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Comment by simonsarris | Hacker News Reader

Here's another one for you, one of my favorites that used to be drastic: http://jsperf.com/pipetrunc

    blah | 0; // fast!
    Math.floor(blah); // slow(er)! (except on FF nightly)

Caveat: Only works with numbers greater than -2147483649 and less than 2147483648.

Another caveat: for negative numbers, the bit-or rounds to zero while the floor rounds to negative infinity:

    > -0.5
    -0.5

    > (-0.5)|0
    0

    > Math.floor(-0.5)
    -1

A fine example why this sort of "who needs the specs, FULL SPEED AHEAD" hyper-optimization can get people into trouble.

Caveat: if you see numbers in the range of 1,500,000,000 that means your code was thrown away by the optimizing compiler.

Microbenchmarks like that require a lot of care to measure something correctly. Check out my talk from LXJS2013 for more details: slides http://mrale.ph/talks/lxjs2013/ and video https://www.youtube.com/watch?v=65-RbBwZQdU