async等待调用函数node.js

Question

我有一个函数，它查询我的数据库，然后调用getNumOfSessionForTrainerClientList函数：

var select = "select * from USERS WHERE ASSIGNED_TRAINER = ?"
mysqlconn.connect(function(err) {
if (err) {
console.error('Database connection failed: ' + err.stack);
return;
}
mysqlconn.query(select, [req.session.username], async function(err, rows) {
if (err) {
console.log(err);
} else {

let numOfSessionsLeft = {
numberOfSessionsLeftIs: 0
}
for (var i = 0; i < rows.length; i++) {
getNumOfSessionForTrainerClientList(req, res, rows[i], numOfSessionsLeft)
rows[i]["NUM_OF_SESSIONS_PER_MONTH"] = (parseInt(rows[i]["NUM_OF_SESSIONS_PER_MONTH"]) - numOfSessionsLeft.numberOfSessionsLeftIs)
console.log(numOfSessionsLeft)
}

}
mysqlconn.end();
})
})

然后在函数getNumOfSessionForTrainerClientList中，我得到了这个：

async function getNumOfSessionForTrainerClientList(req, res, rows, numOfSessionsLeft) {
var getNumOfSessionForTrainerClientList = "select * from SCHEDULE WHERE CLIENT_USERNAME = ? AND ASSIGNED_TRAINER = ?"
mysqlconn.connect(async function(err) {
if (err) {
console.error('Database connection failed: ' + err.stack);
return;
}
mysqlconn.query(getNumOfSessionForTrainerClientList, [rows["USERNAME"], req.session.username], async function(err, sessionData) {
if (err) {
console.log(err);
} else {
numOfSessionsLeft.numberOfSessionsLeftIs = 1;
console.log(numOfSessionsLeft.numberOfSessionsLeftIs)
}
})
})
}

然而，发生的情况是，这一行：rows[i]["NUM_OF_SESSIONS_PER_MONTH"] = (parseInt(rows[i]["NUM_OF_SESSIONS_PER_MONTH"]) - numOfSessionsLeft.numberOfSessionsLeftIs)实际上是在将0分配给rows[i]["NUM_OF_SESSIONS_PER_MONTH"]，因为变量分配是在getNumOfSessionForTrainerClientList的函数调用完成之前发生的。所以这是不同步的。我不知道如何解决这个问题，我以前遇到过asyncawait的问题，但没有这样的问题。非常感谢您的帮助。

Answer 1

For循环本质上是同步的，因此不需要等待方法getNumOfSessionForTrainerClientList返回来继续迭代。

正确的做法是：

1. 将代码放入异步块中，并在调用getNumOfSessionForTrainerClientList调用时使用await((
2. 其次，您的函数getNumOfSessionForTrainerClientList应该回报承诺

样品：

async function functionA() {
var select = "select * from USERS WHERE ASSIGNED_TRAINER = ?"
mysqlconn.connect(function(err) {
if (err) {
console.error('Database connection failed: ' + err.stack);
return;
}

mysqlconn.query(select, [req.session.username], async function(err, rows) {
if (err) {
console.log(err);
} else {

let numOfSessionsLeft = {
numberOfSessionsLeftIs: 0
}
for (var i = 0; i < rows.length; i++) {
await getNumOfSessionForTrainerClientList(req, res, rows[i], numOfSessionsLeft)
rows[i]["NUM_OF_SESSIONS_PER_MONTH"] = (parseInt(rows[i]["NUM_OF_SESSIONS_PER_MONTH"]) - numOfSessionsLeft.numberOfSessionsLeftIs)
console.log(numOfSessionsLeft)
}
}
mysqlconn.end();
})
})
}

您的函数getNumOfSessionForTrainerClientList应该看起来像：

async function getNumOfSessionForTrainerClientList(req, res, rows, numOfSessionsLeft) {
return new Promise((resolve, reject) => {
var getNumOfSessionForTrainerClientList = "select * from SCHEDULE WHERE CLIENT_USERNAME = ? AND ASSIGNED_TRAINER = ?"
mysqlconn.connect(async function(err) {
if (err) {
console.error('Database connection failed: ' + err.stack);
return reject(err);
}
mysqlconn.query(getNumOfSessionForTrainerClientList, [rows["USERNAME"], req.session.username], async function(err, sessionData) {
if (err) {
console.log(err);
return reject(err);
} else {
numOfSessionsLeft.numberOfSessionsLeftIs = 1;
console.log(numOfSessionsLeft.numberOfSessionsLeftIs)
return resolve();
}
})
})
})
}

然后用你需要的任何参数调用functionA()。

Answer 2

您调用包含异步代码的函数，因此getNumOfSessionForTrainerClientList不依赖于上面的代码(以(rows[i][quot；NUM_OF and console(开头(您应该做的是

1. 通过返回prom 使函数异步

2. 使用等待关键字执行

   async function trainerClientList(){
   return new Promise((resolve,reject)=>{
   var getNumOfSessionForTrainerClientList = "select * from SCHEDULE WHERE CLIENT_USERNAME = ? AND ASSIGNED_TRAINER = ?"
   mysqlconn.connect(async function(err) {
   if (err) {
   console.error('Database connection failed: ' + err.stack);
   resolve()
   }
   mysqlconn.query(getNumOfSessionForTrainerClientList, [rows["USERNAME"], req.session.username], async function(err, sessionData) {
   if (err) {
   console.log(err);
   } else {
   numOfSessionsLeft.numberOfSessionsLeftIs = 1;
   console.log(numOfSessionsLeft.numberOfSessionsLeftIs)
   }
   })
   })
   resolve()
   
   })
   

   }

   for(var i=0；i<rows.length；i++({等待trainerClientList((rows[i][quot；NUM_OF_SESSIONS_PER_MONTH"]=(parseInt(rows[i][quot；NUM_OF_SESIONS_PER_MONTH"(-numOfSessionsLeft.numberOfSessions左撇子(console.log(numOfSessionsLeft(}