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

The ! operator checks whether its operand is "falsy".

The following are true:

• !false
• !0
• !null
• !NaN
• !undefined
• !""

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The == operator checks for loose equality, which has nothing to do with falsiness.

Specifically, a == b will convert to operands to numbers, then compare the numbers.
Strings containing numbers convert to the numbers that they contain; booleans convert to 0 and 1.
Objects are converted by calling valueOf, if defined.

Thus, all of the following are true:

• "1" == 1
• "0" == false
• "1" == true
• "2" != true
• "2" != false
• ({ valueOf:function() { return 2; } }) == 2
• ({ valueOf:function() { return 1; } }) == true