如何使用promise转换此代码？ [英] How do I convert this code using promises?
问题描述
我是Node.js的新客户。我想将这个代码转换为promises（nodejs）。这包括四个回调函数get（），abc1（），pqr（），xyz（）。它给出错误的无法读取属性然后未定义任何人可以解决它？我试图将其转换为来自此示例的promise here
I am new to Nodejs.I want to convert this code to promises (nodejs).This includes four callback function get(),abc1(),pqr(),xyz(). It gives error of "cannot read property 'then' of undefined" Can anyone fix it? I am trying it to convert into promises from this example here
var jwt = require('jwt-simple');
var mysql = require('mysql');
var async = require('async');
var Promise = require('bluebird');
var connection = mysql.createConnection({
host: 'localhost',
port: 3306,
database: 'abc',
user: 'root',
password: ''
});
var auth = {
login: function (req, res) {
var username = req.body.username || '';
var password = req.body.password || '';
console.log(req.body);
//This is get() function
function get(err, rows) {
connection.connect(function (err) {
if (err) {
console.error('error connecting: ' + err.stack);
return;
} else {
console.log("connected to database");
connection.query("SELECT count(id) as count from abc where name=? AND email=?", [username, password], function (err, rows) {
if (err) {
throw err
} else {
b = rows;
console.log(b);
}
});
}
});
}
//this is abc1() function
function abc1(err, rows) {
console.log("onresult");
if (err) {
throw err;
console.log("error in Query");
} else {
if (rows[0].count != 1) {
//xyz(result,xyz);
res.json({
"status": 401,
"message": "Invalid credentials"
});
} else {
connection.query("SELECT email from abc where name=? AND email=?", [username, password], function (err, rows) {});
//res.json(genToken("ayush"));
}
}
}
//This is xyz() function
function xyz(err, rows) {
if (err) {
console.log(err);
}
console.log(rows);
connection.query("SELECT name from abc where name=? AND email=?", [username, password], function (err, rows) {});
}
//This is pqr() Function
function pqr(err, rows) {
if (err) {
console.log(err);
}
console.log(rows);
}
get()
.then(function (rows) {
// do something...
//console.log("hellow1");
return abc1();
})
.then(function (rows) {
// do something...
console.log("hellow2");
return xyz();
})
.then(function (rows) {
// the third and final async response
console.log("hellow3");
return pqr();
})
.then(function (rows) {
// the third and final async response
console.log("hellow4");
})
.fail(function (err) {
// handle any error resulting from any of the above calls
throw err;
})
.done();
}
}
推荐答案
参数（err，rows），所有四个函数看起来都是作为nodebacks写的，我们可以对原来的工作代码看起来像一个精明的猜测。
With arguments (err, rows), all four functions appear to be written as nodebacks and we can make a shrewd guess as to what the original working code looked like.
没有修改，nodeback只能用作nodeback。
Without modification, nodebacks can only be used as nodebacks. Inserting them raw into a promise chain won't work.
但是你的函数将在promise链后面工作：
But your functions will work in a promise chain after :
- 确认潜在的asycn函数，即
连接的方法。 - 函数，
connection.methodAsync（）版本的方法，而不是connection.method（）。 - 返回每个函数的promise。
- Promisifying the underlying asycn functions, ie
connection's methods. - Calling, in the functions, the
connection.methodAsync()version of the methods instead ofconnection.method(). - Returning a promise from each of your functions.
重构以 connect（）函数而不是 get（）开始。
The whole thing actually works better with a little renaming/refactoring to start with a connect() function rather than get().
var jwt = require('jwt-simple');
var mysql = require('mysql');
var async = require('async');
var Promise = require('bluebird');
var connection = mysql.createConnection({
host: 'localhost',
port: 3306,
database: 'abc',
user: 'root',
password: ''
});
//Promisify `connection`.
//Lucky you, Bluebird has a `.promisifyAll()` method.
Promise.promisifyAll(connection);
//Now, `auth` will be something like this :
var auth = {
login: function (req, res) {
var username = req.body.username || '';
var password = req.body.password || '';
//Let's assume you want to build an object containing various user details
var authObj = {
'userName': userName,
'email': null, //to be set later
'name': null //to be set later
};
function connect() {
return connection.connectAsync().catch(function(err) {
throw 'Error connecting: ' + err.stack;
});
}
function abc1() {
return connection.queryAsync("SELECT count(id) as count from abc where name=? AND email=?", [username, password]).then(function (rows) {
if (rows[0].count != 1) {
throw new Error('Invalid credentials');
}
});
}
function xyz() {
return connection.queryAsync("SELECT email from abc where name=? AND email=?", [username, password]).then(function(rows) {
return rows[0].email;
});
}
function pqr() {
return connection.queryAsync("SELECT name from abc where name=? AND email=?", [username, password]).then(function(rows) {
return rows[0].name;
});
}
return connect()
.then(abc1)//check credentials
.then(xyz)//get email
.then(function (email) {
authObj.email = email;
return pqr();//get name
})
.then(function (name) {
authObj.name = name;
return authObj;//make authObj available to auth.login()'s caller.
})
.catch(function (err) {
// handle any error resulting from any of the above calls.
console.log(err);
throw err;//make the error available to auth.login()'s caller.
});
}
}
正如你将看到的， err 不在代码中，直到 auth.login（）的底部。它是promisified nodebacks的性质，错误传播下承诺链的错误路径，并且不需要处理中链，除非你需要做一些像中链错误恢复。
As you will see, err doesn't feature in the code until the bottom of auth.login(). It is the nature of promisified nodebacks that errors propagate down the promise chain's error path, and don't need to be handled mid-chain unless you need to do something like mid-chain error recovery.
虽然这对于学习如何构造一个promise链是有好处的，但在实践中你似乎已经知道名字和电子邮件在一开始，你可能会选择登录并获得其他用户的详细信息， SQL。
Whereas this is good for learning how to construct a promise chain, in practice you appear already to know name and email at the outset, and you would probably choose to login and obtain other user details in one hit with suitable SQL.
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