В чём разница между `any`, `unknown` и `never`? Когда что использовать?

// any: the compiler shuts off and the error arrives at runtime
const anyData: any = "hello";
anyData.toFixed(2); // compiles... and blows up at runtime
const spread = anyData + 1; // spread is also any

// unknown: accepts everything, but demands a check before use
function processInput(input: unknown): string {
  // input.toUpperCase();
  // Error: 'input' is of type 'unknown'.

  if (typeof input === "string") {
    return input.toUpperCase(); // we've verified: here it's string
  }
  if (typeof input === "number") {
    return input.toFixed(2);
  }
  return "unsupported type";
}

// Real-world unknown: JSON.parse returns any, better to treat it as unknown
const raw: unknown = JSON.parse('{"name":"Ana"}');

// never: functions that never return
function fail(message: string): never {
  throw new Error(message);
}

// never in exhaustiveness: guarantees we cover every case
type Role = "admin" | "user";

function describeRole(role: Role): string {
  switch (role) {
    case "admin": return "Full access";
    case "user": return "Limited access";
    default: {
      const nothing: never = role; // if Role grows, this stops compiling
      return nothing;
    }
  }
}

// A junior does this to silence the error:
const response: any = await fetch("/api/user").then((r) => r.json());
console.log(response.name.toUpperCase()); // can blow up at runtime

// Better: unknown + verification
const data: unknown = await fetch("/api/user").then((r) => r.json());
if (typeof data === "object" && data !== null && "name" in data) {
  console.log(data.name); // safe access after narrowing
}