The Undefined Type

The Undefined type has only one value, undefined. When a variable is declared without initialization, it is assigned the value of undefined:

 
<!DOCTYPE html>
<html>
<head>
    <title>Undefined Example</title>
    <script type="text/javascript">
          
        var aString;
        document.writeln(aString == undefined);    //true


      
    </script>

</head>
<body>
  
</body>
</html>
Click to view the demo

We can explicitly assign undefined to a variable.

 
<!DOCTYPE html>
<html>
<head>
    <title>Undefined Example</title>
    <script type="text/javascript">
          
        var aString = undefined;
        document.writeln(aString == undefined);    //true

    </script>
</head>
<body>
</body>
</html>
Click to view the demo

A variable containing undefined value is different from an undefined variable.

 
<!DOCTYPE html>
<html>
<head>
    <title>Undefined Example</title>
    <script type="text/javascript">
          
        var aString;     
        
        document.writeln(aString);           //"undefined"
        document.writeln(anotherValue);      //causes an error
      
    </script>

</head>
<body>
</body>
</html>
Click to view the demo
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Data Types:
  1. JavaScript Data Types
  2. typeof Operator
  3. The Undefined Type
  4. null Type
  5. null vs undefined
  6. Boolean Type
  7. Boolean() casting function
  8. The Literials of Number Type
  9. Octal Integer
  10. Hexadecimal
  11. Floating-Point Values
  12. Value range
  13. NaN
  14. Number Conversions:Number(), parseInt() and parseFloat()
  15. Number() function
  16. parseInt()
  17. parseFloat()
  18. The String Type
  19. String Literals and Escapes
  20. Get the String Length
  21. Converting to a String with toString() method
  22. Convert Number to String with radix
  23. Convert to String with String() casting function