Typed Functions — Parameters & Return Types

// ── Required parameters ───────────────────────────────────────
function add(a: number, b: number): number {
  return a + b;
}

// ── Optional parameters — must come AFTER required ─────────────
function greet(name: string, greeting?: string): string {
  return `${greeting ?? "Hello"}, ${name}!`;
}
greet("Alice");            // ✅ "Hello, Alice!"
greet("Bob", "Hey");       // ✅ "Hey, Bob!"

// ── Default parameter values ───────────────────────────────────
function createTask(
  title: string,
  priority: Priority = "medium",
  status: TaskStatus = "todo",
): Task {
  return {
    id: Date.now(),
    title,
    priority,
    status,
    description: null,
    assignee: null,
    dueDate: null,
    tags: [],
    createdAt: new Date(),
    updatedAt: new Date(),
  };
}

// ── Rest parameters ────────────────────────────────────────────
function addTags(task: Task, ...tags: string[]): Task {
  return { ...task, tags: [...task.tags, ...tags] };
}
addTags(task, "design", "urgent", "backend"); // ✅

// ── Named / destructured parameters ───────────────────────────
function createUser({
  name,
  email,
  role = "member",
}: {
  name: string;
  email: string;
  role?: "member" | "admin";
}): User {
  return { id: Date.now(), name, email, role, createdAt: new Date() };
}

// ── Explicit return type annotation (recommended for public APIs) ──
// Without annotation, TS infers — but explicit prevents accidental return type drift:
export function getTaskById(id: number, tasks: Task[]): Task | undefined {
  return tasks.find((t) => t.id === id);
}

// ── void vs never return types ────────────────────────────────
function logError(message: string): void {
  console.error(message);
  // void = function returns, but callers shouldn't use the return value
}

function throwError(message: string): never {
  throw new Error(message);
  // never = function never returns (throws or loops forever)
}