Functions: params, default/optional/rest; return types
Define functions with parameters, defaults, optionals, rest args, and proper return types.
Functions: params, default/optional/rest; return types is a free TypeScript Academy lesson on CoddyKit — lesson 2 of 3. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the TypeScript Academy learning path, one of 3 lessons in the course, and your progress syncs across the web and the CoddyKit app.
Overview
Goal: Learn to declare functions with parameters, optional and default values, rest arguments, and return types.
Basic function
Functions declare parameter and return types. Here, greet takes a string and returns a string.
function greet(name: string): string {
return "Hello " + name;
}
console.log(greet("Ada"));
Default params
Default params assign a fallback value when the argument is missing. Example: exponent defaults to 2.
function power(base: number, exp: number = 2): number {
return base ** exp;
}
console.log(power(3)); // default exp=2 → 3**2 = 9
console.log(power(2, 3)); // exp=3 → 2**3 = 8
Optional params
Optional params use "?" to indicate an argument may be omitted. Handle undefined safely.
function lengthOf(text?: string): number {
return text ? text.length : 0;
}
console.log(lengthOf("TS")); // 2
console.log(lengthOf()); // 0
Rest params
Rest params collect multiple args into an array. Useful for flexible calls.
function sum(...nums: number[]): number {
return nums.reduce((a, b) => a + b, 0);
}
console.log(sum(1, 2, 3, 4)); // 10
Return types
Always specify return types when clarity matters. TS infers simple cases but explicitness helps APIs.
function isEven(n: number): boolean {
return n % 2 === 0;
}
console.log(isEven(4)); // true
Optional param check
Quick check: How do you declare an optional parameter?
Recap
Recap: You learned function parameters, defaults, optional args, rest arrays, and explicit return types. These keep your code predictable.
Frequently asked questions
Is the “Functions: params, default/optional/rest; return types” lesson free?
Yes — the full text of “Functions: params, default/optional/rest; return types” is free to read here on the web, and the TypeScript Academy course includes 3 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the TypeScript Academy course, upgrade to CoddyKit PRO.
What will I learn in “Functions: params, default/optional/rest; return types”?
Define functions with parameters, defaults, optionals, rest args, and proper return types. You practise TypeScript Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start TypeScript Academy?
No prior experience is required. TypeScript Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 2 of 3, so you can start here or from the beginning and move at your own pace.
How long does the “Functions: params, default/optional/rest; return types” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this TypeScript Academy lesson?
Yes. Every TypeScript Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- If/else, switch, loops; truthiness & strict checks
- Functions: params, default/optional/rest; return types
- Arrow functions & contextual typing