Part 5.3Fetching data safely

Calling response.json() gives data from outside your program. A TypeScript annotation cannot prove that a server actually returned that shape.

JSON is unknown

const response = await fetch("/api/user");
const data: unknown = await response.json();

Validate at the boundary

Use a type guard to test the data before your application uses it. This keeps uncertainty at the network boundary instead of pretending a cast has made untrusted data safe.