alternative to multiple try-catch blocks in a function

Question

Imagine we have a function like so:

async function doSomethingWithFriends() { 

  let user, friends, friendsOfFriends = null;

   try { 
       user = await getUser();
   }  catch(err){
       return [401, err]
   } 

    try {
      friends = await getFriends(user); 
    } catch (err){
       return [500, err];
    }

    try {
      friendsOfFriends = await getFriendsOfFriends(friends); 
    } catch(err){
       return [500, err]
    }

    return doFriendsOfFriends(friendsOfFriends);
}

instead is there some established pattern to avoid this somewhat boilerplate code?

There are two problems with the above code:

it's noisy
we cannot use const

One solution is to only use try-catch in the caller functions, but I am wondering if there is a way to solve it all the way down/up.

You can write multiple awaits in a single try catch block. so your catch block will receive the error if any of the above await fails. — Hyunjune Kim
– Hyunjune Kim, Commented Feb 22, 2022 at 7:18
@HyunjuneKim From the code, however, it looks like the failure of different awaited functions shall correspond to different error messages and HTTP status codes. You may lose the ability to differentiate different failures when putting everything in the same try block. — Sebastian Simon
– Sebastian Simon, Commented Feb 22, 2022 at 7:21

Philip Purwoko · Accepted Answer · 2022-02-22 07:13:32Z

Are you using Expressjs or something ? Please provide more information. I will assume you are making an API.

You can create a middleware that would handle all the error in your controller. Then in your controller you only need to throw or pass the error into next middleware. Example below.

error handle middleware

export function errorHandlingController(error: ErrorRequestHandler, _req: Request, res: Response, _next: NextFunction): void {
    let errorCode: number
    if (error instanceof RequestPayloadError) {
        errorCode = 400
    } else if (error instanceof UnauthorizedError) {
        errorCode = 401
    } else if (error instanceof NotFoundError) {
        errorCode = 404
    } else {
        errorCode = 500
    }
    res.status(errorCode).json(<GeneralResponse>{
        status: responseStatus.error,
        message: error.toString()
    })
}

user controller

export function getMyProfile(_req: Request, res: Response, next: NextFunction): void {
    try {
        const user = res.locals.user
        if (!user) throw new UnauthorizedError('Author id not identified')
        res.status(200).json({
            status: responseStatus.success,
            user: user
        })
    } catch (error: unknown) {
        next(error)
    }
}

Lee · Accepted Answer · 2022-02-22 07:20:13Z

1

In this case each function you can define custom exception. In doSomethingWithFriends only one try catch in used. Sample code below:

(async function doSomethingWithFriends() {


  async function getUser() {
    throw 'getUser'
  }

  async function getFriends() {
    throw 'getFriends'
  }

  async function getFriendsOfFriends() {
    throw 'getFriendsOfFriends'
  }

  let user, friends, friendsOfFriends = null;

  try {
    user = await getUser();
    friends = await getFriends(user);
    friendsOfFriends = await getFriendsOfFriends(friends);
  }  catch(err){
    console.log(err)
    if (err) {
      // custom return what you want
    }
  }

  return doFriendsOfFriends(friendsOfFriends);
}())

answered Feb 22, 2022 at 7:20

Lee

4182 silver badges8 bronze badges

2 Comments

Sebastian Simon Over a year ago

A String is not an Error. Other than that, it isn’t necessarily the case that each of these async functions has a unique, easily identifiable error message.

Lee Over a year ago

this is demo code, you should throw custom error

ibrahim tanyalcin · Accepted Answer · 2022-02-22 07:47:01Z

1

The catch phrase of Promise API can be combined with switch/if statements. Something like below (did not test it for syntax err etc.):

async function doSomethingWithFriends() {    
  let errID = 0;
  return getUser()
   .then(user => (++errID,getFriends(user)))
   .then(friends => (++errID, getFriendsOfFriends(friends)))
   .then(fof => (++errID, doFriendsOfFriends(fof)))
   .catch(err => {switch (errID) {
     case 0:
       return [401, err];
     case 1:
       return [500, err];
     ...
   }});
}

edited Feb 22, 2022 at 7:47

answered Feb 22, 2022 at 7:40

ibrahim tanyalcin

6,5634 gold badges21 silver badges27 bronze badges

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