Math.round vs hack

JavaScript performance comparison

Revision 53 of this test case created by Christopher Hall

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

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Testing in unknown unknown
Test Ops/sec
"proper" rounding
rounded = Math.round(somenum);
pending…
Hack rounding
rounded = ~~ (0.5 + somenum);
pending…
Proper hack rounding
rounded = ~~ (somenum + (somenum > 0 ? .5 : -.5));
pending…
Hack with bitwise OR
rounded = (0.5 + somenum) | 0;
pending…
Hack with bitwise shift
rounded = (0.5 + somenum) << 0;
pending…
Hack rounding with if statement
if (somenum > 0) {
  rounded = ~~ (0.5 + somenum);
} else {
  rounded = ~~ (-0.5 + somenum);
}
pending…

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