Most efficient way to prepend a value to an array

Question

Assuming I have an array that has a size of N (where N > 0), is there a more efficient way of prepending to the array that would not require O(N

Answer 1

Example of prepending in-place:

var A = [7,8,9]
var B = [1,2,3]

A.unshift(...B)

console.log(A) // [1,2,3,7,8,9]

Answer 2

f you need to preserve the old array, slice the old one and unshift the new value(s) to the beginning of the slice.

var oldA=[4,5,6];
newA=oldA.slice(0);
newA.unshift(1,2,3)

oldA+'\n'+newA

/*  returned value:
4,5,6
1,2,3,4,5,6
*/

Answer 3

There is special method:

a.unshift(value);

But if you want to prepend several elements to array it would be faster to use such a method:

var a = [1, 2, 3],
    b = [4, 5];

function prependArray(a, b) {
    var args = b;
    args.unshift(0);
    args.unshift(0);
    Array.prototype.splice.apply(a, args);
}

prependArray(a, b);
console.log(a); // -> [4, 5, 1, 2, 3]