Math.round vs hack

JavaScript performance comparison

Revision 22 of this test case created by icrusade

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 mround = Math.round;
  var hround = function(n){
    return ~~ (0.5 + n)
  };
  var phround = function(n){
    return ~~ (n + (n > 0 ? .5 : -.5));
  };
  var orround = function(n){
    return (0.5 + n) | 0;
  };
  var shround = function(n){
    return (0.5 + n) << 0;
  };
 
  var somenum = -500 + (Math.random() * 1000);
  var rounded;
</script>

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Testing in unknown unknown
Test Ops/sec
"proper" rounding
rounded = mround(somenum);
pending…
Hack rounding
rounded = hround(somenum);
pending…
Proper hack rounding
rounded = phround(somenum);
pending…
Hack with bitwise OR
rounded = orround(somenum);
pending…
Hack with bitwise shift
rounded = shround(somenum);
pending…

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