var arr1 = [1,2,3,4,5];
var arr2 = [1,2,3,4,5];
function mean(arr1, arr2) {
	if (arr1.length == 0 && arr2.length == 0)
		return 0;
	var sum = 0;
	for (var i in arr1) sum += arr1[i];
	for (var i in arr2) sum += arr2[i];
	return sum/(arr1.length + arr2.length);
}
document.write(mean(arr1, arr2));