Optional and Default Parameters

A parameter followed by ? is optional — callers may omit it, and its type automatically includes undefined. A default value makes a parameter optional too, while giving it a fallback.

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

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

Common mistakes

• Putting a required parameter after an optional one — TypeScript requires optional/default parameters to come last.