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Danger Zone

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There’s one aspect of constructors you need to be very careful about: don’t forget to use the new keyword. It’s easy to do because a constructor is, after all, a function, and you can call it without new. But if you forget new on a constructor it can...


Cover of Head First JavaScript Programming


What does "undefined" mean?
It means that what are you are calling for is not defined anywhere in the scope which you are checking for what you are looking for.