# Javascript Data Structure Tutorial - Javascript Mergesort

The Mergesort algorithm works by merging sorted sublists together to form a larger, completely sorted list.

## Example

``````
function mergeSort(arr) { //from ww w .j ava2  s . c  o  m
if (arr.length < 2) {
return;
}
var step = 1;
var left, right;
while (step < arr.length) {
left = 0;
right = step;
while (right + step <= arr.length) {
mergeArrays(arr, left, left+step, right, right+step);
left = right + step;
right = left + step;
}
if (right < arr.length) {
mergeArrays(arr, left, left+step, right, arr.length);
}
step *= 2;
}
}
function mergeArrays(arr, startLeft, stopLeft, startRight, stopRight) {
var rightArr = new Array(stopRight -startRight + 1);
var leftArr = new Array(stopLeft -startLeft + 1);
k = startRight;
for (var i = 0; i < (rightArr.length-1); ++i) {
rightArr[i] = arr[k];
++k;
}
k = startLeft;
for (var i = 0; i < (leftArr.length-1); ++i) {
leftArr[i] = arr[k];
++k;
}
rightArr[rightArr.length-1] = Infinity; // a sentinel value
leftArr[leftArr.length-1] = Infinity; // a sentinel value
var m = 0;
var n = 0;
for (var k = startLeft; k < stopRight; ++k) {
if (leftArr[m] <= rightArr[n]) {
arr[k] = leftArr[m];
m++;
} else {
arr[k] = rightArr[n];
n++;
}
}
}
var nums = [6,10,1,9,4,8,2,7,3,5];
console.log(nums);
console.log();
mergeSort(nums);
console.log();
console.log(nums);
``````

The code above generates the following result.