JS利用扩展Array原型实现指定区间求和

<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>JS求数组之和</title>
</head>
<body>
<script >
Array.prototype.sum= function(l,r){
l=l==undefined ? 0 : l;
r=r==undefined ? this.length - 1 : r;
var s = 0;
for(var i = l;i <= r;i++){s+=this[i];}
return s;
}
var ar = new Array(1,3,4,-1,-2,3,4,-2,4);
console.log(ar.sum(0,2))
</script>
</body>
</html>