MergeSort.js

function merge(array, lower, half, upper){
    var left = [], i = 0;
    while (i < half - lower + 1){
        left.push(array[lower + i++]);
    }
    left.push(Number.MAX_VALUE);

    var right = [], j = 0;
    while(j < upper - half){
    	right.push(array[half + j++ + 1]);
    }
    right.push(Number.MAX_VALUE);

    i = j = 0; 
    for(k = lower; k <= upper; k++){
    	array[k] = left[i] < right[j] ? left[i++] : right[j++];
    }
}

function mergeSort(array, lower, upper){
    if(lower === undefined && upper === undefined){
        lower = 0;
        upper = array.length - 1;
    }
	
    if(lower < upper){
        var half = Math.floor((lower + upper)/2);
		
        mergeSort(array, lower, half);
        mergeSort(array, half + 1, upper);
        merge(array, lower, half, upper);
    }
}