Array.prototype.flat(depth) returns a new array with nested arrays pulled up. Default depth is 1. Use a larger number or Infinity for deeper nests. It does not mutate the original (ES2019).
array.flat(depth = 1)
[1, [2, 3]].flat() → [1, 2, 3]
[1, [2, [3]]].flat() → [1, 2, [3]]
[1, [2, [3]]].flat(2) → [1, 2, 3]
[1, [2, [3]]].flat(Infinity) → [1, 2, 3]
returns a new array — original unchangedCall array.flat. Start with this line.
array.flat(depth = 1)| Call | What it does |
|---|---|
array.flat(depth = 1) | Primary signature. |
array.flat(...) | Call with the arguments your case needs. |
Built into JavaScript. Call flat(...) on an array instance — no import required.
array.flat(depth = 1)| Parameter | Description |
|---|---|
| Array | Required. Source that may be nested |
| depth | Optional. How many nesting levels to unwrap |
| Return Value | Description |
|---|---|
| * | New flattened array |
1)One call unwraps a single nesting level.
const nested = [1, [2, 3], 4];
console.log(nested.flat());
// [1, 2, 3, 4][2, 3] is one level deep.flat() means flat(1).1 and 4.nested is unchanged.InfinityDeeper nests need a larger depth—or flatten everything at once.
const nested = [1, [2, [3, [4]]]];
console.log(nested.flat(2));
// [1, 2, 3, [4]]
console.log(nested.flat(Infinity));
// [1, 2, 3, 4][4] still remains nested.Infinity.Prove flat() returns a copy and does not mutate the source.
const nested = [1, [2, 3]];
const flatCopy = nested.flat();
console.log(flatCopy); // [1, 2, 3]
console.log(nested); // [1, [2, 3]]flatCopy.nested still holds [2, 3] inside.Array.prototype.flat() is part of ES2019. It has been widely available in evergreen browsers since 2018–2019; Node.js added it in version 11.
Supported in Chrome 69+, Firefox 62+, Safari 12+, Edge 79+, and Node 11+. Not available in Internet Explorer.
Bottom line: Safe for modern web apps and current Node releases. For IE11, feature-detect with if ("flat" in Array.prototype) and use a polyfill or a recursive helper.
Learn how to map and flatten one level in a single pass.
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