Short: There is no difference in the global context**.
Long: The Variable object for the global context is the global context itself. That is why we can just access global variables and methods within a function-, or eval context. However, the var statement just makes sure you're defining a variable in the current context, so it makes no difference by omitting that in the global context. Exception: ES5 strict mode will probably throw an error when it sees a declaration without var.
**
the only difference is, that var will declare a variable without definition in the current (execution) context. This happens at js parse time, so the engine knows that there is a variable with that name available in the context. Omitting var will end up in a direct property access from the global object.
