Math.round vs hack

JavaScript performance comparison

Revision 63 of this test case created by Sergey

Info

Math.round() has a function call overhead, so using the ~~ hack (truncate towards 0) and adding 0.5 works quicker, but if you want to handle negative numbers too then you have to check whether to add or subtract 0.5 .... and this wipes out the speed advantage. So ~~(0.5+num) is only worth it if you know your numbers always have the same sign...

Preparation code

<script>
  var somenum = -500 + (Math.random() * 1000);
  var rounded;
</script>

Test runner

Warning! For accurate results, please disable Firebug before running the tests. (Why?)

Java applet disabled.

Testing in unknown unknown
Test Ops/sec
"proper" rounding
rounded = Math.round(somenum);
pending…
Hack rounding
rounded = ~~ (0.5 + somenum);
pending…
"proper" floor
rounded = Math.floor(somenum);
pending…
Hack floor
rounded = somenum << 0;
pending…
Hack with bitwise shift
 
pending…
xor with 0
 
pending…
bitwise and
 
pending…
bitwise and 64-bit
 
pending…

Compare results of other browsers

Revisions

You can edit these tests or add even more tests to this page by appending /edit to the URL. Here’s a list of current revisions for this page:

0 comments

Add a comment