Math.round vs hack

JavaScript performance comparison

Revision 5 of this test case created by Miller Medeiros

Info

Test of different ways of rounding numbers, results vary a lot according to browsers and JS engines are getting smarter each day, so it is usually a better approach to don't try to be smarter than the engines and use the native feature (in this case, Math.round) since there is a bigger chance of performance improvements in the future and especially because bitwise operations reduces code readability.

Preparation code

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

Test runner

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Testing in unknown unknown
Test Ops/sec
"proper" rounding
rounded = Math.round(somenum);
pending…
Bitwise NOT
rounded = ~~ (0.5 + somenum);
pending…
Proper hack rounding
rounded = ~~ (somenum + (somenum > 0 ? .5 : -.5));
pending…
Bitwise OR
rounded = (0.5 + somenum) | 0;
pending…
Bit shift left
rounded = (0.5 + somenum) << 0;
pending…
Native casting
rounded = parseInt(0.5 + somenum)
pending…
Bitwise AND
rounded = (0.5 + somenum) & (0.5 + somenum)
pending…
Bitshift right
rounded = (0.5 + somenum) >> 0;
pending…
Bitshift right (0 fill)
rounded = (0.5 + somenum) >>> 0;
pending…
Bitwise XOR
rounded = (0.5 + somenum) ^ 0;
pending…
Cached Reference
rounded = round(somenum);
pending…
Math.floor
rounded = Math.floor(somenum + 0.5);
pending…
Math.ceil
rounded = Math.ceil(somenum - 0.5);
pending…

Compare results of other browsers

Revisions

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