Array.prototype.forEach(callback) runs your function once for every element, in order. It is for side effects (log, update UI, push into another list). It always returns undefined—use map() when you need a new array (ES5).
array.forEach(callback)
callback(element, index, array)
visits each item in order
returns undefined (not a new array!)
return inside callback → skip rest of that call only
cannot break / continue like a for loopCall array.forEach. Start with this line.
array.forEach(callback, thisArg?)| Call | What it does |
|---|---|
array.forEach(callback, thisArg?) | Primary signature. |
array.forEach(...) | Call with the arguments your case needs. |
Built into JavaScript. Call forEach(...) on an array instance — no import required.
array.forEach(callback, thisArg?)| Parameter | Description |
|---|---|
| Array | Required. Source to iterate |
| callback | Required. Runs once per element |
| thisArg | Optional. Optional this for the callback |
| Return Value | Description |
|---|---|
| * | Always undefined |
Run a callback for every item in order.
const languages = ["JavaScript", "Python", "Java"];
languages.forEach((lang) => {
console.log(lang);
});0 … 2.lang is the current element.console.log prints; nothing is collected.forEach finishes and returns undefined.Use forEach for side effects like summing into a variable.
const numbers = [10, 20, 30];
let total = 0;
numbers.forEach((n) => {
total += n;
});
console.log(total); // 60total lives outside the callback.n.total holds the sum.reduce() is often clearer.forEach() vs map()forEach returns undefined; map returns a new array.
const numbers = [1, 2, 3];
const fromForEach = numbers.forEach((n) => n * 2);
const fromMap = numbers.map((n) => n * 2);
console.log(fromForEach); // undefined
console.log(fromMap); // [2, 4, 6]n * 2.undefined.map; need actions → forEach.Array.prototype.forEach() is part of ES5 (2009). It has been available in all modern browsers for many years, including Internet Explorer 9+.
Supported in Chrome, Firefox, Safari, Edge, IE9+, and all current Node.js versions.
Bottom line: Safe everywhere you ship modern or legacy web apps. No polyfill is needed for IE9+ or any current Node LTS release.
Learn how to check whether an array contains a value.
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