JavaScript type casting

Question

Consider empty JavaScript array:

var a = [];
alert(a == false); // shows true
alert(!a); // shows false!

How to explain this? What are the

Answer 1

From http://forums.whirlpool.net.au/archive/966449:

a == false:

In this case, the type of the left-hand side is object, the type of the right-hand side is boolean. Javascript first converts the boolean to a number, yielding 0. Then it converts the object to a "primitive", yielding the empty string. Next it compares the empty string to 0. The empty string is converted to a number, yielding 0, which is numerically equal to the 0 on the right-hand side, so the result of the entire expression is true.

See §11.9.3 of the ECMAScript spec for all the gory details.

(!a):

In this case Javascript converts the object to the boolean true, then inverts it, resulting in false.