What is NaN in javascript

Sometimes we are getting NaN output when we are performing some calculation on values.NaN output means we are performing operation on those values which is not integer.
To avoid this problem Javascrip providing a isNaN() function determines whether a value is an illegal number (Not-a-Number). This function returns true if the value is NaN, and false if not.

For example:

var a = '',
b= 2;
var c = a + parseint(b)

In above example when you alert c value, you will get NaN value into c variable.

How to check value is number on not.
if(NaN(b)) {
alert('Not Number');
} else {