var a = [149, 520, 50, 76, 270, 684, 128, 584, 290];

function merge(left, right)

{

var result = [];

while (left.length && right.length) {

if (left[0] <= right[0]) {

result.push(left.shift());

} else {

result.push(right.shift());

}

}

while (left.length)

result.push(left.shift());

while (right.length)

result.push(right.shift());

return result;

}

function mergeSort(arr)

{

if (arr.length < 2)

return arr;

var middle = parseInt(arr.length / 2);

var left = arr.slice(0, middle);

var right = arr.slice(middle, arr.length);

return merge(mergeSort(left), mergeSort(right));

}

console.log(mergeSort(a));