Why it didn't work?

JavaScript
1

Tell us what’s happening:

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 the bookName one

// Add your code below this line
function add (list,bookName) {
  
  return list.push(bookName);
  
  // 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) {
  if (list.indexOf(bookName) >= 0) {
    return list.filter((item) => item !== bookName);
    }
}

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 6.1; Win64; x64; rv:69.0) Gecko/20100101 Firefox/69.0.

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

2

Try adding check for falsy values i.e.

if(bookName){
//your code
}
3

It is because The push() method adds one or more elements to the end of an array and returns the new length of the array.

Source: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/push

It seems that you expect return list.push(bookName); to return the modified array while it doesn’t, hence the error list.indexOf is not a function. list as the returned value of add(bookList, 'A Brief History of Time') in this case will be a number, therefore it doesn’t have the indexOf method.

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