Math.round vs hack (utility function)

JavaScript performance comparison

Revision 21 of this test case created by JeffMo

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...

This compares Math.round() with the hack methods in http://jsperf.com/math-round-vs-hack/3 wrapped in a utility function.

Preparation code

<script>
  var somenum = -500 + (Math.random() * 1000);
  var rounded;
 
  function hack_rounding_utility(value) {
    return ~~ (0.5 + value);
  }
 
  function proper_hack_rounding_utility(value) {
    return ~~ (value + (value > 0 ? .5 : -.5));
  }
 
  function hack_rounding_bitwise_or(value) {
    return (0.5 + value) | 0;
  }
 
  function hack_rounding_bitwise_shift(value) {
    return (0.5 + value) << 0;
  }
</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 (utility function)
rounded = hack_rounding_utility(somenum);
pending…
Proper hack rounding (utility function)
rounded = proper_hack_rounding_utility(somenum);
pending…
Hack with bitwise OR (utility function)
rounded = hack_rounding_bitwise_or(somenum);
pending…
Hack with bitwise shift (utility function)
rounded = hack_rounding_bitwise_shift(somenum);
pending…

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