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 await
ing 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
catch
blockreturn
early or re- throw
an exception from the catch
blockcatch
block caught an exception, and test for it in an if
condition 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 return
s 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
})
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);
}
You can also add a default "catch to an error object" function to the Promise
object like so:
const createdUser = await this.User.create(userInfo).catchToObj();
if (createdUser && createdUser.error) {
console.error(createdUser.error);
}
And then use it as follows:
const createdUser = await this.User.create(userInfo).catchToObj(); if (createdUser && createdUser.error) { console.error(createdUser.error); }
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 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
Likely, one will want to throw. So the complete answer as follows:
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
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.
@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);
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))
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