Node.js/Express项目中Promise .then语句未按顺序执行问题求助

核心错误原因 #

• 你对Promise链式调用的用法理解有误：.then() 方法要求传入回调函数作为参数，只有当前序Promise执行完成后，才会调用这个回调函数。
• 你现有代码直接把db.checkEmail(client)、traffic.hashPassword(creds)这类异步方法的执行结果（Promise实例）传给了.then()，这些方法会在代码解析到此处时立刻并行执行，完全不会等待前序步骤完成，所以执行顺序完全乱了。
• 额外逻辑错误：你的checkEmail方法逻辑写反了：当前只要SQL执行不报错，哪怕邮箱不存在（results.rows[0]为undefined）也会走resolve，你写的打印“邮箱不存在”的分支实际是SQL执行报错才会触发，和你的预期完全不符。

为什么你的日志顺序异常 #

你现有代码中checkEmail、hashPassword、hashUsername、createUser四个异步任务几乎同时启动：

• 哈希是本地计算，速度最快，所以先打印PASSWORD HASHED、NAME HASHED
• createUser虽然也是数据库操作，但此时newHash、newName还没被赋值，你传了空值给数据库，插入操作很快就报错，所以打印UNABLE TO CREATE NEW USER
• checkEmail需要远程查询Postgres，网络IO耗时最长，所以最后才打印邮箱校验的日志

修复方案 #

第一步：修正checkEmail逻辑 #

把queries.js里的checkEmail改成符合你预期的逻辑：

//queries.js
const checkEmail = function(mail) {
  return new Promise(function(resolve, reject) {
    pool.query('SELECT * FROM clients WHERE email = $1', [mail], function(error, results) {
      if (error) {
        // SQL执行错误才reject
        reject(new Error('Query error when checking email: ' + error.message));     
      } else {
        if (results.rows.length > 0) {
          // 邮箱已存在，resolve返回用户信息
          resolve(results.rows[0]);
        } else {
          // 邮箱不存在，reject提示
          reject(new Error('User email not exist'));
        }
      }
    })
  }); 
}

第二步：修正Promise链式调用 #

有两种修正方式，推荐用更易读的async/await写法：

方式1：修正原有Promise链写法

给每个.then传入回调函数，保证前序执行完再调用下一个异步方法：

// server.js 路由部分修改
app.post('/_join_up', function(req, res) {
  if (!req.body) {
    console.log('ERROR: req.body has NOT been returned...');
    return res.sendStatus(400)
  }

  var newHash, newName;
  var client = req.body.client_email;
  var creds = req.body.client_pword;
  var newToken = shortid.generate();
  var firstname = req.body.client_alias; 

  db.sanitation(client, creds, firstname)
  .then(function(direction) {
    console.log('USER-SUPPLIED DATA HAS PASSED INSPECTION');
    // 这里返回checkEmail的Promise，下一个then等它完成
    return db.checkEmail(client);
  })
  .then(function(foundUser) {
    console.log('HEY THERE IS ALREADY A USER WITH THAT EMAIL!', foundUser);
    // 邮箱已存在，直接抛出错误中断后续流程
    throw new Error('Email already registered');
  }, function(error) {
    console.log('USER EMAIL NOT CURRENTLY IN DATABASE...THEREFORE IT IS OK...');
    // 邮箱不存在，返回密码哈希的Promise
    return traffic.hashPassword(creds);
  })
  .then(function(hashedPassword) {
    console.log('PASSWORD HASHED');
    newHash = hashedPassword;
    // 返回用户名哈希的Promise
    return traffic.hashUsername(firstname);
  }, function(error) {
    console.log('UNABLE TO HASH PASSWORD...' + error);
    throw error;
  })
  .then(function(hashedName) {
    console.log('NAME HASHED');
    newName = hashedName;
    // 返回创建用户的Promise
    return db.createUser(client, newName, newHash, newToken);
  }, function(error) {
    console.log('UNABLE TO HASH NAME...' + error);
    throw error;
  })
  .then(function(data) {
    console.log('REGISTERED A NEW CLIENT JOIN...!!!');
    res.redirect('/landing');
  }, function(error) {
    console.log('UNABLE TO CREATE NEW USER...' + error);
    throw error;
  })
  .catch(function(error) {
    console.log('THERE WAS AN ERROR IN THE SEQUENTIAL PROCESSING OF THE USER-SUPPLIED INFORMATION...' + error);
    res.redirect('/');  
  });
});

方式2：更简洁的async/await写法（推荐）

// server.js 路由部分修改
app.post('/_join_up', async function(req, res) {
  if (!req.body) {
    console.log('ERROR: req.body has NOT been returned...');
    return res.sendStatus(400)
  }

  try {
    const client = req.body.client_email;
    const creds = req.body.client_pword;
    const newToken = shortid.generate();
    const firstname = req.body.client_alias; 

    // 按顺序执行异步步骤
    await db.sanitation(client, creds, firstname);
    console.log('USER-SUPPLIED DATA HAS PASSED INSPECTION');

    try {
      await db.checkEmail(client);
      // 没有抛错说明邮箱存在
      console.log('HEY THERE IS ALREADY A USER WITH THAT EMAIL!');
      throw new Error('Email already registered');
    } catch(e) {
      console.log('USER EMAIL NOT CURRENTLY IN DATABASE...THEREFORE IT IS OK...');
    }

    const newHash = await traffic.hashPassword(creds);
    console.log('PASSWORD HASHED');

    const newName = await traffic.hashUsername(firstname);
    console.log('NAME HASHED');

    await db.createUser(client, newName, newHash, newToken);
    console.log('REGISTERED A NEW CLIENT JOIN...!!!');
    res.redirect('/landing');
  } catch(error) {
    console.log('THERE WAS AN ERROR IN THE SEQUENTIAL PROCESSING OF THE USER-SUPPLIED INFORMATION...' + error);
    res.redirect('/');  
  }
});

内容的提问来源于stack exchange，提问作者Pangit