Array.prototype.flatMap()

callbackFn

A function to execute for each element in the array. It should return an array containing new elements of the new array, or a single non-array value to be added to the new array. The function is called with the following arguments:

element

The current element being processed in the array.

index

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

array

The array flatMap() was called upon.

thisArg Optional

A value to use as this when executing callbackFn. See iterative methods.

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

The flatMap() method is an iterative method. See Array.prototype.map() for a detailed description of the callback function. The flatMap() method is identical to map(callbackFn, thisArg) followed by flat(1) — for each element, it produces an array of new elements, and concatenates the resulting arrays together to form a new array. Read the iterative methods section for more information about how these methods work in general.

The flatMap() method is generic. It only expects the this value to have a length property and integer-keyed properties. However, the value returned from callbackFn must be an array if it is to be flattened.

Pre-allocate and explicitly iterate

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

arr.flatMap((x) => [x, x * 2]);
// is equivalent to
const n = arr.length;
const acc = new Array(n * 2);
for (let i = 0; i < n; i++) {
  const x = arr[i];
  acc[i * 2] = x;
  acc[i * 2 + 1] = x * 2;
}
// [1, 2, 2, 4, 3, 6, 4, 8]

Note that in this particular case the flatMap approach is slower than the for-loop approach — due to the creation of temporary arrays that must be garbage collected, as well as the return array not needing to be frequently resized. However, flatMap may still be the correct solution in cases where its flexibility and readability are desired.

const 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.

const 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
const a = [5, 4, -3, 20, 17, -33, -4, 18];
//         |\  \  x   |  | \   x   x   |
//        [4,1, 4,   20, 16, 1,       18]

const result = a.flatMap((n) => {
  if (n < 0) {
    return [];
  }
  return n % 2 === 0 ? [n] : [n - 1, 1];
});
console.log(result); // [4, 1, 4, 20, 16, 1, 18]

The array argument is useful if you want to access another element in the array, especially when you don't have an existing variable that refers to the array. The following example first uses filter() to extract operational stations and then uses flatMap() to create a new array where each element contains a station and its next station. On the last station, it returns an empty array to exclude it from the final array.

const stations = ["New Haven", "West Haven", "Milford (closed)", "Stratford"];
const line = stations
  .filter((name) => !name.endsWith("(closed)"))
  .flatMap((name, idx, arr) => {
    // Without the arr argument, there's no way to easily access the
    // intermediate array without saving it to a variable.
    if (idx === arr.length - 1) return []; // last station has no next station
    return [`${name} - ${arr[idx + 1]}`];
  });
console.log(line); // ['New Haven - West Haven', 'West Haven - Stratford']

The array argument is not the array that is being built — there is no way to access the array being built from the callback function.

The callbackFn won't be called for empty slots in the source array because map() doesn't, while flat() ignores empty slots in the returned arrays.

console.log([1, 2, , 4, 5].flatMap((x) => [x, x * 2])); // [1, 2, 2, 4, 4, 8, 5, 10]
console.log([1, 2, 3, 4].flatMap((x) => [, x * 2])); // [2, 4, 6, 8]

The flatMap() method reads the length property of this and then accesses each property whose key is a nonnegative integer less than length. If the return value of the callback function is not an array, it is always directly appended to the result array.

const arrayLike = {
  length: 3,
  0: 1,
  1: 2,
  2: 3,
  3: 4, // ignored by flatMap() since length is 3
};
console.log(Array.prototype.flatMap.call(arrayLike, (x) => [x, x * 2]));
// [1, 2, 2, 4, 3, 6]

// Array-like objects returned from the callback won't be flattened
console.log(
  Array.prototype.flatMap.call(arrayLike, (x) => ({
    length: 1,
    0: x,
  })),
);
// [ { '0': 1, length: 1 }, { '0': 2, length: 1 }, { '0': 3, length: 1 } ]

• Polyfill of Array.prototype.flatMap in core-js
• es-shims polyfill of Array.prototype.flatMap
• Indexed collections guide
• Array
• Array.prototype.concat()
• Array.prototype.flat()
• Array.prototype.map()
• Array.prototype.reduce()