Property '...' does not exist on type '...'

Access on a type that doesn't declare this property. 1. Check if you should use a discriminated union + narrowing. 2. Extend the type: `interface Bar { foo?: string }`. 3. For dynamic objects: `(obj as Record<string, unknown>).foo`. 4. If from a 3rd-party lib missing types: install `@types/<pkg>` or add `declare module '<pkg>'` shim.