each : _.each(list, iteratee, [context]) Alias: forEach
Iterates over a list of elements, yielding each in turn to an iteratee function. The iteratee is bound to the context object, if one is passed. Each invocation of iteratee is called with three arguments: (element, index, list). If list is a JavaScript object, iteratee's arguments will be (value, key, list). Returns the list for chaining.
_.each([1, 2, 3], alert);
=> alerts each number in turn...
_.each({one: 1, two: 2, three: 3}, alert);
=> alerts each number value in turn...
map _.map(list, iteratee, [context]) Alias: collect
Produces a new array of values by mapping each value in list through a transformation function (iteratee). The iteratee is passed three arguments: the value, then the index (or key) of the iteration, and finally a reference to the entire list.
_.map([1, 2, 3], function(num){ return num * 3; });
=> [3, 6, 9]
_.map({one: 1, two: 2, three: 3}, function(num, key){ return num * 3; });
=> [3, 6, 9]
_.map([[1, 2], [3, 4]], _.first);
=> [1, 3]
reduce _.reduce(list, iteratee, [memo], [context]) Aliases : inject, foldl
Also known as inject and foldl, reduce boils down a list of values into a single value. Memo is the initial state of the reduction, and each successive step of it should be returned by iteratee. The iteratee is passed four arguments: the memo, then the value and index (or key) of the iteration, and finally a reference to the entire list.
If no memo is passed to the initial invocation of reduce, the iteratee is not invoked on the first element of the list. The first element is instead passed as the memo in the invocation of the iteratee on the next element in the list.
var sum = _.reduce([1, 2, 3], function(memo, num){ return memo + num; }, 0);
=> 6
reduceRight _.reduceRight(list, iteratee, [memo], [context]) Alias : foldr
The right-associative version of reduce. Foldr is not as useful in JavaScript as it would be in a language with lazy evaluation.
var list = [[0, 1], [2, 3], [4, 5]];
var flat = _.reduceRight(list, function(a, b) { return a.concat(b); }, []);
=> [4, 5, 2, 3, 0, 1]
​
find _.find(list, predicate, [context]) Alias : detect
Looks through each value in the list, returning the first one that passes a truth test (predicate), or undefined if no value passes the test. The function returns as soon as it finds an acceptable element, and doesn't traverse the entire list. 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
filter _.filter(list, predicate, [context]) Alias : select
Looks through each value in the list, returning an array of all the values that pass a truth test (predicate). 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 _.findWhere(list, properties)
Looks through the list and returns the first value that matches all of the key-value pairs listed in properties.
If no match is found, or if list is empty, undefined will be returned.
_.findWhere(publicServicePulitzers, {newsroom: "The New York Times"});
=> {year: 1918, newsroom: "The New York Times",
reason: "For its public service in publishing in full so many official reports,
documents and speeches by European statesmen relating to the progress and
conduct of the war."}
where _.where(list, properties)
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 _.reject(list, predicate, [context])
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]
every _.every(list, [predicate], [context]) Alias : 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
some _.some(list, [predicate], [context]) Alias : any
Returns true if any of the values in the list pass the predicate truth test. Short-circuits and stops traversing the list if a true element is found. predicate is transformed through iteratee to facilitate shorthand syntaxes.
_.some([null, 0, 'yes', false]);
=> true
