Array.prototype.flatMap()

The flatMap() method returns a new array formed by applying a given callback function to each element of the array, and then flattening the result by one level. It is identical to a map() followed by a flat() of depth 1, but slightly more efficient than calling those two methods separately.

// Arrow function
flatMap((currentValue) => { ... } )
flatMap((currentValue, index) => { ... } )
flatMap((currentValue, index, array) => { ... } )

// Callback function
flatMap(callbackFn)
flatMap(callbackFn, thisArg)

// Inline callback function
flatMap(function callbackFn(currentValue) { ... })
flatMap(function callbackFn(currentValue, index) { ... })
flatMap(function callbackFn(currentValue, index, array){ ... })
flatMap(function callbackFn(currentValue, index, array) { ... }, thisArg)

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.

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

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.

reduce() and concat()

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

arr.flatMap(x => [x, x * 2]);
// is equivalent to
arr.reduce((acc, x) => acc.concat([x, x * 2]), []);
// [1, 2, 2, 4, 3, 6, 4, 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.

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.

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. 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]

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