Drill 4: Invalid input — Interfaces and type aliases

interface Lesson {
  id: string
  title: string
  minutes: number
  published?: boolean
}

interface Course extends Lesson {
  readonly slug: string
  lessons: Lesson[]
}

const html: Course = {
  id: 'html',
  slug: 'html',
  title: 'HTML Foundations',
  minutes: 120,
  lessons: [],
}