var foo=[0];
if(foo) alert('first');
if(foo==true) alert('second');
Tell me please, why the second alert doesn't work? In first alert foo casts to Boolean, so
Boolean(foo);
//true
If "foo" is "true", why doesn't the second alert work?
3 Answers
Here,
if( foo) alerts because foo isn't null and the condition evaluates to true.
However, this doesn't mean that foo itself is equal to true, hence the second alert doesn't show.
answered Mar 16, 2011 at 10:16
Shamim Hafiz - MSFT
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Comments
Because initially foo is an Array, not a Boolean and you are comparing foo to a boolean value. The if (...)-statement evaluates to true or false and foo == true evaluates to false here. If you used if (!!foo == true) (or just if (!!foo)) or if (foo != null) or if (foo) the second alert would have fired.
Comments
Because there's a difference between the conversion of foo to boolean (which works for an Array) and the comparison of foo to true.
In the latter case, it's a comparison without a conversion, and foo clearly is not the same as true. Note that a conversion does still take place: foo == true is false which is finally "converted" to false for the if. :)
answered Mar 16, 2011 at 10:53
Lightness Races in Orbit
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7 Comments
Scalar
There is a special operator "compare with type check" === in JS and PHP. So operator "==" makes comparison WITH type casting.
Lightness Races in Orbit
@Scalar: I like how you're trying to teach me something that directly contradicts the behaviour you've observed that you asked for help on in the first place.
Scalar
@Tomalak Geret'kal: Sorry, may be i don't understand you, cause my English is not very good :-[ I thougth that you dont see difference between "==" and "===". My first question is the last thing i don't understand in JS type system.
Lightness Races in Orbit
=== is the strict equal operator and only returns a Boolean true if both the operands are equal and of the same type. == is the equal operator and returns a boolean true if both the operands are equal... which they're still not.Scalar
But, for example
[1]==true;//true, wow! and [0]==true;//false and [0]==false;//true, but [2]==true;//false and [2]==false;//false. I understand how it works, now, when i read ECMA-262 (ecma-international.org/publications/files/ECMA-ST/ECMA-262.pdf), but i still don't understand algorithm of if statement expression check. |
true?