Does it mutate 😱

Array.prototype.copyWithin()

Mutates

Description

The copyWithin() method shallow copies part of an array to another location in the same array and returns it, without modifying its size.

arr.copyWithin(target)
arr.copyWithin(target, start)
arr.copyWithin(target, start, end)

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/copyWithin

Example

var 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"]