JavaScript 异步编程

// 同步代码
console.log('开始');
const result = doSomething();
console.log('结果:', result);
console.log('结束');

// 异步代码
console.log('开始');
doSomethingAsync().then(result => {
    console.log('结果:', result);
});
console.log('结束');

// 基本回调示例
function fetchData(callback) {
    setTimeout(() => {
        const data = { id: 1, name: 'JavaScript' };
        callback(data);
    }, 1000);
}

fetchData(data => {
    console.log('获取的数据:', data);
});

// 回调地狱示例
getUser(userId, user => {
    getOrders(user.id, orders => {
        getOrderDetails(orders[0].id, details => {
            console.log('订单详情:', details);
        });
    });
});

const promise = new Promise((resolve, reject) => {
    // 异步操作
    setTimeout(() => {
        const random = Math.random();
        if (random > 0.5) {
            resolve('成功');
        } else {
            reject('失败');
        }
    }, 1000);
});

promise
    .then(result => console.log(result))
    .catch(error => console.error(error));

function fetchUser(id) {
    return new Promise((resolve) => {
        setTimeout(() => {
            resolve({ id, name: 'Alice' });
        }, 1000);
    });
}

function fetchOrders(userId) {
    return new Promise((resolve) => {
        setTimeout(() => {
            resolve([{ id: 1, userId, product: 'Book' }]);
        }, 1000);
    });
}

fetchUser(1)
    .then(user => fetchOrders(user.id))
    .then(orders => console.log('订单:', orders))
    .catch(error => console.error('错误:', error));

const promise1 = new Promise(resolve => setTimeout(() => resolve(1), 1000));
const promise2 = new Promise(resolve => setTimeout(() => resolve(2), 2000));
const promise3 = new Promise(resolve => setTimeout(() => resolve(3), 3000));

// 等待所有 Promise 完成
Promise.all([promise1, promise2, promise3])
    .then(results => console.log('全部完成:', results))
    .catch(error => console.error('有一个失败:', error));

// 等待第一个完成的 Promise
Promise.race([promise1, promise2, promise3])
    .then(result => console.log('第一个完成:', result))
    .catch(error => console.error('第一个失败:', error));

async function fetchUserData() {
    try {
        const user = await fetchUser(1);
        const orders = await fetchOrders(user.id);
        console.log('用户订单:', orders);
    } catch (error) {
        console.error('错误:', error);
    }
}

fetchUserData();

async function fetchAllData() {
    try {
        const [users, products] = await Promise.all([
            fetchUsers(),
            fetchProducts()
        ]);
        console.log('用户:', users);
        console.log('产品:', products);
    } catch (error) {
        console.error('错误:', error);
    }
}

class DataService {
    async fetchUserProfile(userId) {
        try {
            const user = await this.fetchUser(userId);
            const orders = await this.fetchOrders(user.id);
            const preferences = await this.fetchPreferences(user.id);

            return {
                user,
                orders,
                preferences
            };
        } catch (error) {
            console.error('获取用户资料失败:', error);
            throw error;
        }
    }

    async fetchUser(id) {
        // 模拟 API 调用
        return new Promise(resolve => {
            setTimeout(() => {
                resolve({ id, name: 'Alice' });
            }, 1000);
        });
    }

    async fetchOrders(userId) {
        return new Promise(resolve => {
            setTimeout(() => {
                resolve([{ id: 1, userId, product: 'Book' }]);
            }, 1000);
        });
    }

    async fetchPreferences(userId) {
        return new Promise(resolve => {
            setTimeout(() => {
                resolve({ theme: 'dark', language: 'zh' });
            }, 1000);
        });
    }
}

// 使用示例
const service = new DataService();
service.fetchUserProfile(1)
    .then(profile => console.log('用户资料:', profile))
    .catch(error => console.error('错误:', error));

fetchData()
    .then(data => processData(data))
    .then(result => saveData(result))
    .catch(error => {
        console.error('发生错误:', error);
        // 错误恢复逻辑
        return fallbackData;
    })
    .finally(() => {
        // 清理工作
        console.log('处理完成');
    });

async function processUserData() {
    try {
        const data = await fetchData();
        const result = await processData(data);
        await saveData(result);
    } catch (error) {
        console.error('处理用户数据时出错:', error);
        // 错误处理逻辑
    } finally {
        // 清理工作
    }
}