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 when one of the operands if Boolean, type-converts the other to Number.

[] == 0;

Is equivalent to:

0 == 0;

You can see the complete details of The Abstract Equality Comparison Algorithm on the specification.

As you can see, an empty array object, when converted to Number, produces 0:

+[]; // 0
Number(0);

This is really because its toString method produces an empty string, for example:

[].toString(); // ""

+""; // 0
Number(""); // 0