Javascript: Why is 0 equal to -0?


I want to check if a value is negative or not (I am doing it with Math.sign ).

Math.sign(30) // 1
Math.sign(-10) // -1
Math.sign(0) // 0
Math.sign(-0) // -0

But if the value is 0 or -0 I cannot know exactly if it is negative or positive, because:

Math.sign(30) === 1 -> true
Math.sign(-10) === -1 -> true
Math.sign(0) === -0 -> true // ¿Por qué? ¿No debería ser false?


According to the documentation:

  • Same-value-zero equality :

    Similar to same-value equality, but +0 and -0 are considered equal.

  • Abstract equality, strict equality and same value equality in the specification.

    In the ES5 specification, the == comparison is described in Section 11.9.3, The Abstract Equality Algorithm . The === comparison in 11.9.6, The Strict Equality Algorithm . (Look it up and read it, they are short and easy to read. Note: read the strict equality algorithm first.) ES5 also describes, in Section 9.12, The SameValue Algorithm for internal use by the JS engine.

  • The Strict Equality Comparison Algorithm

    ( Translated )
    Comparison x === y , where x and y are values, returns true or false . This comparison is made as follows:
    1. If Type (x) is different from Type (y) , it returns false .
    2. If Type (x) is Undefined , it returns true .
    3. If Type (x) is Null , it returns true .
    4. If Type (x) is Number , then:
    — If x is NaN , it returns false .
    — If y is NaN , it returns false .
    — If x is the same numeric value as y , it returns true .
    If x is +0 and y is −0, return true .
    If x is −0 and y is +0, return true .
    — Otherwise it returns false .
    5. If Type (x) is String , it returns true if x and y are exactly the same sequence of characters (the same length and the same characters in the corresponding positions); otherwise it returns false.
    6. If Type (x) is Bool , return true if x and y are both true or both false; otherwise, it returns false .
    7. Returns true if x and y refer to the same object. Otherwise, it returns false .

How to differentiate 0 from -0 ?

You could use number.toLocaleString()

The toLocaleString() method returns a string with a language-sensitive representation of this number.


let a = (0).toLocaleString('en'),
  b = (-0).toLocaleString('en');
console.log(`${a} === ${b} =>`, a === b);
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