Array.prototype.flatMap(callback) maps each element, then flattens the result by exactly one level. Think map(…).flat(1) in one step (ES2019). It returns a new array and does not mutate the source.
array.flatMap(callback)
callback returns [a, b] → both items join the result
callback returns [] → drop that element
callback returns value → kept as one item (not nested)
≈ map(callback).flat(1)
only one flatten level — not InfinityCall array.flatMap. Start with this line.
array.flatMap(callback, thisArg?)| Call | What it does |
|---|---|
array.flatMap(callback, thisArg?) | Primary signature. |
array.flatMap(...) | Call with the arguments your case needs. |
Built into JavaScript. Call flatMap(...) on an array instance — no import required.
array.flatMap(callback, thisArg?)| Parameter | Description |
|---|---|
| Array | Required. Source to map over |
| callback | Required. Return a value or array per item |
| thisArg | Optional. Optional this for the callback |
| Return Value | Description |
|---|---|
| * | New array, flattened one level |
Return an array from the callback; those items join the result.
const numbers = [1, 2, 3];
const expanded = numbers.flatMap((n) => [n, n * 2]);
console.log(expanded);
// [1, 2, 2, 4, 3, 6][n, n * 2].numbers is unchanged.flatMap() vs map()map keeps nesting; flatMap unwraps one level.
const numbers = [1, 2, 3];
const pair = (n) => [n, n * 10];
console.log(numbers.map(pair));
// [[1, 10], [2, 20], [3, 30]]
console.log(numbers.flatMap(pair));
// [1, 10, 2, 20, 3, 30][n, n * 10] per item.flatMap over map(…).flat().[]Drop items with an empty array; keep others with [value].
const numbers = [1, 2, 3, 4, 5];
const evensDoubled = numbers.flatMap((n) =>
n % 2 === 0 ? [n * 2] : []
);
console.log(evensDoubled);
// [4, 8][n * 2] so the doubled value joins the result.[]; nothing is added for that item.filter then map also works; flatMap is a compact option.Array.prototype.flatMap() is part of ES2019, shipped with flat(). It is available in evergreen browsers and Node.js 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 ("flatMap" in Array.prototype) and fall back to arr.map(fn).flat() or a polyfill.
Learn how to run a side-effect for every array element.
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