JavaScript Fundas: null, undefined, nan, false

What does ==, ===, false, undefined and nan mean in Javascript?

Comparison via ==

== indicates Equality, regardless of type.

Comparison via ===

=== indicates identity, types must match.

Pitfalls using comparison
For boolean operations it evaluates to false. Use === when comparing to the number 0.

var x = 0;
var y = false;

x == y → true
x === y → false


"" (empty string)

Evaluates to false in boolean operations. Use === when comparing to an empty string.

var x = "";
var y = false;

x == y → true
x === y → false


null

Evaluates to false in boolean operations. Use === when comparing to null.

var x = null;
var y = false;

x == y → true
x === y → false


undefined

If a variable hasn't been declared or assigned yet (e.g: an argument to a function which never received a value, an object property that hasn't been assigned a value) then that variable will be given a special undefined value.

Evaluates to false in boolean operations. Always use === when comparing to undefined.

var x;
var y = false;

typeof(x) → undefined (as a string)

x == y → true
x === y → false


NaN

Not a Number, generated when arithmetic operations return invalid results.

Evaluates to false in boolean operations. Always use isNaN() when comparing to NaN.

var x = 10/seventeen;

x → NaN

NOTE: NaN is never equal to itself!

var x = 10/seventeen;

x == x → false


Courtesy: http://www.hunlock.com/blogs/Essential_Javascript_--_A_Javascript_Tutorial
This is a very good JavaScript site. Many of my misconceptions were cleared when I went thru this site.

Hope these concepts help you!