TS TypeScript

Functions

Lesson 3 of 5 ~7 min

Functions are reusable blocks of code. In TypeScript, you can add types to function parameters and return values, making your functions self-documenting and safer to use.

Basic function syntax

// Type annotations on parameters AND return value
function add(a: number, b: number): number {
  return a + b;
}

// TypeScript can also infer the return type
function greet(name: string) {
  return `Hello, ${name}!`; // return type inferred as string
}

Arrow functions

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

// Single-parameter arrow functions
const double = (x: number) => x * 2;

Optional & default parameters

// Optional parameter — marked with ?
function greet(name: string, greeting?: string) {
  return `${greeting || "Hello"}, ${name}!`;
}

// Default parameter — uses = to set a default
function greet2(name: string, greeting: string = "Hello") {
  return `${greeting}, ${name}!`;
}

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

Return types

// Explicit return type
function divide(a: number, b: number): number {
  return a / b;
}

// Void — function doesn't return anything
function logMessage(msg: string): void {
  console.log(msg);
}

// Never — function never returns (throws or loops forever)
function throwError(msg: string): never {
  throw new Error(msg);
}
Editor
TypeScript
Console

        

      

    


    
Quiz


    

      
How do you define a function's return type in TypeScript?

      

        
        
        
        
      

      

    


    

      
Challenge

      
Write a function called sum that takes two numbers and returns their sum. Add a return type annotation. Then write an arrow function version of the same function.