miscellaneous.md

Miscellaneous

• Error handling
• Iterator and Iterable

Error handling

Only two cases below will make errors catchable

• Promise.reject(new Error('something wrong happened')).catch(err => console.log('err caught'));
• throw new Error('something wrong happened');

Return a new error object will not trigger catching process.

iterator-iterable

An iterable is a data structure that wants to make its elements accessible to the public. It does so by implementing a method whose key is Symbol.iterator. That method is a factory for iterators. That is, it will create iterators.

An iterator is a pointer for traversing the elements of a data structure.

To manually convert an Object into interable:

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]() {
    // Get all the authors in an array
    const genres = Object.values(this.allAuthors);

    // Store the current genre and author index
    let currentAuthorIndex = 0;
    let currentGenreIndex = 0;

    return {
      // Implementation of next()
      next() {
        // authors according to current genre index
        const authors = genres[currentGenreIndex];

        // doNotHaveMoreAuthors is true when the authors array is exhausted.
        // That is, all items are consumed.
        const doNothaveMoreAuthors = !(currentAuthorIndex < authors.length);
        if (doNothaveMoreAuthors) {
          // When that happens, we move the genre index to the next genre
          currentGenreIndex++;
          // and reset the author index to 0 again to get new set of authors
          currentAuthorIndex = 0;
        }

        // if all genres are over, then we need tell the iterator that we
        // can not give more values.
        const doNotHaveMoreGenres = !(currentGenreIndex < authors.length);
        if (doNotHaveMoreGenres) {
          // Hence, we return done as true.
          return {
            value: undefined,
            done: true
          };
        }

        // if everything is correct, return the author from the
        // current genre and incerement the currentAuthorindex
        // so next time, the next author can be returned.
        return {
          value: genres[currentGenreIndex][currentAuthorIndex++],
          done: false
        }
      }
    };
  }
};

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

console.log(...myFavouriteAuthors)