`Array.prototype.copyWithin()`

Copies part of an array to another location in the same array, in place.

Since ES2015 Spec ↗

Syntax

arr.copyWithin(target, start, end)

Name	Type	Required	Description
`target`	`number`	Yes	Index at which to paste the copied sequence. Negative counts from the end.
`start`	`number`	No	Index to start copying from. Defaults to 0. Negative counts from the end.
`end`	`number`	No	Index before which to stop copying (exclusive). Defaults to array length.

Array — The modified array (same reference).

console.log([1, 2, 3, 4, 5].copyWithin(0, 3));

Output

[ 4, 5, 3, 4, 5 ]

console.log([1, 2, 3, 4, 5].copyWithin(1, 3, 4));

Output

[ 1, 4, 3, 4, 5 ]

Mutates the array in place and never changes its length. Rarely used outside of performance-sensitive typed-array code.