Optional properties are marked with ? and may or may not be present in the object:

interface Config {
  apiUrl: string;
  timeout?: number; // Optional
  retries?: number; // Optional
  debug?: boolean; // Optional
}

const config: Config = {
  apiUrl: "https://api.example.com"
  // Other properties can be omitted
};

// Function with optional parameters
function createUser(name: string, age?: number): User {
  return {
    name,
    age: age ?? 18 // Default value if not provided
  };
}