Baseline Widely available
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.
const array1 = ["a", "b", "c", "d", "e"];
// Copy to index 0 the element at index 3
console.log(array1.copyWithin(0, 3, 4));
// Expected output: Array ["d", "b", "c", "d", "e"]
// Copy to index 1 all elements from index 3 to the end
console.log(array1.copyWithin(1, 3));
// Expected output: Array ["d", "d", "e", "d", "e"]
copyWithin(target, start)
copyWithin(target, start, end)
target
Zero-based index at which to copy the sequence to, converted to an integer. This corresponds to where the element at start will be copied to, and all elements between start and end are copied to succeeding indices.
-array.length <= target < 0, target + array.length is used.target < -array.length, 0 is used.target >= array.length, nothing is copied.target is positioned after start after normalization, copying only happens until the end of array.length (in other words, copyWithin() never extends the array).start
Zero-based index at which to start copying elements from, converted to an integer.
-array.length <= start < 0, start + array.length is used.start < -array.length, 0 is used.start >= array.length, nothing is copied.end Optional
Zero-based index at which to end copying elements from, converted to an integer. copyWithin() copies up to but not including end.
-array.length <= end < 0, end + array.length is used.end < -array.length, 0 is used.end >= array.length or end is omitted or undefined, array.length is used, causing all elements until the end to be copied.end implies a position before or at the position that start implies, nothing is copied.The modified array.
DescriptionThe 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.
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]
copyWithin() will propagate empty slots.
console.log([1, , 3].copyWithin(2, 1, 2)); // [1, empty, empty]
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
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