Functional merge sort in JS

mergeSort.js

'use strict';

var mergesort = (function () {
    "use strict";

    function sort(array) {
        if (array.length < 2) {
            return array;
        }

        const length = array.length
            , mid    = Math.floor(length * 0.5)
            , left   = array.slice(0, mid)
            , right  = array.slice(mid, length);

        return merge(sort(left), sort(right));
    }

    function merge(left, right) {
        function iter(current, other, acc) {
            if (current.length === 0) {
                return acc.concat(other);
            } else if(other.length === 0) {
                return acc.concat(current);
            }

            const [newCurrent, newOther] = current[0] <= other[0] ?
                [current, other] : [other, current];

            return iter(newCurrent.slice(1), newOther, acc.concat([newCurrent[0]]));
        };

        return iter(left, right, []);
    }

    return {
        sort: sort
    };
})();