Are +0 and -0 the same?

Question

Reading through the ECMAScript 5.1 specification, +0 and -0 are distinguished.

Why then does +0 === -0 evaluate to true?

Answer 1

I just came across an example where +0 and -0 behave very differently indeed:

Math.atan2(0, 0);  //returns 0
Math.atan2(0, -0); //returns Pi

Be careful: even when using Math.round on a negative number like -0.0001, it will actually be -0 and can screw up some subsequent calculations as shown above.

Quick and dirty way to fix this is to do smth like:

if (x==0) x=0;

or just:

x+=0;

This converts the number to +0 in case it was -0.

Answer 2

I'd blame it on the Strict Equality Comparison method ( '===' ). Look at section 4d

see 7.2.13 Strict Equality Comparison on the specification

Answer 3

Wikipedia has a good article to explain this phenomenon: http://en.wikipedia.org/wiki/Signed_zero

In brief, it both +0 and -0 are defined in the IEEE floating point specifications. Both of them are technically distinct from 0 without a sign, which is an integer, but in practice they all evaluate to zero, so the distinction can be ignored for all practical purposes.

Answer 4

Answering the original title Are +0 and -0 the same?:

brainslugs83 (in comments of answer by Spudley) pointed out an important case in which +0 and -0 in JS are not the same - implemented as function:

var sign = function(x) {
    return 1 / x === 1 / Math.abs(x);
}

This will, other than the standard Math.sign return the correct sign of +0 and -0.

Answer 5

I'll add this as an answer because I overlooked @user113716's comment.

You can test for -0 by doing this:

function isMinusZero(value) {
  return 1/value === -Infinity;
}

isMinusZero(0); // false
isMinusZero(-0); // true

Answer 6

In the IEEE 754 standard used to represent the Number type in JavaScript, the sign is represented by a bit (a 1 indicates a negative number).

As a result, there exists both a negative and a positive value for each representable number, including 0.

This is why both -0 and +0 exist.