undefined check not working in javascript

Checking undefined is not work when we go normal if statement, As like below

var id=$('#txt').val();

if(id != 'undefined')
// Our Code

"id" is var type, i.e mean unknown type, such as whether "id" is string, numeric, or even undefined.  
if "txt" is not defined in the page, then id's value is  undefined, so we shall check the type of operand with value.

By using "typeof" operator we resolve above issue. The "typeof" operator in JavaScript/Jquery allows you to probe the data type of its operand.

var item = 10;
alert(typeof item); This returns the type of item not value.

if(typeof id != 'undefined')
// Our Code

I hope this will help you to resolve the problem.