipAlias: transpose
The opposite of zip. Given an array 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
Converts arrays into objects. Pass either a single list of pairs, or a list of keys, and a list of values. Passing by pairs is the reverse of pairs. 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
Chunks an array into multiple arrays, each containing length or fewer items.

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

indexOf
Returns the index at which value can be found in the array, or -1 if value is not present in the array. If you're working with a large array, and you know that the array is already sorted, pass for isSorted to use a faster binary search ... or, pass a number as the third argument in order to look for the first matching value in the array after the given index. If is , this function uses operator (note).

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

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

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

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

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

findIndex
Similar to , returns the first index where the predicate truth test passes; otherwise returns -1.

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

findLastIndex
Like but iterates the array in reverse, returning the index closest to the end where the predicate truth test passes.

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

range
A function to create flexibly-numbered lists of integers, handy for and loops. start, if omitted, defaults to 0; step defaults to 1. Returns a list of integers from start (inclusive) to stop (exclusive), incremented (or decremented) by step. Note that ranges that stop before they start are considered to be zero-length instead of negative — if you'd like a negative range, use a negative step.

_.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 a function to an object, meaning that whenever the function is called, the value of this will be the object. Optionally, pass arguments to the function to pre-fill them, also known as partial application. For partial application without context binding, use partial.

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

bindAll
Binds a number of methods on the object, specified by methodNames, to be run in the context of that object whenever they are invoked. Very handy for binding functions that are going to be used as event handlers, which would otherwise be invoked with a fairly useless this. methodNames are required.

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

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

memoize
Memoizes a given function by caching the computed result. Useful for speeding up slow-running computations. If passed an optional hashFunction, it will be used to compute the hash key for storing the result, based on the arguments to the original function. The default hashFunction just uses the first argument to the memoized function as the key. The cache of memoized values is available as the property on the returned function.

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

delay
Much like setTimeout, invokes function after wait milliseconds. If you pass the optional arguments, they will be forwarded on to the function when it is invoked.

var log = _.bind(console.log, console); _.delay(log, 1000, 'logged later'); => 'logged later' // Appears after one second.

defer
Defers invoking the function until the current call stack has cleared, similar to using setTimeout with a delay of 0. Useful for performing expensive computations or HTML rendering in chunks without blocking the UI thread from updating. If you pass the optional arguments, they will be forwarded on to the function when it is invoked.

throttle
Creates and returns a new, throttled version of the passed function, that, when invoked repeatedly, will only actually call the original function at most once per every wait milliseconds. Useful for rate-limiting events that occur faster than you can keep up with.

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

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

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

debounce
Creates and returns a new debounced version of the passed function which will postpone its execution until after wait milliseconds have elapsed since the last time it was invoked. Useful for implementing behavior that should only happen after the input has stopped arriving. For example: rendering a preview of a Markdown comment, recalculating a layout after the window has stopped being resized, and so on.

At the end of the wait interval, the function will be called with the arguments that were passed most recently to the debounced function.

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

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

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

once
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
Creates a wrapper of function that does nothing at first. From the count-th call onwards, it starts actually calling function. 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
Creates a wrapper of function that memoizes its return value. From the count-th call onwards, the memoized result of the last invocation is returned immediately instead of invoking function again. So the wrapper will invoke function at most count - 1 times.

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

wrap
Wraps the first function inside of the wrapper function, passing it as the first argument. This allows the wrapper to execute code before and after the function runs, adjust the arguments, and execute it conditionally.

var hello = function(name) { return "hello: " + name; }; hello = _.wrap(hello, function(func) { return "before, " + func("moe") + ", after"; }); hello(); => 'before, hello: moe, after'

negate
Returns a new negated version of the predicate function.

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

compose
Returns the composition of a list of functions, where each function consumes the return value of the function that follows. In math terms, composing the functions f(), g(), and h() produces f(g(h())).

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

restArguments
Returns a version of the function that, when called, receives all arguments from and beyond startIndex collected into a single array. If you don’t pass an explicit startIndex, it will be determined by looking at the number of arguments to the function itself. Similar to ES6’s rest parameters syntax.

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

Object Functions

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

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

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

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

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

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

mapObject
Like map, but for objects. Transform the value of each property in turn.

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

pairs
Convert an object into a list of pairs. The opposite of object.

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

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

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

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

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

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

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

findKey
Similar to but for keys in objects. Returns the key where the predicate truth test passes or undefined. predicate is transformed through iteratee to facilitate shorthand syntaxes.

extend
Shallowly copy all of the properties in the source objects over to the destination object, and return the destination 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}

extendOwnAlias: assign
Like extend, but only copies own properties over to the destination object.

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

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

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

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

defaults
Returns object after filling in its properties with the first value present in the following list of defaults objects.

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

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

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

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

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

toPath
Ensures that path is an array. If path 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)

is used internally in , , , , and , as well as in iteratee and all functions that depend on it, in order to normalize deep property paths. You can override if you want to customize this behavior, for example to enable Lodash-like string path shorthands. Be advised that altering 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
Returns the specified property of object. 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 is , the optional default is returned.

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

has
Does the object contain the given key? Identical to , but uses a safe reference to the function, in case it's been overridden accidentally.

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

property
Returns a function that will return the specified property of any passed-in object. 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); => true var stooges = {moe: {fears: {worst: 'Spiders'}}, curly: {fears: {worst: 'Moe'}}}; var curlysWorstFear = _.property(['curly', 'fears', 'worst']); curlysWorstFear(stooges); => 'Moe'

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

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

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

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

isEqual
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
Tells you if the keys and values in properties are contained in object.

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

isEmpty
Returns true if collection has no elements. For strings and array-like objects checks if the length property is 0. For other objects, it returns true 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
Returns true if object is a DOM element.

