Array.prototype.flatMap()
Baseline Widely available
This feature is well established and works across many devices and browser versions. It’s been available across browsers since January 2020.
The flatMap()
method of Array
instances 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 (arr.map(...args).flat()
), but slightly more efficient than calling those two methods separately.
Try it
Syntax
flatMap(callbackFn)
flatMap(callbackFn, thisArg)
Parameters
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:
thisArg
Optional-
A value to use as
this
when executingcallbackFn
. See iterative methods.
Return value
A new array with each element being the result of the callback function and flattened by a depth of 1.
Description
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.
Alternative
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.
Examples
map() and flatMap()
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.
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.
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]
Using the third argument of callbackFn
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.
Using flatMap() on sparse arrays
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]
Calling flatMap() on non-array objects
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 } ]
Specifications
Specification |
---|
ECMAScript Language Specification # sec-array.prototype.flatmap |
Browser compatibility
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