variable === undefined vs. typeof variable === "undefined"

Question

The jQuery Core Style Guidelines suggest two different ways to check whether a variable is defined.

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

Why does jQuery use one approach for global variables and another for locals and properties?

Answer 1

For undeclared variables, typeof foo will return the string "undefined", whereas the identity check foo === undefined would trigger the error "foo is not defined".

For local variables (which you know are declared somewhere), no such error would occur, hence the identity check.