Typing Functions

10 min read

Typing Functions

TypeScript lets you annotate function parameter types and the return type. Annotating the return type is especially valuable — it prevents accidentally returning the wrong thing.

// Named function
function add(a: number, b: number): number {
  return a + b;
}

// Arrow function
const multiply = (a: number, b: number): number => a * b;

// Function type alias
type MathOp = (a: number, b: number) => number;
const divide: MathOp = (a, b) => a / b;

Optional and default parameters

function greet(name: string, greeting?: string): string {
  return `${greeting ?? "Hello"}, ${name}!`;
}

// Default parameter (implies optional)
function createUser(name: string, role: string = "student") {
  return { name, role };
}

greet("Alice");           // "Hello, Alice!"
greet("Alice", "Hi");    // "Hi, Alice!"

Rest parameters and void

function logAll(...messages: string[]): void {
  messages.forEach((m) => console.log(m));
}

// void vs undefined
// void: the return value won't be used
// undefined: the function explicitly returns undefined
function noop(): void {}
function returnsUndefined(): undefined { return undefined; }

TypeScript infers return types automatically, but explicitly annotating them is good practice for exported functions. It catches mistakes like forgetting a return in a branch.

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