Why is "0" true only in if-statement [duplicate]

This question already has an answer here:

• In Javascript, why is “0” equal to false, but when tested by 'if' it is not false by itself? 9 answers

This expression

if("0"){ alert("true");}else{alert("false");}

results "true".

Why is that, when

if("0" == true){alert("true");}else{alert("false");}

results "false"?

I'm used to

"0" == 0 == false

Seems like that does not hold true only in an if-statement.

Is this the intendet behavoir? Can some one shed some light on it?