When analyzing the code in the shared playground (Playground Link), the compiler is showing an error indicating that Object is possibly 'null'.
Could there be any scenario where the refresh function could be called, leading to a situation where viewer ends up being null despite having an early return check for null in the topLayer function?
If not, have you considered why TypeScript fails to narrow down this possibility in cases involving nested functions?
Here's an example that is similar but not identical and somewhat contrived:
declare var user: { id: string } | null;
let _user = { id: 'id' } as { id: string } | null
Object.defineProperty(window, 'user', {
get() {
const val = _user;
_user = null;
return val
}
})
function displayInfo() {
if (!user) return;
function showId() {
console.log(user.id); // displays `Cannot read property 'id' of null` error
}
showId();
}
displayInfo();
Although TypeScript attempts to narrow types based on heuristics, it doesn't check all possible values of the user variable during execution. The new function showId creates a new scope for the user free variable (not local or parameter) where its type is still not narrowed.
If you rewrite it like this:
function displayInfo() {
if (!user) return;
function showId(user: { id: string }) {
console.log(user.id);
}
showId(user);
}
Everything functions as intended.
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