That example looks like a strawman to me. In practice, x would probably originate from outside the function, as a parameter or class member, and there is a good

That example looks like a strawman to me. In practice, x would probably originate from outside the function, as a parameter or class member, and there is a good chance it could hold the value null or undefined. Seems like way too much additional work with little to no benefit to make the compiler track things like that through the whole application. Worse, the variable state could intentionally be hardcoded to try things and it would be weird if the compiler would error because the variable is never set to undefined, when I might be hardcoding it to undefined later on.