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Monthly Archives: May 2011


I learned something interesting about JavaScript undefined. Here is the axioms and proofs.

  1. undefined is a type.
  2. undefined type has only one value. that values is undefined.
  3. There is a global variable called undefined
  4. global undefined variable has default value undefined
  5. global undefined variable is not a constant
  6. undefined is not a keyword
Proof (tested in firebug console)

1. typeof undefined \\undefined

2. undefined = 1; alert(typeof undefined !== 'undefined'); // false

3. var name = undefined;  \\ global variable undefined is assigned to name

4. alert(name); \\ undefined, that means undefined global variable has value undefined

5. undefined = 'rajakvk';  \\ we can change value

6. var undefined = 'rajakvk';  \\ no syntax error
    var if = 'rajakvk';  \\ SyntaxError: missing variable name

This gives very interesting findings and helps to understand strange JavaScript bugs.

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