Why Does JavaScript Treat 0 and -0 as Equal in `===` Comparisons but Not with `Object.is`?

Distinguishing Between 0 and -0 in JavaScript

While the ECMAScript 5.1 specification distinguishes between 0 and -0 numerically, the question arises why the comparison 0 === -0 evaluates to true in JavaScript.

IEEE 754 Standard and Signed Zeros

JavaScript utilizes the IEEE 754 standard for number representation, which allows for the existence of two zeros: 0 (positive zero) and -0 (negative zero). In IEEE 754, zero is typically encoded as 0, but both 0 and -0 are valid representations.

Strict Equality Comparison Algorithm

Section 11.9.6 of the ECMA-262 specification defines the Strict Equality Comparison Algorithm. For numerical comparisons, it explicitly states that 0 and -0 are treated as equal in strict equality comparisons:

If x is 0 and y is −0, return true.
If x is −0 and y is 0, return true.

Therefore, while 0 and -0 may be distinct numerically, they are considered equal in strict equality comparisons for convenience and logical reasons.

Object.is Comparison

ES2015 introduced the Object.is method for more precise comparison. Object.is explicitly distinguishes between -0 and 0, returning false when they are compared:

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

In conclusion, JavaScript uses the IEEE 754 standard and treats 0 and -0 as equal in strict equality comparisons to simplify coding conventions. However, the Object.is method allows for more nuanced comparisons if needed.

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