How to know if a variable is null

[peace2007]

Hi,

I try to check if a variable is null via following code but I don't get an expected result:

 var askedUser = re.responseText;
		    askedUser = askedUser.substr(0, askedUser.indexOf('&',0));
			if(askedUser != null  && askedUser != " ")
			{	do something}

the if() returns true whether askedUser == "" or askedUser = "a value"!
Could anyone guide me please?