Entry

How can I check whether a variable is defined?

How can I check whether a variable is undefined?

Mar 7th, 2000 01:19
Martin Honnen,

You can check whether a variable is undefined with
  if (typeof variableName == 'undefined')
Note that there are a couple of situations causing that condition to be 
true:
1) You have NOT declared
     var variableName;
   before. In that case the above check is the only safe check as
     if (!variableName)
   would throw an error.
2) You have declared
     var variableName;
   before but not assigned it any value. In that case 
     if (!variableName)
   is another valid check for the variable having a undefined value.

That means the above check doesn't allow you to distinguish whether a 
variable has been declared or not. Only JavaScript 1.5 with the 
  in 
operator will allow that as there you can check
  if ("variableName" in window)
as for example in
  var x;
  alert("x" in window); // shows true
  alert("y" in window); // shows false