JavaScript check if variable exists (is defined/initialized)

Question

Which method of checking if a variable has been initialized is better/correct? (Assuming the variable could hold anything (string, int, object, function, etc.))

Answer 1

It is difficult to distinguish between undefined and null. Null is a value you can assign to a variable when you want to indicate that the variable has no particular value. Undefined is a special value which will be the default value of unassigned variables.

var _undefined;
var _null = null;

alert(_undefined); 
alert(_null); 
alert(_undefined == _null);
alert(_undefined === _null);