TypeScript’s function parameter system goes well beyond what plain JavaScript offers. Three features — rest parameters, optional parameters, and default parameters — let you write functions that are flexible to call, self-documenting to read, and safe to maintain. Mastering all three is one of the fastest ways to level up your TypeScript code quality.

This guide walks through each feature in depth with fully annotated code, explains the type-system rules that govern them, highlights the mistakes beginners make, and closes with a single real-world function that combines all three.