Question

84

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

rated 0 times [ 91] [ 7] / answers: 1 / hits: 108520 / 14 Years ago, tue, january 18, 2011, 12:00:00

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?

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tajo

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4 Years ago

answered 14 Years ago kiley · Accepted Answer

For undeclared variables, typeof foo will return the string literal 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.