Promise.then（），但功能运行异步

Question

我新的Javascript和做履带，我创建了4无极这些

var openConfig = new Promise((resolve, reject) => { 
    fs.readFile('./config.json', (err, data) => { 
    if (err) throw err; 
    config = JSON.parse(data); 
    client = new MsTranslator({ 
     client_id: config.translatorId, 
     client_secret: config.translatorSecret 
    }, true) 
    resolve(); 
    }) 
}) 

var openFile = new Promise((resolve, reject) => { 
    console.log('Opening file...') 
    fs.readFile('./writing/writing.json', (err, data) => { 
    if (err) throw err; 
    writing = JSON.parse(data); 
    console.log('Done parsing file'); 
    resolve(); 
    }) 
}) 

var ask = new Promise((resolve, reject) => { 
    var rl = readline.createInterface({ 
    input: process.stdin, 
    output: process.stdout 
    }) 
    rl.question('Which lesson do you want to add? ', (ans) => { 
    lessonId = ans; 
    rl.close(); 
    resolve(); 
    }) 
}) 

var createLesson = new Promise((resolve, reject) => { 
    console.log('Now processing lesson ' + lessonId); 
}) 

然后调用第一承诺

openConfig 
    .then(() => { 
    return openFile; 
    }) 
    .then(() => { 
    return ask; 
    }) 
    .then(() => { 
    return createLesson; 
    }) 

但当我运行，控制台显示

Opening file... 
Which lesson do you want to add? Now processing lesson undefined 
Done parsing file 

我明白我的承诺是错误的，我的功能运行异步。你能帮我解决这个问题吗？

谢谢。

Answer 1

不是指定新的承诺，以增值经销商（这些只要创建运行），你应该将它们包装成函数，这又回到了新的承诺的

为了帮助你了解这里是一个简单的例子：

function p1 (data) { 
    return new Promise(function (resolve, reject) { 
     resolve(Object.assign(data, {a:1})); 
    }); 
} 

function p2 (data) { 
    return new Promise(function (resolve, reject) { 
     resolve(Object.assign(data, {b:2})); 
    }); 
} 

function p3 (data) { 
    return new Promise(function (resolve, reject) { 
     resolve(Object.assign(data, {c:3})); 
    }); 
} 

p1({z:0}) 
.then(p2) 
.then(p3) 
.then((data)=>console.log(data)) 

这导致{ z: 0, a: 1, b: 2, c: 3 }

看到这里，如果你想实验一下上述：https://repl.it/DwNB/0

---

在一个单独的说明，如果您使用的承诺，还应该处理链中的错误在最后.catch()，而不是同步throw荷兰国际集团中途。这就是reject的回拨！

Answer 2

承诺不是“叫”。在您的then连锁店中，您只能按顺序等待它们 - 但是当您的创建了的承诺时，任务已经开始。如果你想排序的行动，把他们的功能。

顺便说一句，你的代码包含多个典型的错误。 Don't use global variables，始终promisify在尽可能低的水平：

function openFile(path) { 
    return new Promise((resolve, reject) => { 
    fs.readFile('./config.json', (err, data) => { 
     if (err) reject(err); // never `throw err` in non-promise callbacks! 
     else resolve(data); 
    }); 
    }); 
} 
function openJSON(path) { 
    return openFile(path).then(JSON.parse); 
} 
function openConfig(path) { 
    return openJSON(path).then(config => 
    new MsTranslator({ 
     client_id: config.translatorId, 
     client_secret: config.translatorSecret 
    }, true) 
) 
} 
function ask(question) { 
    return new Promise((resolve, reject) => { 
    var rl = readline.createInterface({ 
     input: process.stdin, 
     output: process.stdout 
    }) 
    rl.question(question, ans => { 
     rl.close(); 
     resolve(ans); // always resolve to *something* 
    }); 
    }); 
} 

readConfig('./config.json') 
.then(client => { 
    console.log('Opening file...') 
    return openJSON('./writing/writing.json'); 
}) 
.then(writing => { 
    console.log('Done parsing file'); 
    return ask('Which lesson do you want to add? '); 
}) 
.then(lessonId => { 
    console.log('Now processing lesson ' + lessonId); 
});