A compelling title for your article could be: Why Does jQuery Use Different Methods for Checking Variable Existence?

Variable Existence Comparison: === undefined vs. typeof === "undefined"

jQuery's Core Style Guidelines advocate distinct methods for ascertaining the existence of variables:

• Global Variables: typeof variable === "undefined"
• Local Variables: variable === undefined
• Properties: object.prop === undefined

This begs the question: why does jQuery employ differing approaches for global variables versus local variables and properties?

Undecided Variables: typeof vs. ===

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".

Localized Variables: Identity Check

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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