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Its because they don't do the same thing

Math.floor() favors the number equal to/less than the parameter, Math.floor(-15.5) is -16 while (-15.5 | 0) is -15

Also because the returned value is int32[0],

Math.floor(2147483648.5) is 2147483648 while (2147483648.5 | 0) is -2147483648

You are fine if you know the input is less than (2^31) + 1 and you want to truncate, rather than floor.

ECMAScript 2016 adds Math.trunc()[0] which should give the same value as | 0 for inputs that fit in 32 bits.

Hmm, pretty sure floor rounds a number downward to its nearest integer. So Math.floor(-15.5) would be 15.

Nope. Math.floor(-15.5) would be -16. It rounds a number downwards. -16 < -15.

Literally the first example on that page:

> Math.floor( 45.95); // 45

> Math.floor(-45.95); // -46

Its not like its hidden or anything... Its in the center of the page on my 1920x1080 screen.