JavaScript (ES2026) reference · Array Methods

JavaScript Array flat() Method

Return a new array with nested arrays flattened into it, one level deep by default or to a given depth.

JavaScript (ES2026) Since ES2019

Syntax

array.flat(depth)

Parameter	Description
`depth` optional	How many levels to flatten; `Infinity` flattens everything. Default: `1`

A new array.

Flatten nested arraysTry it yourself

const nested = [1, [2, [3, [4]]]];
console.log(nested.flat());
console.log(nested.flat(2));
console.log(nested.flat(Infinity));

Output

[ 1, 2, [ 3, [ 4 ] ] ]
[ 1, 2, 3, [ 4 ] ]
[ 1, 2, 3, 4 ]

Holes are removedTry it yourself

console.log([1, , 3].flat()); // also removes holes

Output

[ 1, 3 ]

Version	Change
JavaScript ES2019	Added

array.flatMap() · Map each element with a function, then flatten the result one level. Useful when one element becomes several.
array.concat() · Return a new array that joins this array with other arrays or values. The original arrays are not changed.
JavaScript Arrays · tutorial
JavaScript Array Methods · tutorial