Refactor Global Variables Out of Functions Something should be wrong

JavaScript
#1

Tell us what’s happening:

Can someone explain to me why my Add function works? Shouldn’t it be wrong? So, because the concat method creates a new array, does it work?

Sorry for the inconvenience, I appreciate your help.

Your code so far


// 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 bookName

// Add your code below this line
function add (list, bookName) {
let addBook = list.concat(bookName);
//var value = copyList.push(bookName);
return addBook;

// 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 (list, bookName) {
var book_index = list.indexOf(bookName);
if (book_index >= 0) {
  let copyList = [...list];
  copyList.splice(book_index, 1);
  //list.splice(book_index, 1);
  return copyList;

  // 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);

Your browser information:

User Agent is: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/78.0.3904.97 Safari/537.36.

Challenge: Refactor Global Variables Out of Functions

Link to the challenge:
https://www.freecodecamp.org/learn/javascript-algorithms-and-data-structures/functional-programming/refactor-global-variables-out-of-functions

#2

Array.prototype.concat creates a new array.

#3

Because the .concat() method creates a new array is my code working?

#4

I thought I answered the question in my response. As long as you are not mutating the original array, you will pass. You are passing.

#5