JavaScript - Detecting the difference between positive zero and negative zero [duplicate]

Question

I am using MomentJS to calculate the difference between two times. The weird thing is about this library is the difference between today and today is 0. The difference between today and tomorrow is -0.

My question is, how do I differentiate between 0 and -0. It seems JavaScript treats them the same.

So for example if I write the following code:

if (tomorrow === -0)
    console.log('It is tomorrow!');
else if (tomorrow === 0)
    console.log('It is today!');

Here's an example on JSFiddle as to how it handles the returned values (I am in Australia so depending on where you are in the word you may have to adjust the today and tomorrow dates)

Answer 1

1/val > 0 will do what you want. Returns true for positive and false for negative.

This works because 1/-0 returns negative infinity, and 1/0 returns positive infinity, which are then comparable. You could also do something like 1/val == Infinity.

Answer 2

As pointed out in this Stack Overflow question (and in the comments), in JavaScript +0 === -0 indeed evaluates to true by design.

You can use Infinity/-Infinity to see a difference (technically, +0/-0 might not be the only numbers to produce infinite values, so I left the additional check in):

var positiveZero = +0; var negativeZero = -0;  console.log(positiveZero === 0 && 1/positiveZero === Infinity); // true console.log(positiveZero === 0 && 1/positiveZero === -Infinity); // false  console.log(negativeZero === 0 && 1/negativeZero === Infinity); // false console.log(negativeZero === 0 && 1/negativeZero === -Infinity); // true 

JSFiddle

See also this blog post for more details and yet another solution with ECMAScript 5:

function isNegative0(x) {    if (x!==0) return false;    var obj=Object.freeze({z:-0});    try {       Object.defineProperty(obj,'z',{value:x});    } catch (e) {return false};    return true; }