Function Basics

Function Basics

Functions in TypeScript can have typed parameters and typed return values.

Typed Parameters

const greet = (name: string, age: number) => {
    console.log(`Hello ${name}, you are ${age}`);
};

greet("Mario", 30);   // ✅
greet("Mario");        // ❌ Expected 2 arguments
greet("Mario", "30");  // ❌ Argument of type 'string' not assignable

Return Types

TypeScript infers return types, but you can be explicit:

const add = (a: number, b: number): number => {
    return a + b;
};

// void — function doesn't return anything
const log = (message: string): void => {
    console.log(message);
};

Optional Parameters

const greet = (name: string, greeting?: string) => {
    console.log(`${greeting || "Hello"}, ${name}`);
};

greet("Mario");           // "Hello, Mario"
greet("Mario", "Hi");     // "Hi, Mario"

Default Parameters

const add = (a: number, b: number = 10): number => {
    return a + b;
};
add(5);    // 15
add(5, 3); // 8

Key Takeaways

• Add types to parameters: (name: string)
• Add return types: (): number
• Use void for functions with no return
• Optional params use ? suffix