Merge sort in Javascript

mergesort.js

function mergesort(array) {
    return sort(array, 0, array.length - 1);
    
    function sort(array, start, end) {
        if (start === end) return [array[start]];    
        
        var middle = Math.floor((start + end) / 2);
        return merge(sort(array, start, middle), sort(array, middle + 1, end));
    }

    function merge(left, right) {
        var result = [],
            leftIndex = 0,
            rightIndex = 0;
        
        while (leftIndex < left.length && rightIndex < right.length) {
            if (left[leftIndex] <= right[rightIndex]) {
                result.push(left[leftIndex]);
                leftIndex++;
            }
            else {
                result.push(right[rightIndex]);
                rightIndex++;
            }
        }
        
        while (leftIndex < left.length) {
            result.push(left[leftIndex]);
            leftIndex++;
        }
        
        while (rightIndex < right.length) {
            result.push(right[rightIndex]);
            rightIndex++;
        }
        
        return result;
    }
}