add and assignment(+=) operator
The add and assignment(+=) operator reduces the code little bit.It may not much influential in small codes whereas coming to long codes it's use can't be ignored.
Example
<html>
<body>
<script>
var tot = 0;
var a = [1,45,78,9,78,40];
for(var i = 0; i<a.length;i++){
tot += a[i]
}
document.write(tot);
</script>
</body>
</html>Output
251