Understanding the Distinction: variable === undefined vs. typeof variable === "undefined"

In the jQuery Core Style Guidelines, two methods are proposed to verify if a variable is defined:

• Global Variables: typeof variable === "undefined"
• Local Variables and Properties: variable === undefined

Why this difference?

The explanation lies in the behavior of these operators when dealing with undeclared variables. For undeclared variables, typeof foo will return "undefined" as a string. However, the identity check foo === undefined will raise the error "foo is not defined."

Contrast this with local variables. Since they are explicitly declared somewhere, attempting the identity check variable === undefined will not trigger an error.

Therefore, jQuery uses the typeof operator for global variables, which may or may not be declared, to avoid potential errors. For local variables and properties, where declaration is ensured, the identity check is preferred due to its simplicity.