What are optional properties and how do you define them?

An optional property is one an object may leave out. You mark it with ? after the name, and its type becomes the declared type or undefined.

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
};

When you read config.timeout, its type is number | undefined, so the compiler makes you handle the missing case. Use ?? to supply a default value, or ?. to read deeper into a property that may be missing.

Function parameters can be optional with the same ?, as long as they come after the required ones.