One thing to note here is that if you create the variable in the global space (i.e. outside of a function scope) then it will be global. If this code is run inside a function then the variable created will be local to that function.
This variable will always be global, no matter where it is created.
Give it a go by running the following:
So, bottom line? It's good practice to define your variables local (as in 'var variable;') so that they don't clobber anything else within the scope. Doing this as a matter of course prevents any future code breaks from global variables.