javascript merge sort

* MergeSort.js

/**
 * Created by Mch on 9/15/18.
 */
function MergeSort() {}

Array.prototype.mergeSort = function() {
    var length = this.length, a = new Array(length);
    if (length < 2) {
        return this;
    }
    // copy
    for (var i = 0; i < this.length; i++) {
        a[i] = this[i];
    }
    Array._mergeSort(a, 0, length-1);
    return a;
};

Array._mergeSort = function(/* Array */a, /* Number */left, /* Number */right) {
    if (left >= right) {
        return;
    }
    var mid = Math.floor((left - right) /2 + right);
    Array._mergeSort(a, left, mid);
    Array._mergeSort(a, mid+1, right);
    Array.merge(a, left, mid, right);
};

/**
 * left ... mid mid+1 ... right
 * @param array Array
 * @param left Integer
 * @param mid Integer
 * @param right Integer
 */
Array.merge = function(array, left, mid, right) {
    var result = [], il = left, ir = mid+1;

    while (il <= mid && ir <= right) {
        // compare
        if (array[il] < array[ir]) {
            result.push(array[il++]);
        } else {
            result.push(array[ir++]);
        }
    }
    while (il <= mid) {
        result.push(array[il++]);
    }

    while (ir <= right) {
        result.push(array[ir++]);
    }
    // put back
    for (var i = left; i <= right; i++) {
        array[i] = result.shift();
    }
};

Array.prototype.swap = function(i, j) {
    if (0<=i && i < this.length && 0 <=j && j < this.length) {
        var t = this[i];
        this[i] = this[j];
        this[j] = t;
    } else {
        throw RangeException('index out of range');
    }
};

Array.prototype.shuffle = function() {
    var m = this.length, i;
    while (m) {
        i = Math.floor(Math.random() * m--);
        this.swap(i, m);
    }
    return this;
};

MergeSort.prototype.main = function () {
    // var a = [0,1,2,3,4,5,6,7,8,9];
    // a = a.shuffle();
    // console.log(a);
    var a = [8,2,9,0,1,5,6,3,7,4];
    a = a.mergeSort();
    console.log(a);
};

var m = new MergeSort();
m.main();