Math.round vs hack

JavaScript performance comparison

Revision 54 of this test case created

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 defer="defer" src="http://ajax.googleapis.com/ajax/libs/jquery/1.4.3/jquery.min.js">
</script>
<script>
  var util = {
    round: Math.round
  };

  function hack_round(n) {
    return (n >= 0) ? (n - 0.5) << 0 : (n + 0.5) << 0;
  }

  if ($.browser.mozilla) {
    util.round = function(n) {
      return (n >= 0) ? (n - 0.5) << 0 : (n + 0.5) << 0;
    };
    alert('blah');
  }

  function r(n) {
    return ~~ (0.5 + n);
  }

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

Preparation code output

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 call overhead
rounded = hack_round(somenum);
pending…
util.round
rounded = util.round(somenum);
pending…
Hack with bitwise shift wrapper
rounded = r(somenum);
pending…

Compare results of other browsers

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