Types for functions
You can type function parameters and return value: function add(a: number, b: number): number { return a + b; }. That way you can't accidentally pass a string. Here we write the same in plain JS.
Appy Says…
TypeScript makes functions airtight — you declare what goes in and what comes out, and TypeScript catches any mismatch before you even run the code. No more 'undefined is not a function' surprises.
How do TypeScript Functions work?
In TypeScript you annotate a function's parameter types and return type. TypeScript then verifies every caller passes the right arguments and uses the return value correctly.
- •
function greet(name: string): string { return `Hello, ${name}`; } - •Return type after
):: string,: number,: void - •Arrow functions:
const add = (a: number, b: number): number => a + b; - •Optional params:
function log(msg: string, level?: string) - •Default params:
function log(msg: string, level = 'info') - •TypeScript infers the return type if you omit it — annotation is recommended for clarity
Think of it like a Roblox Remote with typed arguments
A Roblox RemoteEvent that fires can't guarantee the data format — you have to check it yourself. TypeScript function types are like a contract: if the caller doesn't send exactly the right types, the game (compiler) refuses to even start.
How It Works
- •1. Annotate parameters:
(x: number, y: string) - •2. Annotate return:
): booleanafter the closing parenthesis - •3. TypeScript checks every call site: wrong argument type = compile error
- •4. Return type mismatch (returning string from a number function) = compile error
- •5.
void: function returns nothing (implicitly undefined) - •6.
never: function never returns (always throws or infinite loop)
Real-World Examples
- •
function calculateDiscount(price: number, percent: number): number - •
async function fetchUser(id: string): Promise<User> - •
function handleClick(event: React.MouseEvent<HTMLButtonElement>): void - •
const formatDate = (date: Date, locale = 'en-GB'): string => ...
Key Facts
- •TypeScript infers return type from the return statement — annotation is optional but recommended for public APIs
- •Function types as variables:
type Handler = (event: Event) => void; - •Overloads: multiple function signatures for the same function name
- •
neverreturn type is used in functions that always throw or call process.exit()
Watch Out!
Forgetting to annotate return types on async functions is a common mistake. Without it, TypeScript may infer Promise<any> instead of the specific type you intend. Always annotate async function returns: async function loadUser(id: string): Promise<User>.
Remember
function name(param: Type): ReturnType. Use void when there's no return. TypeScript verifies every call site. Annotate return types explicitly for public functions.
What You Learned
- •TypeScript functions: annotate parameter types and return type; compiler checks every call
- •
void= no return;never= never returns; optional params with? - •Unlocks: type-safe APIs, self-documenting functions, catching argument bugs at compile time
Key Facts
- →TypeScript infers return type from the return statement — annotation is optional but recommended for public APIs
- →Function types as variables:
type Handler = (event: Event) => void; - →Overloads: multiple function signatures for the same function name
- →
neverreturn type is used in functions that always throw or call process.exit()
Real-World Examples
Remember
function name(param: Type): ReturnType. Use void when there's no return. TypeScript verifies every call site. Annotate return types explicitly for public functions.
Quick Quiz
What can you type in a function?