Refactor Global Variables SLICE/SPREAD - SPOILERT

I completed the challenge using spread operator, but when I tried using slice() it did not copy the arrays as well into the new variable, can anyone tell me why and when I should use spread operator instead of slice and vice versa

  **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"];

// Change code below this line
function add (list, bookName) {
let news = [...list]
return news;

// Change code above this line

// Change code below this line
function remove (list, bookName) {
var book_index = bookList.indexOf(bookName);
if (book_index >= 0) {
  let arr = [...list]
  arr.splice(book_index, 1);
  return arr;

  // 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');

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Challenge: Refactor Global Variables Out of Functions

Link to the challenge:

can you show an example of code not working?

for example, I did this:

let news = list.slice()
return news;

it should work the same, there is no reason for which why that shouldn’t work

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