CHALLENGE 3: any vs unknown
============================

SOLUTION:

function doubleIfNumber(value: unknown): number {
  if (typeof value === "number") {
    return value * 2;
  }
  return 0;
}

EXPLANATION:

Step 1: Accept unknown
  - "value: unknown" means we don't know what type it is
  - This is safer than 'any' because it forces us to check

Step 2: Check the type
  - "typeof value === "number"" narrows the type
  - Inside the if block, TypeScript KNOWS value is a number

Step 3: Use the value safely
  - "return value * 2" is safe because we checked first
  - TypeScript allows this multiplication

Step 4: Default return
  - "return 0" if the value isn't a number
  - We always return a number (the function's return type)

WHY NOT USE 'any'?

If you wrote it like this:

  function doubleIfNumber(value: any): number {
    return value * 2;  // ✅ Technically works, but wrong!
  }

This would:
  - NOT enforce type checking
  - Return NaN if value is a string
  - Hide potential bugs
  - Defeat the entire purpose of TypeScript

TYPE NARROWING:
The "typeof value === 'number'" check is called type narrowing.
After that check, TypeScript knows value must be a number.
This pattern is fundamental to writing safe TypeScript.

TESTING:
doubleIfNumber(5)        // Returns 10 ✅
doubleIfNumber(3.5)      // Returns 7 ✅
doubleIfNumber("5")      // Returns 0 (not a number)
doubleIfNumber(null)     // Returns 0 (not a number)
doubleIfNumber(true)     // Returns 0 (not a number)