TypeScript error: Object is possibly 'undefined' after I already checked it

Question

```ts
const user = users.find(u => u.id === id);
if (user) {
  setTimeout(() => {
    console.log(user.name);
  });
}
const cfg = map.get('x');
if (map.has('x')) cfg.value; // error: possibly undefined
```

Why doesn't TS understand my checks?

Accepted Answer

Two different situations:

1. The `user` example actually **works**. Narrowing a `const` carries into closures. If you see an error there, `user` is probably declared with `let`: TS can't prove it wasn't reassigned before the callback runs. Use `const`.

2. `map.has('x')` does **not** narrow `map.get('x')`. TypeScript doesn't link the two calls. Check the value itself:

```ts
const cfg = map.get('x');
if (cfg !== undefined) {
  cfg.value; // ok
}
```

Avoid `cfg!.value` (the non-null assertion) unless you're truly certain. It silences the compiler without adding a runtime check.

1 réponse