Math.round vs hack

JavaScript performance comparison

Revision 45 of this test case created by Christian


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

  var somenum = (Math.random());
  var rounded;

Test runner

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Testing in unknown unknown
Test Ops/sec
"proper" rounding
rounded = Math.round(somenum);
Hack rounding
rounded = ~~ (0.5 + somenum);
Proper hack rounding
rounded = ~~ (somenum + (somenum > 0 ? .5 : -.5));
Hack with bitwise OR
rounded = (0.5 + somenum) | 0;
Hack with bitwise shift
rounded = (0.5 + somenum) << 0;
Hack with bitwise OR large num first
rounded = (somenum + 0.5) | 0;

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1 comment

MySchizoBuddy commented :

What if the hacks are inside a function call as well? does that eliminate their advantage

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