Async/Await error handling - javascript

I'm trying to handle a custom error that my async method throws, but the try catch block doesn't work appropriately.
I think the way I'm doing it should work but the error is not caught and the program terminates by displaying it in the terminal.
Here is where it throws the error:
async setupTap(tap) {
const model = this.connection.model('Tap', TapSchema);
await model.findOneAndUpdate({ id: tap.id }, tap, (err, result) => {
let error = null;
if (!result) {
throw new Error('Tap doesn\'t exists', 404);
}
return result;
});
}
Then, the error handling code:
async setupTapHandler(request, h) {
const tapData = {
id: request.params.id,
clientId: request.payload.clientId,
beerId: request.payload.beerId,
kegId: request.payload.kegId,
};
try {
await this.kegeratorApi.setupTap(tapData);
} catch (e) {
if (e.code === 404) return h.response().code(404);
}
return h.response().code(204);
}
Can someone help me?
I also looked at other topics:
Correct Try...Catch Syntax Using Async/Await
How to properly implement error handling in async/await case

You can only use await to successfully wait on an async operation if you are awaiting a promise. Assuming you are using mongoose, I don't know mongoose really well, but it appears that model.findOneAndUpdate() does not return a promise if you pass it a callback. Instead, it executes and puts the result in the callback.
In addition, doing a throw from a callback like this just throws into the database (the code that called the callback) and won't do you any good at all. To have a throw make a rejected promise, you need to either be throwing from the top level of an async function or be throwing from inside a .then() or .catch() handler or inside a promise executor function. That's where throw makes a promise rejected.
The key here is that you want to use the promise interface to your database, not the callback interface. If you don't pass a callback, then it returns a query which you can use .exec() on to get a promise which you can then use with await.
In addition, you weren't building an error object that would have a .code property set to 404. That isn't a property that is supported by the Error object constructor, so if you want that property, you have to set it manually.
I'd suggest this:
async setupTap(tap) {
const model = this.connection.model('Tap', TapSchema);
let result = await model.findOneAndUpdate({ id: tap.id }, tap).exec();
if (!result) {
let err = new Error('Tap doesn\'t exists');
err.code = 404;
throw err;
}
return result;
}
Or, with only one async operation here, there's really not much benefit to using await. You could just do this:
setupTap(tap) {
const model = this.connection.model('Tap', TapSchema);
return model.findOneAndUpdate({ id: tap.id }, tap).exec().then(result => {
if (!result) {
let err = new Error('Tap doesn\'t exists');
err.code = 404;
throw err;
}
return result;
});
}

The function findOneAndUpdate returns a promise so there should be no need for the callback. If a callback is needed and you can't update to a newer version then maybe wrap calls in a promise (under To use a callback api as promise you can do:)
Then you want to set code on error, you can't do that with the constructor.
async setupTap(tap) {
const model = this.connection.model('Tap', TapSchema);
const result = await model.findOneAndUpdate({ id: tap.id }, tap);
if (!result) {
const e = new Error('Tap doesn\'t exists');
e.code = 404;
throw(e);
}
return result;
}
async setupTapHandler(request, h) {
const tapData = {
id: request.params.id,
clientId: request.payload.clientId,
beerId: request.payload.beerId,
kegId: request.payload.kegId,
};
try {
await this.kegeratorApi.setupTap(tapData);
} catch (e) {
if (e.code === 404) return h.response().code(404);
}
return h.response().code(204);
}

Related

How can I use async lambdas without a try/catch block and still have custom error messages?

I'm trying to avoid wrapping all my awaited calls in an async lambda with a try catch. I want to catch and send custom error responses, but wrapping each awaited call in a try/catch is syntactically ugly compared to .catch(). Is there a way to do something like this:
exports.hanlder = async (event, context, callback) => {
const foo = await bar(baz).catch((error) => {
eventResponse.statusCode = 503;
eventResponse.body = JSON.stringify({ message: 'unable to bar' , error});
// normally we'd callback(null, eventResponse)
});
Without wrapping in try/catch like this?
exports.hanlder = async (event, context, callback) => {
let foo;
try {
foo = await bar(baz);
} catch (error) {
eventResponse.statusCode = 503;
eventResponse.body = JSON.stringify({ message: 'unable to bar', error});
return eventResponse;
}
// then do something else with foo
if (foo.whatever) {
// some more async calls
}
It's just not pretty to have a bunch of try/catch once you have like 7 awaited calls in a single lambda. Is there a prettier way to do it using the promise built-in .catch()?
The .catch() method is compatible with async/await and often less ugly if you want to rethrow an exception. I think you're looking for
exports.handler = async (event, context, callback) => {
try {
const foo = await bar(baz).catch(error => {
throw {message: 'unable to bar', error};
});
// do something with `foo`, and more `await`ing calls throwing other errors
// return a response
} catch(err) {
eventResponse.statusCode = 503;
eventResponse.body = JSON.stringify(err);
return eventResponse;
}
};

