First of all, never compare things like these for performance reasons. Math.round is obviously easier on the eyes than window.Math.round, and you wouldn't see a noticeable performance increase by using one or the other. So don't obfuscate your code for very slight performance increases.
However, if you're just curious about which one is faster... I'm not sure how the global scope is looked up "under the hood", but I would guess that accessing window is just the same as accessing Math (window and Math live on the same level, as evidenced by window.window.window.Math.round working). Thus, accessing window.Math would be slower.
Also, the way variables are looked up, you would see a performance increase by doing var round = Math.round; and calling round(1.23), since all names are first looked up in the current local scope, then the scope above the current one, and so on, all the way up to the global scope. Every scope level adds a very slight overhead.
But again, don't do these optimizations unless you're sure they will make a noticeable difference. Readable, understandable code is important for it to work the way it should, now and in the future.
Here's a full profiling using Firebug:
<!DOCTYPE html>
<html>
<head>
<title>Benchmark scope lookup</title>
</head>
<body>
<script>
function bench_window_Math_round() {
for (var i = 0; i < 100000; i++) {
window.Math.round(1.23);
}
}
function bench_Math_round() {
for (var i = 0; i < 100000; i++) {
Math.round(1.23);
}
}
function bench_round() {
for (var i = 0, round = Math.round; i < 100000; i++) {
round(1.23);
}
}
console.log('Profiling will begin in 3 seconds...');
setTimeout(function () {
console.profile();
for (var i = 0; i < 10; i++) {
bench_window_Math_round();
bench_Math_round();
bench_round();
}
console.profileEnd();
}, 3000);
</script>
</body>
</html>
My results:
Time shows total for 100,000 * 10 calls, Avg/Min/Max show time for 100,000 calls.
Calls Percent Own Time Time Avg Min Max
bench_window_Math_round
10 86.36% 1114.73ms 1114.73ms 111.473ms 110.827ms 114.018ms
bench_Math_round
10 8.21% 106.04ms 106.04ms 10.604ms 10.252ms 13.446ms
bench_round
10 5.43% 70.08ms 70.08ms 7.008ms 6.884ms 7.092ms
As you can see, window.Math is a really bad idea. I guess accessing the global window object adds additional overhead. However, the difference between accessing the Math object from the global scope, and just accessing a local variable with a reference to the Math.round function isn't very great... Keep in mind that this is 100,000 calls, and the difference is only 3.6ms. Even with one million calls you'd only see a 36ms difference.
Things to think about with the above profiling code:
- The functions are actually looked up from another scope, which adds overhead (barely noticable though, I tried importing the functions into the anonymous function).
- The actual
Math.round function adds overhead (I'm guessing about 6ms in 100,000 calls).