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jQuery's Core Style Guidelines advocate distinct methods for ascertaining the existence of variables:
This begs the question: why does jQuery employ differing approaches for global variables versus local variables and properties?
When an identifier (variable) is not declared, the typeof operator returns the string "undefined". However, an identity comparison with foo === undefined would trigger a JavaScript error: "foo is not defined".
On the other hand, for local variables whose declarations are assumed, there's no danger of error. Consequently, jQuery opts for the identity check in this scenario.
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