How to do error handling with async/await without having to await

Take the following contrived example:
const housekeepingStuff = async function (data) {
const result = await notImportant(data);
result.more = 'yawn';
storeInDatabase(result);
};
const getStuff = async function () {
try {
const data = await getData();
data.extra = 'wow';
housekeepingStuff(data); // <---- don't want to await... but need to for error catching
return Promise.resolve(data);
} catch (err) {
return Promise.reject(err);
}
};
try {
const myData = await doSomeStuff();
res.send(myData);
} catch (err) {
console.log(err);
res.sendStatus(400);
}
I want to return the data from getStuff () ASAP without waiting for housekeepingStuff() but if I don't await that function then I have an uncaught error.
I could call housekeepingStuff() outside the getStuff() function, after getting and sending the data to whoever wants it:
try {
const myData = await doSomeStuff();
res.send(myData);
await housekeepingStuff(data); // <---- am awaiting but who cares because nothing follows
} catch (err) {
console.log(err);
res.sendStatus(400);
}
But that doesn't seem right because I don't want to have to remember to call housekeepingStuff() every time I call doSomeStuff()... it should ideally be handled "internally".
What is the correct approach here?
A promise (or async) function has 2 possible outcomes:
A successful outcome
An error outcome
To get either outcome, you must wait for it. You can't wait for 1 condition and not for the other, because the entire thing needs to execute so you can find out what the outcome was.
Otherwise you're really asking the javascript engine: Please predict for me if the function will fail, and if it does, await it.
The correct approach therefore is to just await it.
However, if you don't care about either successful or failed outcomes of this function, just call the function via another async function that eats all the errors:
async function doSomeStuffAndIgnoreError() {
try {
await doSomeStuff();
} catch (e) {
console.error(e);
}
}

How to properly handle error in MongoDB bulkwrite? [duplicate]

