Math.round vs hack

JavaScript performance comparison

Revision 3 of this test case created by Seb Lee-Delisle

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;
</script>

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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…

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4 comments

Mathias Bynens commented :

Also see http://jsperf.com/rounding-numbers-down:

Note that some of the listed alternatives to Math.floor() use bitwise operators, which convert numbers to a 32-bit sequence.

These alternatives will only work with positive signed 32-bit floats, i.e. numbers from 0 to +2,147,483,647 (2^31-1).

~~2147483647.1; // 2147483647
~~2147483648.1; // -2147483648

Antti commented :

Yes, it seems like that when summing numbers together it is good idea to put bigger number first. (Assuming that somenum is over 0.5) So use rounded = (somenum + 0.5) | 0; instead of rounded = (0.5 + somenum) | 0;

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