如何修复TypeError: books.map不是我的代码生成的函数？

let books = []; // 确保 books 是一个数组

// 假设 books 是从某个 API 获取的
fetch('https://example.com/api/books')
    .then(response => response.json())
    .then(data => {
        if (Array.isArray(data)) {
            books = data;
            const result = books.map(book => {
                // 处理每个 book
                return {
                    id: book.id,
                    title: book.title.toUpperCase()
                };
            });
            console.log('Processed books:', result);
        } else {
            console.error('API response is not an array:', data);
        }
    })
    .catch(error => {
        console.error('Error fetching books:', error);
    });