我正在尝试过滤数组以使用express打印已读书籍，但无法读取或结果为空

Question

我有一个书单，并尝试打印出那些已经以“作者标题”形式阅读过的书

const bookList = [
    {title: "Harry Potter", author: "J.K. Rowling", alreadyRead: true},
    {title: "Never Eat Alone", author: "Keith Ferrazzi", alreadyRead: false},
    {title: "INSPIRED", author: "Marty Cagan", alreadyRead: false},
    {title: "Zero to One", author: "Peter Thiel", alreadyRead: true}
]

首先，我通过传递书单和状态来使用此 for 循环方法来调用函数

printBookList(bookList, true)

，但最终会打印列表中的所有元素

function printBookList(bookList, completedReadingBook){
    for(i=0;i<bookList.length;i++){
        if (bookList[i].alreadyRead == completedReadingBook) {
            console.log(bookList[i].title + ' by ' + bookList[i].author);
        }
    }  
}

然后我也尝试使用下面的

filter()

方法，但最终显示为空

for(i=0; i<bookList.length;i++){
    let filtered = bookList.filter((readbook)=>readbook.alreadyRead==completedReadingBook)
    return filtered;
    }
    console.log(bookList[filtered].title + ' by ' + bookList[filtered].author);

我对这个js很陌生，如果有人可以向我解释这一点，我将不胜感激，非常感谢！

Answer 1

正如 Jaromanda X 指出的：

• 你的第一个简单的 for 循环函数应该可以正常工作；

• Array.filter()

  是调用整个数组的方法；
• 不确定你的第二段代码是否在函数内部，但如果是，确实最后一行（

  console.log()

  ）永远不会被执行，因为它位于

  return

  语句之后；如果不是，那么

  return

  语句不应该存在；

• filtered

  是一个数组，而不是索引，因此

  bookList[filtered]

  是不正确的语义。

您可以阅读

Array.filter()

here上的文档，您的第二段代码应该如下所示：

function printBooklist(bookList, completedReadingBook) {
  const filtered = bookList.filter((readbook)=>readbook.alreadyRead==completedReadingBook);
  for(i=0; i<bookList.length;i++){
    console.log(filtered[i].title + ' by ' + filtered[i].author);
  }
}

您还可以使用内置的 JS 方法，例如

Object.groupBy

和

forEach

:

function printReadBooks() {
  const bookList = [
    {title: "Harry Potter", author: "J.K. Rowling", alreadyRead: true},
    {title: "Never Eat Alone", author: "Keith Ferrazzi", alreadyRead: false},
    {title: "INSPIRED", author: "Marty Cagan", alreadyRead: false},
    {title: "Zero to One", author: "Peter Thiel", alreadyRead: true}
  ];
  // Object.groupBy returns an object grouped by the alreadyRead property.
  const readBooks = Object.groupBy(bookList, ({alreadyRead}) => alreadyRead).true;
 
  console.log("Already read books:");
  readBooks.forEach(({title, author}) => console.log(` - ${title}, by ${author};\n`));
 
  /* The following line will only work in some environments:
  readBooks.forEach(({title, author}) =>
                    console.log(` - %c${title}%c, by ${author};\n`,
                                "font-style: italic", "font-style: normal")); */
}

printReadBooks();