I like the flatness of the new Async/Await feature available in Typescript, etc. However, I'm not sure I like the fact that I have to declare the variable I'm awaiting on the outside of a try...catch block in order to use it later. Like so:
let createdUser
try {
createdUser = await this.User.create(userInfo)
} catch (error) {
console.error(error)
}
console.log(createdUser)
// business
// logic
// goes
// here
Please correct me if I'm wrong, but it seems to be best practice not to place multiple lines of business logic in the try body, so I'm left only with the alternative of declaring createdUser outside the block, assigning it in the block, and then using it after.
What is best practice in this instance?
It seems to be best practice not to place multiple lines of business logic in the try body
Actually I'd say it is. You usually want to catch all exceptions from working with the value:
try {
const createdUser = await this.User.create(userInfo);
console.log(createdUser)
// business logic goes here
} catch (error) {
console.error(error) // from creation or business logic
}
If you want to catch and handle errors only from the promise, you have three choices:
Declare the variable outside, and branch depending on whether there was an exception or not. That can take various forms, like
assign a default value to the variable in the catch block
return early or re-throw an exception from the catch block
set a flag whether the catch block caught an exception, and test for it in an if condition
test for the value of the variable to have been assigned
let createdUser; // or use `var` inside the block
try {
createdUser = await this.User.create(userInfo);
} catch (error) {
console.error(error) // from creation
}
if (createdUser) { // user was successfully created
console.log(createdUser)
// business logic goes here
}
Test the caught exception for its type, and handle or rethrow it based on that.
try {
const createdUser = await this.User.create(userInfo);
// user was successfully created
console.log(createdUser)
// business logic goes here
} catch (error) {
if (error instanceof CreationError) {
console.error(error) // from creation
} else {
throw error;
}
}
Unfortunately, standard JavaScript (still) doesn't have syntax support for conditional exceptions.
If your method doesn't return promises that are rejected with specific enough errors, you can do that yourself by re-throwing something more appropriate in a .catch() handler:
try {
const createdUser = await this.User.create(userInfo).catch(err => {
throw new CreationError(err.message, {code: "USER_CREATE"});
});
…
} …
See also Handling multiple catches in promise chain for the pre-async/await version of this.
Use then with two callbacks instead of try/catch. This really is the least ugly way and my personal recommendation also for its simplicity and correctness, not relying on tagged errors or looks of the result value to distinguish between fulfillment and rejection of the promise:
await this.User.create(userInfo).then(createdUser => {
// user was successfully created
console.log(createdUser)
// business logic goes here
}, error => {
console.error(error) // from creation
});
Of course it comes with the drawback of introducing callback functions, meaning you cannot as easily break/continue loops or do early returns from the outer function.
Another simpler approach is to append .catch to the promise function. ex:
const createdUser = await this.User.create(userInfo).catch( error => {
// handle error
})
Cleaner code
using async/await with Promise catch handler.
From what I see, this has been a long-standing problem that has bugged (both meanings) many programmers and their code. The Promise .catch is really no different from try/catch.
Working harmoniously with await/async, ES6 Promise's catch handler provides a proper solution and make code cleaner:
const createUser = await this.User
.create(userInfo)
.catch(error => console.error(error))
console.log(createdUser)
// business
// logic
// goes
// here
Note that while this answers the question, it gobbles up the error. The intention must be for the execution to continue and not throw. In this case, it's usually always better to be explicit and return false from catch and check for user:
.catch(error => {
console.error(error);
return false
})
if (!createdUser) // stop operation
In this case, it is better to throw because (1) this operation (creating a user) is not expected to failed, and (2) you are likely not able to continue:
const createUser = await this.User
.create(userInfo)
.catch(error => {
// do what you need with the error
console.error(error)
// maybe send to Datadog or Sentry
// don't gobble up the error
throw error
})
console.log(createdUser)
// business
// logic
// goes
// here
Learning catch doesn't seem like worth it?
The cleanliness benefits may not be apparent above, but it adds up in real-world complex async operations.
As an illustration, besides creating user (this.User.create), we can push notification (this.pushNotification) and send email (this.sendEmail).
this.User.create
this.User.create = async(userInfo) => {
// collect some fb data and do some background check in parallel
const facebookDetails = await retrieveFacebookAsync(userInfo.email)
.catch(error => {
// we can do some special error handling
// and throw back the error
})
const backgroundCheck = await backgroundCheckAsync(userInfo.passportID)
if (backgroundCheck.pass !== true) throw Error('Background check failed')
// now we can insert everything
const createdUser = await Database.insert({ ...userInfo, ...facebookDetails })
return createdUser
}
this.pushNotifcation and this.sendEmail
this.pushNotification = async(userInfo) => {
const pushed = await PushNotificationProvider.send(userInfo)
return pushed
})
this.sendEmail = async(userInfo) => {
const sent = await mail({ to: userInfo.email, message: 'Welcome' })
return sent
})
Compose the operations:
const createdUser = await this.User
.create(userInfo)
.catch(error => {
// handle error
})
// business logic here
return await Promise.all([
this.pushNotification(userInfo),
this.sendEmail(userInfo)
]).catch(error => {
// handle errors caused
// by pushNotification or sendEmail
})
No try/catch. And it's clear what errors you are handling.
I usually use the Promise's catch() function to return an object with an error property on failure.
For example, in your case i'd do:
const createdUser = await this.User.create(userInfo)
.catch(error => { error }); // <--- the added catch
if (Object(createdUser).error) {
console.error(error)
}
If you don't like to keep adding the catch() calls, you can add a helper function to the Function's prototype:
Function.prototype.withCatcher = function withCatcher() {
const result = this.apply(this, arguments);
if (!Object(result).catch) {
throw `${this.name}() must return a Promise when using withCatcher()`;
}
return result.catch(error => ({ error }));
};
And now you'll be able to do:
const createdUser = await this.User.create.withCatcher(userInfo);
if (Object(createdUser).error) {
console.error(createdUser.error);
}
EDIT 03/2020
You can also add a default "catch to an error object" function to the Promise object like so:
Promise.prototype.catchToObj = function catchToObj() {
return this.catch(error => ({ error }));
};
And then use it as follows:
const createdUser = await this.User.create(userInfo).catchToObj();
if (createdUser && createdUser.error) {
console.error(createdUser.error);
}
#Bergi Answer is good, but I think it's not the best way because you have to go back to the old then() method, so i think a better way is to catch the error in the async function
async function someAsyncFunction(){
const createdUser = await this.User.create(userInfo);
console.log(createdUser)
}
someAsyncFunction().catch(console.log);
But what if we have many await in the same function and need to catch every error?
You may declare the to() function
function to(promise) {
return promise.then(data => {
return [null, data];
})
.catch(err => [err]);
}
And then
async function someAsyncFunction(){
let err, createdUser, anotherUser;
[err, createdUser] = await to(this.User.create(userInfo));
if (err) console.log(`Error is ${err}`);
else console.log(`createdUser is ${createdUser}`);
[err, anotherUser] = await to(this.User.create(anotherUserInfo));
if (err) console.log(`Error is ${err}`);
else console.log(`anotherUser is ${anotherUser}`);
}
someAsyncFunction();
When reading this its: "Wait to this.User.create".
Finally you can create the module "to.js" or simply use the await-to-js module.
You can get more information about to function in this post
await this.User.create(userInfo).then(async data => await this.emailService.sendEmail(data.email), async error => await this.sentryService.sendReport(error))

