refactorGlobalVariablesOutofFunctions.js

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

/* This function should add a book to the list and return the list */
// New parameters should come before the bookName one

// Add your code below this line
function add (arr,bookName) {
  let emtyarr = [...arr];
  emtyarr.push(bookName);
  return emtyarr;
  
  // Add your code above this line
}

/* This function should remove a book from the list and return the list */
// New parameters should come before the bookName one

// Add your code below this line
function remove (arr,bookName) {
  let emtyarr = [...arr];
  if (emtyarr.indexOf(bookName) >= 0) {
    
     emtyarr.splice(emtyarr.indexOf(bookName),1);
     return emtyarr;
    
    // Add your 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);