js-negative-zero.md

JavaScript: Negative Zero (-0)

In JavaScript, negative zero -0 is not the same as a positive zero +0.

This is because numbers in JavaScript are represented using the IEEE 754 floating-point standard which requires zeros to have an associated sign. Floating point numbers include a sign bit (0 for positive, 1 for negative). In the case of +0, the sign bit is 0 while in the case of -0 the sign bit is 1.

How does JavaScript handle comparison?

+0 === -0 // true
-0 === +0 // true

This is because of ECMAScript's Strict Equality Comparison Algorithm:

If Type(x) is Number, then
a. If x is NaN, return false.
b. If y is NaN, return false.
c. If x is the same Number value as y, return true.
d. If x is +0 and y is −0, return true.
e. If x is −0 and y is +0, return true.
f. Return false.

How to distinguish between the two?

Object.is() can be used:

Object.is(+0, -0); // false
Object.is(-0, +0); // false

How are strings handled?

Both +0 and -0 will return "0".

const negativeZero = -0;
negativeZero.toString() // "0"

const positiveZero = +0;
positiveZero.toString() // "0"

JSON.stringify({"negativeZero": -0}); // '{"negativeZero":0}'