Narrowing Unknown Data Payload with Type Predicates

interface UserProfile {
  id: string;
  role: "admin" | "member";
}
 
function isUserProfile(data: unknown): data is UserProfile {
  return (
    typeof data === "object" &&
    data !== null &&
    "id" in data &&
    typeof (data as Record<string, unknown>).id === "string" &&
    "role" in data &&
    ((data as Record<string, unknown>).role === "admin" || (data as Record<string, unknown>).role === "member")
  );
}
 
function processPayload(payload: unknown) {
  if (isUserProfile(payload)) {
    console.log(`Valid user: ${payload.id} (${payload.role})`);
  }
}