🧪 Manual Testing — JavaScript Arithmetic

You're tasked with testing and fixing four basic arithmetic functions.

But here's the twist:

• ❌ You may not use any test library (no Jest, Mocha, etc.)
• ✅ You must write all tests by hand, directly in this file
• ✅ You are encouraged to invent your own helpers (e.g. assertEqual, expect, etc.)

Your job is to:

1. Write tests that reveal the bugs
2. Fix the broken functions
3. Improve your test readability as you go

💡 Run your file using: node 01-manual-test.js