Why in Javascript '' == '0' is false but 0 == '' is true?

Question

I saw an exciting and confusing issue. Based on "JavaScript, the good parts" :

'' == '0'           // false
0 == ''             // true

So, why?!

tl;dr type coercion. http://stackoverflow.com/questions/19915688/what-exactly-is-type-coercion-in-javascript or http://stackoverflow.com/questions/359494/does-it-matter-which-equals-operator-vs-i-use-in-javascript-comparisons for more. — Dan Smith, Feb 09 '15 at 16:47
@Dom Interestingly, PHP considers `"0"` to be a falsy string, presumably because databases pass all values as strings and so a "false" value from a database needed to be considered falsy. Really, really hacky and annoying at times XD — Niet the Dark Absol, Feb 09 '15 at 16:49
Not sure why it confuses you that comparing different values gives a different result. — , Feb 09 '15 at 16:57

score 2 · Answer 1 · answered Feb 09 '15 at 16:48

Simple answer: Because it is.

Advanced answer:

'' == '0' compares the items as strings, since they're both strings. No type changes needed, just compare. They're blatantly different, so false.
0 == '' compares the items as numbers. '' converts to 0 so they are the same.

But really, who cares?

score 1 · Answer 2 · answered Feb 09 '15 at 16:48

In the first case, both are of type String and hence do not equate, as they are compared by their values.

In the second case, the left hand side is a Number, so there is a conversion occurring, which makes '' to 0 and hence true

== results in typecasting and then comparing, if needed. === doesn't typecast, so

0 === '' // false

2 Answers2