関数型プログラミング: 関数からグローバル変数をリファクタリングする

var bookList = ["The Hound of the Baskervilles", "On The Electrodynamics of Moving Bodies", "Philosophiæ Naturalis Principia Mathematica", "Disquisitiones Arithmeticae"];

// Change code below this line
function add (bookName) {

  bookList.push(bookName);
  return bookList;

  // Change code above this line
}

// Change code below this line
function remove (bookName) {
  var book_index = bookList.indexOf(bookName);
  if (book_index >= 0) {

    bookList.splice(book_index, 1);
    return bookList;

    // Change code above this line
    }
}

var newBookList = add(bookList, 'A Brief History of Time');
var newerBookList = remove(bookList, 'On The Electrodynamics of Moving Bodies');
var newestBookList = remove(add(bookList, 'A Brief History of Time'), 'On The Electrodynamics of Moving Bodies');

console.log(bookList);

var bookList = ["The Hound of the Baskervilles", "On The Electrodynamics of Moving Bodies", "Philosophiæ Naturalis Principia Mathematica", "Disquisitiones Arithmeticae"];

function add (books, bookName) {
  let copyBooks = [...books] <--- we used a copy of the array in our functions.
  copyBooks.push(bookName);
  return copyBooks ;

}

function remove (books, bookName) {
  let copyBooks = [...books]
  var book_index = copyBooks.indexOf(bookName); // <-- is just finding if the book exists in the copyBooks(bookList) in this case for `newestBookList` book_index would be on position 1.
  if (book_index >= 0) { // <-- (1 >= 0)
    copyBooks.splice(book_index, 1); // <-- (1, 1) removes 'On The Electrodynamics of Moving Bodies' 
    return copyBooks;

    }
}

var newBookList = add(bookList, 'A Brief History of Time');
var newerBookList = remove(bookList, 'On The Electrodynamics of Moving Bodies');
var newestBookList = remove(add(bookList, 'A Brief History of Time'), 'On The Electrodynamics of Moving Bodies');

console.log(newestBookList); // will display ["The Hound of the Baskervilles", "Philosophiæ Naturalis Principia Mathematica", "Disquisitiones Arithmeticae", "A Brief History of Time"]