Testing for -0 (negative zero) in JavaScript

http://www.wirfs-brock.com/allen/posts/128

by mbrubeck • 15 years ago

39 10 15 years ago

pzxc• 15 years ago

But what's the point? What is this useful for, or is it only academic?

sp332• 15 years ago

Negative 0 means that you have a negative value, but the value is too small to be represented with available precision. It's a way of noting the sign of the value even if the magnitude is unavailable. It could be useful if you're testing edge cases, or if you're looking for signs of an infinity (or negative infinity) in your program.

jckarter• 15 years ago

There are lots of edge cases where the sign of zero affects computation. In floating-point, zero values represent not only the exact value zero but the result of an underflowed computation too small to represent as a non-zero floating point value, so retaining the sign is often important to at least keep such computations on the correct side of a branch cut or limit. For example:

x/+0 = +infinity

x/-0 = -infinity

sqrt(+0) = 0

sqrt(-0) = nan

log(+0) = -infinity

log(-0) = nan

joshzayin2• 15 years ago

I'll admit I'm not familiar with JS, but assuming that it conforms to the IEEE standard for floating point and you can do bitwise logical operations, can't you just AND it with a 1 in the sign bit place and 0s in all other places, then do a logical right shift of it and see if it's 1? (In the IEEE floating point standard, negative numbers have a 1 in the most significant bit.)

whatanaccount• 15 years ago

Bitwise operations don't really make sense on floating point data, especially not in javascript. Based on a quick one minute test it appears that javascript just uses the integer part of the number if you try bitwise operations (e.g. 2.6 & 6.2 == 2)

joshzayin2• 15 years ago

There's no way to force it to interpret the bits logically, rather than as a floating point number?

pilif• 15 years ago

What concerns me about the ES5 solution is that with freeze, apparently we got yet another different way to evaluate equality.

if(value) behaves differently from if (value == true), behaves differently from if (value === true), which again behaves differently from that check in freeze.

IMHO, that freeze check should have behaved the same as ===, so the proposed solution should not have worked.

Oh well.

Testing for -0 (negative zero) in JavaScript

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