Array.prototype.flat()
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.
Theflat() method ofArray instances creates a new array with all sub-arrayelements concatenated into it recursively up to the specified depth.
Try it
const arr1 = [0, 1, 2, [3, 4]];console.log(arr1.flat());// expected output: Array [0, 1, 2, 3, 4]const arr2 = [0, 1, [2, [3, [4, 5]]]];console.log(arr2.flat());// expected output: Array [0, 1, 2, Array [3, Array [4, 5]]]console.log(arr2.flat(2));// expected output: Array [0, 1, 2, 3, Array [4, 5]]console.log(arr2.flat(Infinity));// expected output: Array [0, 1, 2, 3, 4, 5]Syntax
flat()flat(depth)Parameters
depthOptionalThe depth level specifying how deep a nested array structure should be flattened.Defaults to 1.
Return value
A new array with the sub-array elements concatenated into it.
Description
Theflat() method is acopying method. It does not alterthis but instead returns ashallow copy that contains the same elements as the ones from the original array.
Theflat() method removes empty slots if the array being flattened issparse. For example, ifdepth is 1, both empty slots in the root array and in the first level of nested arrays are ignored, but empty slots in further nested arrays are preserved with the arrays themselves.
Theflat() method isgeneric. It only expects thethis value to have alength property and integer-keyed properties. However, its elements must be arrays if they are to be flattened.
Examples
Flattening nested arrays
const arr1 = [1, 2, [3, 4]];arr1.flat();// [1, 2, 3, 4]const arr2 = [1, 2, [3, 4, [5, 6]]];arr2.flat();// [1, 2, 3, 4, [5, 6]]const arr3 = [1, 2, [3, 4, [5, 6]]];arr3.flat(2);// [1, 2, 3, 4, 5, 6]const arr4 = [1, 2, [3, 4, [5, 6, [7, 8, [9, 10]]]]];arr4.flat(Infinity);// [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]Using flat() on sparse arrays
Theflat() method removesempty slots in arrays:
const arr5 = [1, 2, , 4, 5];console.log(arr5.flat()); // [1, 2, 4, 5]const array = [1, , 3, ["a", , "c"]];console.log(array.flat()); // [ 1, 3, "a", "c" ]const array2 = [1, , 3, undefined, ["a", , ["d", , "e"]], null];console.log(array2.flat()); // [ 1, 3, undefined, "a", ["d", empty, "e"], null ]console.log(array2.flat(2)); // [ 1, 3, undefined, "a", "d", "e", null ]Calling flat() on non-array objects
Theflat() method reads thelength property ofthis and then accesses each property whose key is a nonnegative integer less thanlength. If the element is not an array, it's directly appended to the result. If the element is an array, it's flattened according to thedepth parameter.
const arrayLike = { length: 3, 0: [1, 2], // Array-like objects aren't flattened 1: { length: 2, 0: 3, 1: 4 }, 2: 5, 3: 3, // ignored by flat() since length is 3};console.log(Array.prototype.flat.call(arrayLike));// [ 1, 2, { '0': 3, '1': 4, length: 2 }, 5 ]Specifications
| Specification |
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| ECMAScript® 2026 Language Specification # sec-array.prototype.flat |