Math.round vs hack

JavaScript performance comparison

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

Preparation code

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

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Testing in CCBot 2.0.0 / Other 0.0.0
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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Revisions

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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;