Why is 0 == "" true in JavaScript

Question

Why is 0 == "" true in JavaScript? I have found a similar post here, but why is a number 0 similar an empty string? Of course, 0 === "" is false.

You mean 0 is "casted" in false, "" is casted in false and the check is false == false => true — Horst Walter, Sep 30 '11 at 01:12
I believe it's because Javascript automagically coerces strings and numbers in some contexts, the == operator is one of those contexts, and "" coerces to 0. — millimoose, Sep 30 '11 at 01:14
@HorstWalter No. The sting is coerced to the Number type. See my answer. — Šime Vidas, Sep 30 '11 at 01:15
@ŠimeVidas: That comment with all the up votes is remarkably different from your answer (which is much better). — user113716, Sep 30 '11 at 01:28
@ŠimeVidas: Ah, so you were thinking logically. Don't forget we're talking about JavaScript here. ;) — user113716, Sep 30 '11 at 01:33
@Ӫ_._Ӫ Yea, I had the idea that a `==` comparison of two falsy values always evaluates to true. But then I remembered `NaN != NaN`... and that whole idea collapsed lol. — Šime Vidas, Sep 30 '11 at 01:42
@ŠimeVidas: Yep, `NaN` is the one value that will never pass a `==` test (or it will *always* pass a `!=` test). — user113716, Sep 30 '11 at 01:50

score 74 · Accepted Answer · answered Sep 30 '11 at 01:12

74

0 == ''

The left operand is of the type Number.
The right operand is of the type String.

In this case, the right operand is coerced to the type Number:

0 == Number('')

which results in

0 == 0

From the Abstract Equality Comparison Algorithm (number 4):

If Type(x) is Number and Type(y) is String, return the result of the comparison x == ToNumber(y).

answered Sep 30 '11 at 01:12

Šime Vidas

Thanks, I did just expect it the other way round, the 0 converted to string and then false. – Horst Walter Sep 30 '11 at 01:20
Yes, on the other hand this means that saying both are falsy - as in the other answers, is not quite correct. Because - as you very well showed - the reason is that "" => 0. Thanks for your support! – Horst Walter Sep 30 '11 at 01:26
1

@Horst - yes, that's right. Both _are_ falsy, as you can see if you use them alone in `if (0)` or `if("")`, but in the case of an `==` comparison that's not what's happening. (Not sure why Šime also left a comment above saying "Because both value are falsy".) – nnnnnn Sep 30 '11 at 01:29
3

Ooh, I didn't know about es5.github.com. Much handier than typing out page numbers into the PDF. – millimoose Sep 30 '11 at 01:32
@nnnnnn That was my first reaction. It turned out to be incorrect. – Šime Vidas Sep 30 '11 at 01:47
Which sometimes makes me hate Javascript's conversion system. In a language like R, it is 0 which would be converted to character "0" and the result would be, as expected, false. Converting 0 to "0" does have some logic, but converting "" to 0 doesn't have any logic at all... – Adrian Jan 01 '17 at 14:34

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