This is my implementation of mergesort.

mergeSort.js

function merge(left, right){
    const list = [];
    while(left.length && right.length){
        if(left[0] < right[0]){
            list.push(left[0])
            left.shift();
        } else {
            list.push(right[0])
            right.shift();
        }
    }

    if(left.length){
        return list.concat(left);
    } else {
        return list.concat(right);
    }
}

function mergeSort(arr){
    if(arr.length <= 1){
        return arr;
    } else {
        const middle = Math.floor(arr.length / 2);
        let left = arr.slice(0, middle);
        let right = arr.slice(middle, arr.length)

        left = mergeSort(left)
        right = mergeSort(right)

        if(left[left.length-1] <= right[0] ){
            return left.concat(right);
        }

        return merge(left, right);
    }
}

mergeSort([4,3,5,6,9,8,4,5,11,1,2])
// (11) [ 1, 2, 3, 4, 4, 5, 5, 6, 8, 9, 11 ]