TypeScript Academy · Lesson

Optional and Default Parameters

Handle missing arguments with ? and default values.

Welcome

TypeScript lets you mark function parameters as optional with `?` or give them default values. This makes APIs flexible while staying type-safe.

Add `?` after a parameter name to make it optional. The parameter type becomes T | undefined inside the function.

function greet(name: string, greeting?: string): string {
  return (greeting ?? 'Hello') + ', ' + name;
}