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

isArray
Returns true if object is an Array.

isObject
Returns true if value is an Object. Note that JavaScript arrays and functions are objects, while (normal) strings and numbers are not.

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

isArguments
Returns true if object is an Arguments object.

isFunction
Returns true if object is a Function.

_.isFunction(alert); => true

isString
Returns true if object is a String.

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

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

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

isFinite
Returns true if object is a finite Number.

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

isBoolean
Returns true if object is either true or false.

_.isBoolean(null); => false

isDate
Returns true if object is a Date.

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

isRegExp
Returns true if object is a RegExp.

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

isError
Returns true if object inherits from an Error.

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

isSymbol
Returns true if object is a Symbol.

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

isMap
Returns true if object is a Map.

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

isWeakMap
Returns true if object is a WeakMap.

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

isSet
Returns true if object is a Set.

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

isWeakSet
Returns true if object is a WeakSet.

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

isArrayBuffer
Returns true if object is an ArrayBuffer.

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

isDataView
Returns true if object is a DataView.

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

isTypedArray
Returns true if object is a TypedArray.

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

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

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

isNull
Returns true if the value of object is null.

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

isUndefined
Returns true if value is undefined.

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

Utility Functions

noConflict
Give control of the global variable back to its previous owner. Returns a reference to the Underscore object.

var underscore = _.noConflict();

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

identity
Returns the same value that is used as the argument. In math:
This function looks useless, but is used throughout Underscore as a default iteratee.

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

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

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

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

obj.initialize = _.noop;

times
Invokes the given iteratee function n times. Each invocation of iteratee is called with an argument. Produces an array of the returned values.

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

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

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

mixin
Allows you to extend Underscore with your own utility functions. Pass a hash of 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
Generates a callback that can be applied to each element in a collection. supports a number of shorthand syntaxes for common callback use cases. Depending upon 's type, 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 : , , , , , , , , , , , , , , , , , , and

You may overwrite 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
Generate a globally-unique id for client-side models or DOM elements that need one. If prefix is passed, the id will be appended to it.

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

escape
Escapes a string for insertion into HTML, replacing , , , , , and characters.

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

unescape
The opposite of escape, replaces , , , , and with their unescaped counterparts.

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

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

now
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
Compiles JavaScript templates into functions that can be evaluated for rendering. Useful for rendering complicated bits of HTML from JSON data sources. Template functions can both interpolate values, using , as well as execute arbitrary JavaScript code, with . If you wish to interpolate a value, and have it be HTML-escaped, use . When you evaluate a template function, pass in a data object that has properties corresponding to the template's free variables. The settings argument should be a hash containing any that should be overridden.

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

You can also use from within JavaScript code. This is sometimes more convenient than using .

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

If ERB-style delimiters aren't your cup of tea, you can change Underscore's template settings to use different symbols to set off interpolated code. Define an interpolate regex to match expressions that should be interpolated verbatim, an escape regex to match expressions that should be inserted after being HTML-escaped, and an evaluate regex to match expressions that should be evaluated without insertion into the resulting string. 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 perform Mustache.js-style templating:

_.templateSettings = { interpolate: /\{\{(.+?)\}\}/g }; var template = _.template("Hello {{ name }}!"); template({name: "Mustache"}); => "Hello Mustache!"

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

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

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

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

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.

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

Chaining

Calling will cause all future method calls to return wrapped objects. When you've finished the computation, call to retrieve the final value. Here's an example of chaining together a map/flatten/reduce, in order to get the word count of every word in a song.

var lyrics = [ {line: 1, words: "I'm a lumberjack and I'm okay"}, {line: 2, words: "I sleep all night and I work all day"}, {line: 3, words: "He's a lumberjack and he's okay"}, {line: 4, words: "He sleeps all night and he works all day"} ]; _.chain(lyrics) .map(function(line) { return line.words.split(' '); }) .flatten() .reduce(function(counts, word) { counts[word] = (counts[word] || 0) + 1; return counts; }, {}) .value(); => {lumberjack: 2, all: 4, night: 2 ... }

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

chain
Returns a wrapped object. Calling methods on this object will continue to return wrapped objects until 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
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: , , , , , , and more.

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

Ruby's Enumerable module.

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

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

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

Python's itertools.

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 and , use JavaScript’s built-in relational 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 being 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:

• out all unsortable values first.
• Pick a target type, i.e., either string or number, and pass an 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 contain or , you can control whether you want to sort these before or after all numbers by passing an to that returns or for such values, respectively. Or maybe you want to treat them as zeros; it is up to you. The same can also be passed to other Underscore functions to ensure that the behavior is consistent.

Change Log

1.12.0 — November 24, 2020 — Diff — Docs

• Adds the and 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 and by using that function from Underscore-contrib to override .
• Fixes a bug in that caused typed arrays to compare equal when viewing different segments of the same underlying .
• Improves the compatibility of , , , and with some older browsers, especially IE 11.
• Significantly enhances the performance of and several members of the isType family of functions.
• Speeds up comparison of typed arrays and s with idential , and .
• 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 6 notation in version 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. is still present and the UMD bundle is still recommended for most users.
  Since the modularization obfuscates the diff, piecewise diffs are provided below.
• Adds a monolithic bundle in EcmaScript 6 module format, , 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 of , because 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 , and functions, as well as support for the corresponding value types to .
• Adds the option to flatten arrays to a specific depth: .
• Adds as an alias to .
• Fixes an inconsistency where methods on the Underscore wrapper would error when the wrapped value is or . These methods now perform a no-op on null values like the other Underscore functions.
• Fixes a bug that caused and to return instead of for empty arrays when used as an iteratee.
• Fixes a regression introduced in version 1.9.0 that caused to return 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 with , while using the legacy Node.js require API:

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