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What is the difference between null and undefined in
Warn About == null About debugging code About unsafe for..in About arguments.caller and .callee About assignments if/for/ About functions inside loops About eval
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Note: There is no built-in language literal for undefined. Thus (x == undefined) is not a foolproof way to check whether a variable is undefined, because in versions before ECMAScript 5, it is legal for someone to write var undefined = "Im defined now";.
@JamiePate: Just to be clear, I disagree that xyz in window is a better answer than typeof xyz == "undefined" because it is testing the wrong thing.
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