Why Does jQuery Use Different Methods for Checking Undefined Variables in Global and Local Scopes?

Why jQuery Distinguishes Undefined Checks for Globals and Locals

The jQuery Core Style Guidelines provide two distinct methods to verify whether a variable is defined:

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

Let's explore the rationale behind jQuery's approach.

For undeclared variables, typeof foo returns "undefined". However, the identity comparison foo === undefined would lead to the error "foo is not defined" because foo does not exist in the runtime.

In contrast, for local variables (which are explicitly declared), no such error would arise. Therefore, jQuery employs the identity check (variable === undefined) for local variables and local properties of objects. This approach is safe to use because declared variables and properties are always defined, even if they have no initial value.

However, when dealing with global variables, typeof variable === "undefined" is used instead. This is because global variables may or may not be declared, and using variable === undefined would result in a runtime error if the variable is undeclared.

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