What is the best way to compare a value against 'undefined'?

Question

Is there any differences between

var a;
(a == undefined)
(a === undefined)
((typeof a) == \"undefined\")
((typeof a) === \"undefined\")

Whi

Answer 1

var a;
(a == undefined) //true
(a === undefined) //true
((typeof a) == "undefined") //true
((typeof a) === "undefined") //true

BUT:

var a;
(a == "undefined") //false
(a === "undefined") //false
((typeof a) == undefined) //false
((typeof a) === undefined) //false