Misunderstanding JavaScript type system

var foo=[0];
if(foo) alert('first');
if(foo==true) alert('second');

Tell me please, why the second ale开发者_运维技巧rt 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?

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.

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.

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. :)