Math.round vs hack

JavaScript performance comparison

Revision 37 of this test case created by Hao Wu

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;
var MyMath = {
        round: function( a ) {
                return (a + (a > 0 ? .5 : -.5)) << 0;
        }
};
round = MyMath.round;
</script>

Test runner

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Testing in unknown unknown
Test Ops/sec
"proper" rounding in function call
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…
proper in function call
rounded = round(somenum);
pending…
Proper hack wrapped in function call
rounded = MyMath.round(somenum);
pending…
Proprer hack with bitwise
rounded = (somenum + (somenum > 0 ? .5 : -.5)) | 0;
pending…
Proprer hack with bitwise <<
rounded = (somenum + (somenum > 0 ? .5 : -.5)) << 0;
pending…
Proprer hack with bitwise REVERSED
rounded = 0 | (somenum + (somenum > 0 ? .5 : -.5));
pending…
-~(num-0.5)
rounded = -~(somenum -0.5)
pending…

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