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Sometimes you have a variable in Javascript and do not know whether it is a number or a string. You can test the type of a variable by using the typeof() operator like this:

     Var x     = 1.234;
     Var y     = "Hello";
     Var Type1 = typeof(x); 
     Var Type2 = typeof(y);  
 
     Alert(Type1 + " and " + Type2);

...

 "undefined" is useful because it tells you that something does not exist yet (ie: it's undefined) so sometimes you see code like this:

     if (typeof(objGlobal.CustomerNumber) = "undefined")) objGlobal.CustomerNumber = "12345";