Array.prototype.flatMap()

The flatMap() method first maps each element using a mapping function, then flattens the result into a new array. It is identical to a map() followed by a flat() of depth 1, but flatMap() is often quite useful, as merging both into one method is slightly more efficient.

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Syntax

var new_array = arr.flatMap(function callback(currentValue[, index[, array]]) {
    // return element for new_array
}[, thisArg])

Parameters

callback

Function that produces an element of the new Array, taking three arguments:

currentValue

The current element being processed in the array.

indexOptional

The index of the current element being processed in the array.

arrayOptional

The array map was called upon.

thisArgOptional

Value to use as this when executing callback.

Return value

A new array with each element being the result of the callback function and flattened to a depth of 1.

Description

See Array.prototype.map() for a detailed description of the callback function. The flatMap method is identical to a map followed by a call to flat of depth 1.

Examples

map() and flatMap()

let arr1 = [1, 2, 3, 4];

arr1.map(x => [x * 2]); 
// [[2], [4], [6], [8]]

arr1.flatMap(x => [x * 2]);
// [2, 4, 6, 8]

// only one level is flattened
arr1.flatMap(x => [[x * 2]]);
// [[2], [4], [6], [8]]

While the above could have been achieved by using map itself, here is an example that better showcases the use of flatMap.

Let's generate a list of words from a list of sentences.

let arr1 = ["it's Sunny in", "", "California"];

arr1.map(x => x.split(" "));
// [["it's","Sunny","in"],[""],["California"]]

arr1.flatMap(x => x.split(" "));
// ["it's","Sunny","in", "", "California"]

Notice, the output list length can be different from the input list length.

For adding and removing items during a map()

flatMap can be used as a way to add and remove items (modify the number of items) during a map. In other words, it allows you to map many items to many items (by handling each input item separately), rather than always one-to-one. In this sense, it works like the opposite of filter. Simply return a 1-element array to keep the item, a multiple-element array to add items, or a 0-element array to remove the item.

// Let's say we want to remove all the negative numbers and split the odd numbers into an even number and a 1
let a = [5, 4, -3, 20, 17, -33, -4, 18]
//       |\  \  x   |  | \   x   x   |
//      [4,1, 4,   20, 16, 1,       18]

a.flatMap( (n) =>
  (n < 0) ?      [] :
  (n % 2 == 0) ? [n] :
                 [n-1, 1]
)

// expected output: [4, 1, 4, 20, 16, 1, 18]

Alternative

reduce() and concat()

var arr1 = [1, 2, 3, 4];

arr1.flatMap(x => [x * 2]);
// is equivalent to
arr1.reduce((acc, x) => acc.concat([x * 2]), []);
// [2, 4, 6, 8]

Note, however, that this is inefficient and should be avoided for large arrays: in each iteration, it creates a new temporary array that must be garbage-collected, and it copies elements from the current accumulator array into a new array instead of just adding the new elements to the existing array.

Polyfill

This polyfill is based on the one of Array.prototype.map().

if (!Array.prototype.flatMap) {
    Object.defineProperty(Array.prototype, 'flatMap', {
        value: function(callback, thisArg) {
            var self = thisArg || this;
            if (self === null) {
                throw new TypeError( 'Array.prototype.flatMap ' +
                'called on null or undefined' );
            }
            if (typeof callback !== 'function') {
                throw new TypeError( callback +
                ' is not a function');
            }

            var list = [];

            // 1. Let O be ? ToObject(this value).
            var o = Object(self);

            // 2. Let len be ? ToLength(? Get(O, "length")).
            var len = o.length >>> 0;

            for (var k = 0; k < len; ++k) {
                if (k in o) {
                    var part_list = callback.call(self, o[k], k, o);
                    list = list.concat(part_list);
                }
            }

            return list;
        }
    });
}

Specifications

Browser compatibility

See also

• Array.prototype.flat()
• Array.prototype.map()
• Array.prototype.reduce()
• Array.prototype.concat()