Duplicate an array an arbitrary number of times (javascript)

Question

Let's say I'm given an array. The length of this array is 3, and has 3 elements:

var array = ['1','2','3'];

Eventually I will need to check if this array is equal to an array with the same elements, but just twice now. My new array is:

var newArray = ['1','2','3','1','2','3'];

I know I can use array.splice() to duplicate an array, but how can I duplicate it an unknown amount of times? Basically what I want is something that would have the effect of

var dupeArray = array*2;

Answer 1

const duplicateArr = (arr, times) =>
    Array(times)
        .fill([...arr])
        .reduce((a, b) => a.concat(b));

This should work. It creates a new array with a size of how many times you want to duplicate it. It fills it with copies of the array. Then it uses reduce to join all the arrays into a single array.

Answer 2

The simplest solution is often the best one:

function replicate(arr, times) {
     var al = arr.length,
         rl = al*times,
         res = new Array(rl);
     for (var i=0; i<rl; i++)
         res[i] = arr[i % al];
     return res;
}

(or use nested loops such as @UsamaNorman).

However, if you want to be clever, you also can repeatedly concat the array to itself:

function replicate(arr, times) {
    for (var parts = []; times > 0; times >>= 1) {
        if (times & 1)
            parts.push(arr);
        arr = arr.concat(arr);
    }
    return Array.prototype.concat.apply([], parts);
}