---
id: 587d7b8f367417b2b2512b60
title: Refactor Global Variables Out of Functions
challengeType: 1
forumTopicId: 301235
dashedName: refactor-global-variables-out-of-functions
---

# --description--

So far, we have seen two distinct principles for functional programming:

1) Don't alter a variable or object - create new variables and objects and return them if need be from a function. Hint: using something like `const newArr = arrVar`, where `arrVar` is an array will simply create a reference to the existing variable and not a copy. So changing a value in `newArr` would change the value in `arrVar`.

2) Declare function parameters - any computation inside a function depends only on the arguments passed to the function, and not on any global object or variable.

Adding one to a number is not very exciting, but we can apply these principles when working with arrays or more complex objects.

# --instructions--

Rewrite the code so the global array `bookList` is not changed inside either function. The `add` function should add the given `bookName` to the end of the array passed to it and return a new array (list). The `remove` function should remove the given `bookName` from the array passed to it.

**Note:** Both functions should return an array, and any new parameters should be added before the `bookName` parameter.

# --hints--

`bookList` should not change and still equal `["The Hound of the Baskervilles", "On The Electrodynamics of Moving Bodies", "Philosophiæ Naturalis Principia Mathematica", "Disquisitiones Arithmeticae"]`.

```js
add(bookList, "Test");
remove(bookList, "The Hound of the Baskervilles");
assert(
  JSON.stringify(bookList) ===
    JSON.stringify([
      'The Hound of the Baskervilles',
      'On The Electrodynamics of Moving Bodies',
      'Philosophiæ Naturalis Principia Mathematica',
      'Disquisitiones Arithmeticae'
    ])
);
```

`add(bookList, "A Brief History of Time")` should return `["The Hound of the Baskervilles", "On The Electrodynamics of Moving Bodies", "Philosophiæ Naturalis Principia Mathematica", "Disquisitiones Arithmeticae", "A Brief History of Time"]`.

```js
assert(
  JSON.stringify(add(bookList, "A Brief History of Time")) ===
    JSON.stringify([
      'The Hound of the Baskervilles',
      'On The Electrodynamics of Moving Bodies',
      'Philosophiæ Naturalis Principia Mathematica',
      'Disquisitiones Arithmeticae',
      'A Brief History of Time'
    ])
);
```

`remove(bookList, "On The Electrodynamics of Moving Bodies")` should return `["The Hound of the Baskervilles", "Philosophiæ Naturalis Principia Mathematica", "Disquisitiones Arithmeticae"]`.

```js
assert(
  JSON.stringify(remove(bookList, 'On The Electrodynamics of Moving Bodies')) ===
    JSON.stringify([
      'The Hound of the Baskervilles',
      'Philosophiæ Naturalis Principia Mathematica',
      'Disquisitiones Arithmeticae'
    ])
);
```

`remove(add(bookList, "A Brief History of Time"), "On The Electrodynamics of Moving Bodies");` should equal `["The Hound of the Baskervilles", "Philosophiæ Naturalis Principia Mathematica", "Disquisitiones Arithmeticae", "A Brief History of Time"]`.

```js
assert(
  JSON.stringify(remove(add(bookList, 'A Brief History of Time'), 'On The Electrodynamics of Moving Bodies')) ===
    JSON.stringify([
      'The Hound of the Baskervilles',
      'Philosophiæ Naturalis Principia Mathematica',
      'Disquisitiones Arithmeticae',
      'A Brief History of Time'
    ])
);
```

# --seed--

## --seed-contents--

```js
// The global variable
const 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) {
  const book_index = bookList.indexOf(bookName);
  if (book_index >= 0) {

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

    // Change code above this line
    }
}
```

# --solutions--

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

function add(bookList, bookName) {
  return [...bookList, bookName];
}

function remove(bookList, bookName) {
  const bookListCopy = [...bookList];
  const bookNameIndex = bookList.indexOf(bookName);
  if (bookNameIndex >= 0) {
    bookListCopy.splice(bookNameIndex, 1);
  }
  return bookListCopy;
}
```
