Why Does ` 0 === -0` Evaluate to True in JavaScript, While `Object.is(-0, 0)` Returns False?

The Difference and Equality of 0 and -0

The ECMAScript 5.1 specification distinguishes between 0 and -0, raising the question of why 0 === -0 evaluates to true.

IEEE 754 Standard

JavaScript employs the IEEE 754 standard for representing numbers. According to IEEE 754, signed zero allows for positive zero ( 0) and negative zero (-0). This distinction is required for floating-point arithmetic, as 1/-0 = -∞ and 1/ 0 = ∞.

Strict Equality Comparison Algorithm

Despite the technical distinction, section 11.9.6 of the specification explicitly defines the behavior of the strict equality comparison algorithm for 0 and -0:

• If x is 0 and y is -0, return true.
• If x is -0 and y is 0, return true.

Logical and Practical Rationale

Logically, it makes sense to treat 0 and -0 as equal. Distinguishing between them would complicate code, especially when working with zero.

Object.is Comparison Method

ES2015 introduced Object.is for more precise comparisons. Object.is explicitly distinguishes between 0 and -0:

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

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