Negative Zero in JavaScript

Classical mathematics has no notion of a negative zero. Zero is often termed neither positive nor negative. In computing, however, there is a concept of signed zero.

JavaScript uses the IEEE 754 standard to represent numbers, which results in the language having a -0 and 0.

The IEEE 754 standard for floating point arithmetic (presently used by most computers and programming languages that support floating point numbers) requires both +0 and −0. The zeroes can be considered as a variant of the extended real number line such that 1/−0 = −∞ and 1/+0 = +∞, division by zero is only undefined for ±0/±0 and ±∞/±∞.

Although we often don’t distinguish between -0 and 0 in everyday programming, it might be useful for rare cases. For example, a negative zero can be used to express a sense of direction (left or right, up or down). This could also be helpful in representing a moving object on a map (x and y coordinates).

There are certain quirks when working with negative zeros in JavaScript which I’ll try to summarize in this post.

Equality

Although -0 and 0 should be treated different, the conventional equality operators == and === treat them as same (according to the rules of equality).

const a = 0;
const b = -0;

console.log(a == b); // true
console.log(a === b); // true

In order to correctly compare -0 and 0 we should use the Object.is() method. Object.is() unlike the == operator, doesn’t coerce and unlike the === operator doesn’t treat values differently.

const a = 0;
const b = -0;

console.log(Object.is(a, b)); // false

Object.is() unlike == and === also treats NaN as same.

console.log(NaN == NaN); // false
console.log(NaN === NaN); // false

console.log(Object.is(NaN, NaN)); // true

Sign

Math.sign() can be used to determine the sign of a number. It also exhibits a different behavior when working with zeroes.

// for non zero values
console.log(Math.sign(13)); // 1
console.log(Math.sign(-13)); // -1

// for zeroes
console.log(Math.sign(0)); // 0
console.log(Math.sign(-0)); // -0