How to convert callback code to async/await? [duplicate]

This question already has answers here:
Nodejs Mongoose - how to avoid callback hell?
(3 answers)
Closed 5 years ago.
I am working with Mongoose ODM for MongoDB in a node express-based web application which provides its API in a callback fashion. This fashion creates a callback hell to populate documents.
Edit 1: I have added the current working code.
User.remove({}, (err) => {
if (err) return console.error(err)
let admin = new User({
email: 'admin#dap.com',
password: 'dapdap'
})
admin.save((err, admin) => {
if (err) return console.error(err)
console.info('Success')
mongoose.disconnect()
})
})
Is there an elegant way to convert the callbacks into async/await fashion in javascript in general? And in mongoose environment?
Would you be able to configure mongoose to use bluebird?
mongoose.Promise = require('bluebird');
Then you could get fancy and do:
User.remove().then(() => {
let admin = new User({ ... });
admin.save().then((admin) => {
}).catch((err) => {
//Saving admin error handling
});
}).catch((err) {
//User remove error handling
});
The same code you wrote above can be written as a promise. From the original docs,
"Mongoose queries are not promises. However, they do have a .then() function for yield and async/await. If you need a fully-fledged promise, use the .exec() function."
Thus you can use the async await pattern as follows,
async function removeUser () {
// Surround the following call with a try catch
await User.remove({});
}
EDIT:
One answer here says about plugging in blue bird promises. While this is a good approach, it is optional. You can do the same without plugging in your own Promise library.
You can use a Promise:
const userRemovePromise = new Promise((resolve, reject) => {
User.remove({}, (err) => {
if (err) reject();
let admin = new User({
email: 'admin#dap.com',
password: 'dapdap'
})
admin.save((err, admin) => {
if (err) reject();
resolve();
})
});
});
userRemovePromise.then(function(){
mongoose.disconnect();
});
As mongoose's default promise library mpromise is now deprecated so you can plugin your own promise library to make mongoose's operations work with await/async.
Mongoose queries are not promises. However, they do have a .then() function for yield and async/await. If you need a fully-fledged promise, use the .exec() function.
mongoose.Promise = require('bluebird'); bluebird promise
// mongoose.Promise = global.Promise; // default js promise - 4x- time slower than bluebird
async function removeUser () {
try {
await User.remove({}).exec();
let admin = new User({
email: 'admin#dap.com',
password: 'dapdap'
})
await admin.save();
} catch (e) {
console.log(e);
}
}
Official Doc: mongoosejs.com/docs/promises.html
Recent versions of Mongoose returns a Promise as well as providing the regular callback-style pattern. Since async functions are syntatic sugar over Promises, you can await calls to Mongoose methods.
async function clearUsers () {
try {
await User.remove({})
let admin = new User({
email: 'admin#dap.com',
password: 'dapdap'
})
await admin.save()
console.info('Success')
mongoose.disconnect()
} catch (e) {
console.error(e)
}
}
Some things to keep in mind:
async functions always returns a Promise. If you don't return anything from an async function, it will still return a Promise that resolves when execution of that function finishes, but will not resolve with any value.
try/catch in an async function works the same as for regular, synchronous code. If any function call in the try body throw or return a Promise that rejects, execution stops right on that line and proceeds to the catch body.
Rejected promises "trickle" up the function call chain. This means that the top-most callee can handle errors. See the below example:
This is an anti-pattern that should be avoided unless you absolutely need to handle an error in a specific function, possibly to provide a return value from a different source:
async function fn1 () {
throw new Error("Something went wrong")
}
async function fn2 () {
try {
await fn1()
} catch (e) {
throw e
}
}
async function fn3 () {
try {
await fn2()
} catch (e) {
throw e
}
}
async function run () {
try {
await fn3()
} catch (e) {
console.error(e)
}
}
The above could rather be implemented like the below and still catch the error, not resulting in a runtime panic/crash:
async function fn1 () {
throw new Error("Something went wrong")
}
function fn2 () {
return fn1()
}
function fn3 () {
return fn2()
}
async function run () {
try {
await fn3()
} catch (e) {
console.error(e)
}
}
There are multiple ways to write the above code that will all be valid, so I recommend that you explore these.
Keeping the above examples in mind, your function clearUsers() could then be rewritten to this:
async function clearUsers () {
await User.remove({})
let admin = new User({
email: 'admin#dap.com',
password: 'dapdap'
})
await admin.save()
mongoose.disconnect()
}
And then possibly called in two different ways;
By interacting with the Promise returned directly:
clearUsers()
.then(() => {
console.log('Success')
})
.catch((e) => {
console.error(e)
})
Or from within another async function:
(async function () {
try {
await clearUsers()
console.log('Success')
} catch (e) {
console.error(e)
}
})()
If any function call within the clearUsers() function throws, for example await admin.save(), execution will stop on that line and return a rejected Promise which will be caught in the corresponding catch blocks in both variants.
Using Bluebird is a good option to handle callbacks in promises, It makes it easier to understand to an extent.
However I would recommend you to try
Async.js
It is built particularly for managing asynchronous nature of javascript.
This particular function of the library would do exactly what you want.

