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JavaScript is different from other programming languages by containin both undefined and null values, which may cause confusion.

null is a special value meaning "no value". null is a special object because typeof null returns 'object'.

On the other hand, undefined means that the variable has not been declared, or has not been given a value.

Following snippets display 'undefined':

// i is not declared anywhere in code
alert(typeof i);

var i;
alert(typeof i);

Although null and undefined are slightly different, the == (equality) operator considers them equal, but the === (identity) operator doesn't.

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Comments

June 20. 2008 11:36

Interesting article. Succinct.

Simon

Simon | Reply

October 21. 2009 16:32

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