UNB/ CS/ David Bremner/ teaching/ cs2613/ books/ mdn/ Reference/ Global Objects/ Array/ Array.prototype.copyWithin()

The copyWithin() method of Array instances shallow copies part of this array to another location in the same array and returns this array without modifying its length.

Syntax

copyWithin(target, start)
copyWithin(target, start, end)

Parameters

Return value

The modified array.

Description

The copyWithin() method works like C and C++'s memmove, and is a high-performance method to shift the data of an Array. This especially applies to the TypedArray method of the same name. The sequence is copied and pasted as one operation; the pasted sequence will have the copied values even when the copy and paste region overlap.

Because undefined becomes 0 when converted to an integer, omitting the start parameter has the same effect as passing 0, which copies the entire array to the target position, equivalent to a right shift where the right boundary is clipped off and the left boundary is duplicated. This behavior may confuse readers of your code, so you should explicitly pass 0 as start instead.

console.log([1, 2, 3, 4, 5].copyWithin(2));
// [1, 2, 1, 2, 3]; move all elements to the right by 2 positions

The copyWithin() method is a mutating method. It does not alter the length of this, but it will change the content of this and create new properties or delete existing properties, if necessary.

The copyWithin() method preserves empty slots. If the region to be copied from is sparse, the empty slots' corresponding new indices are deleted and also become empty slots.

The copyWithin() method is generic. It only expects the this value to have a length property and integer-keyed properties. Although strings are also array-like, this method is not suitable to be applied on them, as strings are immutable.

Examples

Using copyWithin()

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

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

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

Using copyWithin() on sparse arrays

copyWithin() will propagate empty slots.

console.log([1, , 3].copyWithin(2, 1, 2)); // [1, empty, empty]

Calling copyWithin() on non-array objects

The copyWithin() method reads the length property of this and then manipulates the integer indices involved.

const arrayLike = {
  length: 5,
  3: 1,
};
console.log(Array.prototype.copyWithin.call(arrayLike, 0, 3));
// { '0': 1, '3': 1, length: 5 }
console.log(Array.prototype.copyWithin.call(arrayLike, 3, 1));
// { '0': 1, length: 5 }
// The '3' property is deleted because the copied source is an empty slot

Specifications

Browser compatibility

See also