如何从另一个函数内的Promise返回值? [英] How to return value from a promise that's inside another function?
问题描述
我知道有关异步编程和Promises的问题很多,但是我确实需要一个带有此特定代码的示例,以弄清楚我应该如何返回在另一个函数中返回的Promise.
I know that a lot of questions have been asked about async programming and Promises, but I really need an example with this specific code to figure out how I should go about returning promises that's being returned in another function.
我有两个职能.GET'ing到路由时调用第一个.此路线应创建一个付款链接,并将预订保存到数据库中.
I have two functions. The first one is called upon GET'ing to a route. This route should create a payment link and save a booking to a database.
exports.create_booking = function(req, res) {
req.body.payment = exports.create_booking_payment(req.body.total_amount);
console.log(req.body.payment); // This returns ' Promise { <pending> } '
var new_booking = new Booking(req.body);
new_booking.save(function(err, booking) {
if (err)
res.send(err);
res.json(booking);
});
};
但是,创建付款链接是通过异步方法进行的.我的第一个问题是我只能在方法回调函数中访问付款.
However creating the payment link happens with an asynchronous method. My first problem was that I could only access the payment inside the methods callback function.
现在,我将方法包装在另一个(异步)方法中,在其中创建并解决了Promise.该方法将通过await语句返回到我的第一个方法,但是返回的所有内容都是:'Promise {}'.
Now I have wrapped the method inside another (async) method in which a Promise is created and resolved. This method is being returned to my first method with an await statement, but all this returns is: ' Promise { } '.
我知道发生这种情况是因为在解决诺言之前就已经返回了该方法.但是我不明白为什么会这样.我的假设是,"await"语句可确保在异步功能完成之前等待返回方法.
I know that this happens because the method is being returned before the promise is resolved. But I don't understand why this is. My assumption is that the 'await' statement makes sure to wait returning the method before the async function is completed.
exports.create_booking_payment = async function() {
function asyncPayment() {
return new Promise (function(resolve, reject) {
mollie.payments.create({
amount: 20.00,
description: "Reservation code: ",
redirectUrl: "https://www.website.com/",
webhookUrl: ""
}, function(payment) {
if (payment.error) reject(payment.error)
else { resolve({
id: payment.id,
link: payment.getPaymentUrl(),
status: payment.status
})
}
});
});
}
return await asyncPayment();
}
我希望有人可以在这里帮助我...
I hope someone can help me out here...
推荐答案
您似乎错过了 async
函数仍然返回promise,而不是实际值的想法.因此,当您调用 create_booking_payment()
时,您将得到一个承诺,即您需要将 .then()
与或将 await
与.跨职能边界没有免费的午餐. await
允许您以类似同步的方式在函数内部进行编程,但仍然不允许您从函数中返回值.当您看起来好像要从 async
函数返回值时,实际上是在返回解析为该值的promise.
You seem to have missed that an async
function still returns a promise, not the actual value. So, when you call create_booking_payment()
, you are getting back a promise that you need to use either .then()
with or await
with. There's no free lunch across a function boundary. await
allows you to program in a synchronous-like fashion inside a function, but still does not allow you to return the value from the function. When it looks like you're returning the value from the async
function, you're actually returning a promise that resolves to that value.
因此,您可以使用 async
和 await
:
exports.create_booking = async function(req, res) {
try{
req.body.payment = await exports.create_booking_payment(req.body.total_amount);
console.log(req.body.payment);
var new_booking = new Booking(req.body);
new_booking.save(function(err, booking) {
if (err)
res.status(500).send(err);
else
res.json(booking);
});
} catch(e) {
res.status(500).send(err);
}
};
或使用 .then()
:
exports.create_booking = function(req, res) {
exports.create_booking_payment(req.body.total_amount).then(payment => {
console.log(payment);
req.body.payment = payment;
var new_booking = new Booking(req.body);
new_booking.save(function(err, booking) {
if (err)
res.status(500).send(err);
else
res.json(booking);
});
}).catch(err => {
res.status(500).send(err);
});
};
注意,我还为这两种情况添加了更完整的错误处理.另外,如果您承诺"或对 .save()
方法使用已经承诺的接口,则此代码将更加简洁.我非常不喜欢在基于Promise的代码中使用普通的回调异步代码,因为它最终会导致重复的错误处理(就像您在本例中看到的那样).
Note, I also added more complete error handling to both scenarios. Also, this code would be a lot cleaner if you "promisfied" or used an already promisified interface for your .save()
method. I strongly dislike using plain callback async code inside of promise-based code because it ends up duplicating error handling (like you see in this case).
此外, create_booking_payment()
不必是 async
或使用 await
,因为您所需要做的就是返回您的承诺已经知道该怎么做:
Also, create_booking_payment()
doesn't need to be async
or use await
since all you need it to do is to return your promise which it already knows how to do:
exports.create_booking_payment = function() {
return new Promise (function(resolve, reject) {
mollie.payments.create({
amount: 20.00,
description: "Reservation code: ",
redirectUrl: "https://www.website.com/",
webhookUrl: ""
}, function(payment) {
if (payment.error) reject(payment.error)
else { resolve({
id: payment.id,
link: payment.getPaymentUrl(),
status: payment.status
})
}
});
});
}
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