function mergeSort(arr) {

    mergeSortHelper(arr, 0, arr.length - 1);

}

​

function mergeSortHelper(arr, left, right) {

    if(left < right) {

        var middle = Math.floor((right + left) / 2);

        mergeSortHelper(arr, left, middle);

        mergeSortHelper(arr, middle + 1, right);

        merge(arr, left, middle, right);

    }

}

​

function merge(arr, left, middle, right) {

    var i = left;

    var j = middle + 1;

    var tem = [];

    while(i <= middle && j <= right) {

          if(arr[i] <= arr[j]) {

              tem.push(arr[i]);

              i++;

          }else {

              tem.push(arr[j]);

              j++;

          }

    }

    while(i <= middle) {

          tem.push(arr[i]);

          i++;

    }

    while(j <= right) {

          tem.push(arr[j]);

          j++;

    }

    var count = 0;

    while(left <= right) {

          arr[left] = tem[count];

          left++;

          count++;

    }

}

​

var arr = [10, 2, 3, 4, 9, 100, -1];

console.log(arr);

mergeSort(arr);

console.log(arr);