Error: Callback was already called in loopback - node.js

Question

I have the following code:
"use strict";
const Raven = require("raven");
Raven.config(
"test"
).install();
module.exports = function(Reservation) {
function dateValidator(err) {
if (this.startDate >= this.endDate) {
err();
}
}
function sendEmail(campground) {
return new Promise((resolve, reject) => {
Reservation.app.models.Email.send(formEmailObject(campground),
function(
err,
mail
) {
if (err) {
console.log(err);
Raven.captureException(err);
reject(err);
} else {
console.log(mail);
console.log("email sent!");
resolve(mail);
}
});
});
}
function formEmailObject(campground) {
return {
to: "loopbackintern#yopmail.com",
from: "noreply#optis.be",
subject: "Thank you for your reservation at " + campground.name,
html:
"<p>We confirm your reservation for <strong>" +
campground.name +
"</strong></p>"
};
}
Reservation.validate("startDate", dateValidator, {
message: "endDate should be after startDate"
});
Reservation.observe("after save", async function(ctx, next) {
try {
const campground = await Reservation.app.models.Campground.findById(
ctx.instance.campgroundId
);
const mail = await sendEmail(campground);
next();
} catch (e) {
Raven.captureException(e);
next(e);
}
});
};
Sorry for the poor formatting. When the flow is done I get this error:
(node:3907) UnhandledPromiseRejectionWarning: Unhandled promise rejection (rejection id: 1): Error: Callback was already called.
I am calling the next() callback in two places, one in the try code and one in the catch code. I assume that when it all goes right, next callback is called only once, and the same when it goes wrong. But it seems that it is called twice and I don't know why.
I also tried to call next outside the try/catch code but it results in the same error. If I left only the next that is called inside the catch code it doesn't throw the error.
Any idea? Thanks!

Answer 1

if you are using async function you shouldn't explicitly call next, it gets automatically called.
check out this github issue for loopback async/await
so your hook can be like the following.
Reservation.observe("after save", async ctx => {
try {
const campground = await Reservation.app.models.Campground.findById(
ctx.instance.campgroundId
);
const mail = await sendEmail(campground);
} catch (e) {
Raven.captureException(e);
throw e;
}
});
NB: you don't need to wrap it in try catch unless you want to modify/work with the error.

Answer 2

You should declare your sendEmail method as async as it returns a promise.
async function sendEmail(campground) {
...
}

Answer 3

After reading this article, I created a await-handler.js file which include following code.
module.exports = (promise) =>
promise
.then(data => ({
ok: true,
data
}))
.catch(error =>
Promise.resolve({
ok: false,
error
})
);
Then in MyModel.js file, I created a async function to get a value from database as follow.
const awaitHandler = require("./../await-handler.js")
const getMaxNumber = async (MyModel) => {
let result = await awaitHandler(MyModel.find());
if (result.ok) {
if (result.data.length) {
return result.data.reduce((max, b) => Math.max(max, b.propertyName), result.data[0] && result.data[0].propertyName);
} else {
return 0;
}
} else {
return result.error;
}
}
As per #Mehari's answer, I've commented call to next() method as follow:-
module.exports = function(MyModel) {
MyModel.observe('before save', async(ctx, next) => {
const maxNumber = await getMaxNumber (MyModel);
if(ctx.instance) {
...
set the required property using ctx.instance.*
like createdAt, createdBy properties
...
// return next();
} else {
...
code for patch
...
// return next();
}
})
}
This solves the warning issue whenever saving endpoint is triggered.
But the warning issue still appear when I run the endpoint to load the resource.Like
http://localhost:3000/api/MyModel
Previously, the issue appear only when the before save operation hook gets triggered.
After encountering this issue, I checked adding access and loaded operation hooks and I found that the the warnings are issued after loaded operation hook.
MyModel.observe('access', (ctx, next) => {
return next();
})
MyModel.observe('loaded', (ctx, next) => {
return next();
})
What could have caused this issue and how can it gets resolved?