// ECMA used the name Symbol.iterator. Symbols offer names that are unique and cannot clash 
// with other property names. Also, Symbol.iterator will return an object called an iterator. 
// This iterator will have a method called next which will return an object with keys value and done.

const iterable = {
  [Symbol.iterator]() {
    let step = 0;

    const iterator = {
      next() {
        step++;

        if (step === 1) {
          return { value: 'This', done: false }
        } else if (step === 2) {
          return { value: 'is', done: false }
        } else if (step === 3) {
          return { value: 'iterable', done: true }
        }

        return { value: undefined, done: false };
      }
    }

    return iterator
  }
}

// var iterator = iterable[Symbol.iterator]();
// console.log(iterator.next());
// console.log(iterator.next());
// console.log(iterator.next());
// console.log(iterator.next());
// console.log(iterator.next());


const myFavouriteAuthors = {
  allAuthors: {
    fiction: [
      'Agatha Christie',
      'J. K. Rowling',
      'Dr. Seuss'
    ],
    scienceFiction: [
      'Neal Stephenson',
      'Arthur Clarke',
      'Isaac Asimov',
      'Robert Heinlein'
    ],
    fantasy: [
      'J. R. R. Tolkien',
      'J. K. Rowling',
      'Terry Pratchett'
    ],
  },
  [Symbol.iterator]() {
    const genres = Object.values(this.allAuthors);

    // console.log('genres', genres);

    let currentAuthorIndex = 0;
    let currentGenreIndex = 0;

    return {
      next() {
        const authors = genres[currentGenreIndex];

        const doNothaveMoreAuthors = !(currentAuthorIndex < authors.length);
        if (doNothaveMoreAuthors) {
          currentGenreIndex++;
          currentAuthorIndex = 0;
        }

        const doNotHaveMoreGenres = !(currentGenreIndex < genres.length);
        if (doNotHaveMoreGenres) {
          return {
            value: undefined,
            done: true
          };
        }

        return {
          value: genres[currentGenreIndex][currentAuthorIndex++],
          done: false
        }
      }
    };
  }
};

for (const author of myFavouriteAuthors) {
  console.log(author);
}

console.log(...myFavouriteAuthors)