错误：(40，20) TS2339:类型'string | Book | Book[]‘上不存在属性'map’。属性'map‘在类型'string’上不存在

interface Book {
  title: string;
  author: string;
}

function processBooks(books: string | Book | Book[]) {
  if (typeof books === 'string') {
    // 将字符串转换为数组
    const bookArray = books.split(',');
    // 在数组上调用'map'方法
    const processedBooks = bookArray.map((book) => {
      return {
        title: book,
        author: 'Unknown',
      };
    });
    console.log(processedBooks);
  } else if (Array.isArray(books)) {
    // 在数组上调用'map'方法
    const processedBooks = books.map((book) => {
      return {
        title: book.title,
        author: book.author,
      };
    });
    console.log(processedBooks);
  } else {
    // 在对象上调用'map'方法
    const processedBook = {
      title: books.title,
      author: books.author,
    };
    console.log(processedBook);
  }
}

const bookString = 'Book1,Book2,Book3';
const bookObject = { title: 'Book', author: 'Author' };
const bookArray = [
  { title: 'Book1', author: 'Author1' },
  { title: 'Book2', author: 'Author2' },
  { title: 'Book3', author: 'Author3' },
];

processBooks(bookString);
processBooks(bookObject);
processBooks(bookArray);