Which is the preferred or trendy method for JS at this moment?

Library.js

/**
 * Method A: OOP
 */

class Library {
  constructor() {
    this.books = this.loadBooks();
  }
  
  add(newBook) {
    this.books.push(newBook);
  }
  
  loadBooks() {
    return api.books(); // array
  }
}

module.exports = new Library(); // singleton

// -------------------------------------------------------------------------------------

/**
 * Method B: Functional, with state
 */

const books = loadBooks();

function loadBooks() {
  return api.books(); // array
}

function add(newBook) {
  books.push(newBook);
}

module.exports = { books, add };

// -------------------------------------------------------------------------------------

/**
 * Method C: Fully functional
 */

function getBooks() {
  return api.books(); // array
}

function add(books, newBook) {
  books.push(newBook);
  return books;
}

module.exports = { getBooks, add };