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id	title	challengeType	forumTopicId	dashedName
587d7b8f367417b2b2512b60	在函数中重构全局变量	1	301235	refactor-global-variables-out-of-functions

--description--

目前为止，我们已经看到了函数式编程的两个原则：

不要更改变量或对象 - 创建新变量和对象，并在需要时从函数返回它们。提示：使用类似 var newArr = arrVar 时 arrVar 是一个数组，代码只是创建一个对现有变量的引用，而不是副本。所以更改 newArr 中的值会同时更改 arrVar 中的值。
声明函数参数 - 函数内的任何计算仅取决于参数，而不取决于任何全局对象或变量。

给数字增加 1 不够刺激，我们可以在处理数组或更复杂的对象时应用这些原则。

--instructions--

重构代码，使全局数组 bookList 在函数内部不会被改变。 add 函数可以将指定的 bookName 增加到数组末尾并返回一个新的数组（列表）。 remove 函数可以从数组中移除指定 bookName。

注意： 两个函数都应该返回一个数组，任何新参数都应该在 bookName 参数之前添加。

--hints--

bookList 应等于 ["The Hound of the Baskervilles", "On The Electrodynamics of Moving Bodies", "Philosophiæ Naturalis Principia Mathematica", "Disquisitiones Arithmeticae"].

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

newBookList 应等于 ["The Hound of the Baskervilles", "On The Electrodynamics of Moving Bodies", "Philosophiæ Naturalis Principia Mathematica", "Disquisitiones Arithmeticae", "A Brief History of Time"].

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

newerBookList 应等于 ["The Hound of the Baskervilles", "Philosophiæ Naturalis Principia Mathematica", "Disquisitiones Arithmeticae"].

assert(
  JSON.stringify(newerBookList) ===
    JSON.stringify([
      'The Hound of the Baskervilles',
      'Philosophiæ Naturalis Principia Mathematica',
      'Disquisitiones Arithmeticae'
    ])
);

newestBookList 应等于 ["The Hound of the Baskervilles", "Philosophiæ Naturalis Principia Mathematica", "Disquisitiones Arithmeticae", "A Brief History of Time"].

assert(
  JSON.stringify(newestBookList) ===
    JSON.stringify([
      'The Hound of the Baskervilles',
      'Philosophiæ Naturalis Principia Mathematica',
      'Disquisitiones Arithmeticae',
      'A Brief History of Time'
    ])
);

--seed--

--seed-contents--

// 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 (bookName) {

  bookList.push(bookName);
  return bookList;

  // Change code above this line
}

// Change code below this line
function remove (bookName) {
  var book_index = bookList.indexOf(bookName);
  if (book_index >= 0) {

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

    // 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');

console.log(bookList);

--solutions--

// The global variable
var 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;
}

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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