MergeSort in Javascript

gistfile1.js

function merge(left, right){

	var li = 0;
	var ri = 0;
	var result = [];

	while(li < left.length && ri < right.length){

		if(left[li] < right[ri]){
			result.push(left[li++])
		}else{
			result.push(right[ri++])
		}

	}

	return result.concat(left.slice(li)).concat(right.slice(ri));

}

function mergeSort(arr){
    // Terminal case: 0 or 1 item arrays don't need sorting
    if (arr.length < 2) {
        return arr;
    }

		var middle = Math.floor(arr.length/2);
		var left = arr.slice(0,middle);
		var right = arr.slice(middle);
		return merge(mergeSort(left), mergeSort(right));

}

var arr_to_sort = [2,5,9,3,1,76,34,23,4];

console.log("arr_to_sort", arr_to_sort);
console.log("arr sorted", mergeSort(arr_to_sort));