javascript - How to distinguish between a variable that is not declared and a varrable that is declared but not be assigned any value?

Question

Both scenario, the typeof the variable will be "undefined". But undeclared variable will raise a exception.

Is there a easy way to handle this?

Answer 1

You may find the question (and my answer) in How to check if a variable or object is undefined? relevant. In general, I view any access to an "undeclared variable" a programming error.

However, this particular case can *only** be detected with the use of detecting for a ReferenceError exception. But, yuck, yuck, yuck! Remember variable declarations are a static lexical construct, ignoring the quirks with property-variables of the global object.

ReferenceError, and now "strict", exist for a reason and I suspect this is an X-Y problem. I do not even recommend the use of typeof for this purpose: fix the code :-)

Happy coding.

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*It has been pointed out that "variable" in window will also [and only] work for global "variables" (they are really just properties that don't need be qualified in all contexts).