Array.prototype.copyWithin(target, start, end) copies a slice of the array onto another position in the same array. It mutates in place, keeps the same length, and returns the array.
array.copyWithin(target, start, end?)
target → where writing begins
start → first source index (inclusive)
end → stop index (exclusive); default = length
mutates the array · length unchanged
negative indexes count from the endCall array.copyWithin. Start with this line.
arr.copyWithin(target, start, end?)| Call | What it does |
|---|---|
arr.copyWithin(target, start, end?) | Primary signature. |
array.copyWithin(...) | Call with the arguments your case needs. |
Built into JavaScript. Call copyWithin(...) on an array instance — no import required.
arr.copyWithin(target, start, end?)| Parameter | Description |
|---|---|
| target | Required. Write start index |
| start | Required. Source start (*defaults to 0 if omitted) |
| end | Optional. Source stop (exclusive); default = length |
| Return Value | Description |
|---|---|
| * | The same array |
Copy indexes 3–5 and paste them starting at index 0.
const numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9];
numbers.copyWithin(0, 3, 6);
console.log(numbers);
// [4, 5, 6, 4, 5, 6, 7, 8, 9]4, 5, 6 (end is exclusive).0.4, 5, 6.start from the TailCopy the last three elements to the front without writing length - 3.
const numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9];
numbers.copyWithin(0, -3);
console.log(numbers);
// [7, 8, 9, 4, 5, 6, 7, 8, 9]-3. On length 9, that is index 6.end. Source runs through the last element → 7, 8, 9.7, 8, 9.Copy the first three elements into the next three slots.
const pattern = [1, 2, 3, 0, 0, 0];
pattern.copyWithin(3, 0, 3);
console.log(pattern);
// [1, 2, 3, 1, 2, 3]1, 2, 3.3.1, 2, 3.Array.prototype.copyWithin() was added in ES2015 (ES6). It is available in all modern browsers and Node.js.
Supported in Chrome 45+, Firefox 32+, Safari 9+, Edge 12+, and Node.js 4+. Not available in Internet Explorer.
Bottom line: Safe for modern apps. For IE11, use a polyfill or a manual loop that copies into a temporary buffer first.
Learn how to iterate index/value pairs with Array.entries().
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