SOLUTION: Challenge 1 - Union & Type Guard
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Challenge: Write a function that accepts either a User object or a user id (number).
Inside, use a type guard to determine which was passed and handle each case differently.

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SOLUTION:

interface User {
  id: number;
  name: string;
  email: string;
}

type UserInput = User | number;

function getUser(input: UserInput): void {
  // Type guard using typeof
  if (typeof input === "number") {
    console.log(`Fetching user with ID: ${input}`);
    // Here you'd typically fetch from a database
  } else {
    // TypeScript now knows input is a User object
    console.log(`User: ${input.name} (${input.email})`);
  }
}

// Test cases
const userId = 42;
getUser(userId);  // Output: Fetching user with ID: 42

const user: User = {
  id: 1,
  name: "Alice",
  email: "alice@example.com"
};
getUser(user);  // Output: User: Alice (alice@example.com)

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EXPLANATION:

1. We define a union type UserInput that can be either User or number.

2. Inside getUser(), we use typeof input === "number" to narrow the type:
   - If true, input is definitely a number (the id)
   - If false, input must be a User object (TypeScript eliminates the number possibility)

3. This is a type guard — it tells TypeScript "after this check, the type is narrowed."

4. Each branch can now safely access type-specific properties or methods.

WHY THIS WORKS:

The typeof operator works for primitives (string, number, boolean, undefined).
For object types, you'd use instanceof or custom type guards with "value is Type".