Why do I have to use the window. prefix here?

Question

Why on earth does this work:

FOO = window.FOO || {isFoo: true};

But this doesn't:

FOO = FOO || {isFoo: true};

Since FOO and window.FOO both reference the same thing (both are running in the global scope).

Answer 1

Because FOO is not declared, but window is. Trying to access an undeclared variable will throw a ReferenceError, but accessing an undefined property will not.

You can get around it by using typeof:

FOO = typeof FOO != 'undefined' ? FOO : {isFoo: true};