Lab 3 — Union Types & Type Narrowing (TypeScript)

Welcome to Lab 3 of the TypeScript Labs! In this lab, you'll gain hands-on experience with union types, type narrowing, and writing functions that safely handle different input types — key tools for writing real-world TypeScript code.

🏆 Learning Goals

• Declare variables that can accept multiple types using union types
• Use type narrowing to write safe logic for different inputs
• Build flexible functions that branch based on input type
• (Bonus) Combine structural and primitive checks in more complex logic

🛠️ What to Do

• Open each file in the src/ folder and complete the tasks listed in the comments
• Use the Mocha + Chai tests in the test/ folder to verify your solutions

✅ How to Run Tests

npm install
npm test

🗂️ Lab Structure & Tasks

🔹 section1_union_types.ts

• Declare a variable called input that can be a string or a number and set it to one of those
• Write a function toUpperOrFixed that:
  • uppercases the input if it's a string
  • returns a string with 2 decimal places if it's a number

🔹 section2_type_narrowing.ts

• Write a function printValue that takes a value which may be:
  • a string
  • a number
  • a boolean
• Use typeof to determine the type
• Print different messages to the console based on the type

⭐ section3_bonus_validateInput.ts

• Write a function validateInput that accepts any of the following:
  • a string (must be non-empty)
  • a number (must be > 0)
  • an object with a valid: boolean property
• Return true if valid, otherwise false
• Use typeof, in, and other narrowing tools to implement this cleanly

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Ready? Start in src/section1_union_types.ts and get some practice with TypeScript’s powerful type flexibility! 🚀