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. This is 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, and your progress syncs across the web and the CoddyKit app. The TypeScript Academy course includes 3 lessons in total.
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.
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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. 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. The TypeScript Academy course includes 3 lessons in total.
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, so you can start here or from the beginning and move at your own pace. This is lesson 2 of 3.
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