Async function always return undefined

im using nodejs 8. I've replaced promise structure code to use async and await.
I have an issue when I need to return an object but await sentence resolve undefined.
This is my controller method:
request.create = async (id, params) => {
try {
let request = await new Request(Object.assign(params, { property : id })).save()
if ('_id' in request) {
Property.findById(id).then( async (property) => {
property.requests.push(request._id)
await property.save()
let response = {
status: 200,
message: lang.__('general.success.created','Request')
}
return Promise.resolve(response)
})
}
}
catch (err) {
let response = {
status: 400,
message: lang.__('general.error.fatalError')
}
return Promise.reject(response)
}
}
In http request function:
exports.create = async (req, res) => {
try {
let response = await Request.create(req.params.id, req.body)
console.log(response)
res.send(response)
}
catch (err) {
res.status(err.status).send(err)
}
}
I tried returning Promise.resolve(response) and Promise.reject(response) with then and catch in the middleware function and is occurring the same.
What's wrong?
Thanks a lot, cheers
You don't necessarily need to interact with the promises at all inside an async function. Inside an async function, the regular throw syntax is the same as return Promise.reject() because an async function always returns a Promise. Another thing I noticed with your code is that you're rejecting promises inside a HTTP handler, which will definitely lead to unexpected behavior later on. You should instead handle all errors directly in the handler and act on them accordingly, instead of returning/throwing them.
Your code could be rewritten like so:
request.create = async (id, params) => {
let request = await new Request(Object.assign(params, { property : id })).save()
if ('_id' in request) {
let property = await Property.findById(id)
property.requests.push(request._id)
await property.save()
}
}
And your http handler:
exports.create = async (req, res) => {
try {
await Request.create(req.params.id, req.body)
res.send({
status: 200,
message: lang.__('general.success.created','Request')
})
} catch (err) {
switch (err.constructor) {
case DatabaseConnectionError: // Not connected to database
return res.sendStatus(500) // Internal server error
case UnauthorizedError:
return res.sendStatus(401) // Unauthorized
case default:
return res.status(400).send(err) // Generic error
}
}
}
Error classes:
class DatabaseConnectionError extends Error {}
class UnauthorizedError extends Error {}
Because you have that try/catch block inside your http handler method, anything that throws or rejects inside the Request.create method will be caught there. See https://repl.it/LtLo/3 for a more concise example of how errors thrown from async function or Promises doesn't need to be caught directly where they are first called from.

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