Array.prototype.reduce(callback, initialValue?) folds an array into one value by updating an accumulator left to right. Use it for sums, max, grouping, and one-level flattens.
array.reduce((acc, value, index, array) => nextAcc, initialValue?)
acc starts as initialValue (recommended)
each step returns the next acc
final return = last acc
empty array + no initialValue → TypeError
read-only · ES5Call array.reduce. Start with this line.
arr.reduce(fn, init?)| Call | What it does |
|---|---|
arr.reduce(fn, init?) | Primary signature. |
array.reduce(...) | Call with the arguments your case needs. |
Built into JavaScript. Call reduce(...) on an array instance — no import required.
arr.reduce(fn, init?)| Parameter | Description |
|---|---|
| Callback | Required. Returns next accumulator |
| initialValue | Optional. Starting acc; avoids empty-array errors |
| Return Value | Description |
|---|---|
| * | Final accumulator |
Start at 0 and add each element to the accumulator.
const numbers = [1, 2, 3, 4, 5];
const sum = numbers.reduce((acc, num) => {
return acc + num;
}, 0);
console.log(sum);
// 15initialValue to 0. Sums should not start from the first element alone.acc + num becomes the next accumulator.sum is 15.Without initialValue, the first element seeds the accumulator.
const scores = [88, 92, 76, 95, 84];
const max = scores.reduce((acc, score) => {
return score > acc ? score : acc;
});
console.log(max);
// 9588 becomes the starting acc.acc and score.95 beats every other value.[] this pattern throws — pass an init or check length.Concatenate each inner array into an accumulator starting as [].
const nested = [[1, 2], [3, 4], [5, 6]];
const flat = nested.reduce((acc, chunk) => {
return acc.concat(chunk);
}, []);
console.log(flat);
// [1, 2, 3, 4, 5, 6][]. The accumulator is an empty array.[1,2], then [3,4], then [5,6].flat() for more.nested is unchanged.Array.prototype.reduce() has been available since ES5 (2009). It is widely supported for accumulation and grouping patterns.
Supported in Chrome 3+, Firefox 3+, Safari 4+, Edge (all versions), IE 9+, and all modern Node.js versions.
Bottom line: Safe for modern projects and IE9+. No polyfill needed for typical targets.
Learn how to reduce an array from right to left.
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