TypeScript any vs unknown vs never: Choose the Right Type

let value: any;
value = 42;
value = "hello";
value.toUpperCase(); // No error at compile time

let value: unknown;
value = 42;

// Must check type first
if (typeof value === "string") {
  console.log(value.toUpperCase()); // Safe!
}

async function fetchUser(id: string) {
  const response = await fetch(`/api/users/${id}`);
  const data: unknown = await response.json();

  if (isUser(data)) {
    return data;
  }
  throw new Error("Invalid data");
}

function isUser(obj: unknown): obj is User {
  return typeof obj === "object" && obj !== null && "id" in obj;
}

function throwError(message: string): never {
  throw new Error(message);
}

// Exhaustive check example
type Shape = { kind: "circle" } | { kind: "square" };

function getArea(shape: Shape): number {
  switch (shape.kind) {
    case "circle":
      return 0;
    case "square":
      return 0;
    default:
      const _exhaustive: never = shape;
      throw new Error("Unhandled shape");
  }
}