UnhandledPromiseRejectionWarning: Callback was already called (Loopback remote method)

user_5721702 0
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I'm getting an error that says 

(node:27301) UnhandledPromiseRejectionWarning: Unhandled promise rejection (rejection id: 1): Error: Callback was already called.

From what I understand about rejecting promises in await's and per the Mozilla description:

If the Promise is rejected, the await expression throws the rejected value.

I reject the error in the callback that's wrapped around my Promise like so:

Airport.nearbyAirports = async (location, cb) => {
  let airports
  try {
    airports = await new Promise((resolve, reject) => {
      Airport.find({
        // code
      }, (err, results) => {
        if (err)
          reject(err) // Reject here
        else
          resolve(results)
      })
    })
  } catch (err) { // Catch here
    cb(err, null)
    return
  }
  if (!airports.empty)
    cb(null, airports)
  }

My question is

  1. Why does it still consider my promise rejection unhandled? I thought the catch statement should silent this error.
  2. Why does it consider my callback already called? I have a return statement in my catch, so both should never be called.
es6-promise loopbackjs async-await node.js
4 Answers

The problem was actually my framework (LoopbackJS), not my function. Apparently at the time of writing this, using promises are not supported:

https://loopback.io/doc/en/lb3/Using-promises.html#setup

Meaning I can't even use await in my function because the remote method wraps my function somewhere else, so async would always be unhandled. I ended up going back to a Promise-based implementation of the inner code:

Airport.nearbyAirports = (location, cb) => {
const settings = Airport.dataSource.settings
const db = DB(settings)
let airports
NAME_OF_QUERY().then((res) => {
  cb(null, res)
}).catch((err) => {
  cb(err, null)
})
user_57217020

If Airport.find() throws an exception, then execution will jump to your catch block and your Promise will never be resolved or rejected. Perhaps you need to wrap it in its own try/catch:

Airport.nearbyAirports = async (location, cb) => {
  let airports
  try {
    airports = await new Promise((resolve, reject) => {
      try {
        Airport.find({
          // code
        }, (err, results) => {
          if (err)
            reject(err) // Reject here
          else
            resolve(results)
        })
      } catch (err) {
        reject(err) // Reject here too
        cb(err, null)
      }
    })
  } catch (err) { // Catch here
    cb(err, null)
    return
  }
  if (!airports.empty)
    cb(null, airports)
  }
user_15196890

We use Loopback 2.31.0 and it also supports simple return for async functions used for remote methods. If you put a break-point somewhere in your remote method and jump one level above it in the call-stack you will see how it is implemented in loopback itself (shared-method.js):

  // invoke
  try {
    var retval = method.apply(scope, formattedArgs);
    if (retval && typeof retval.then === 'function') {
      return retval.then(
        function(args) {
          if (returns.length === 1) args = [args];
          var result = SharedMethod.toResult(returns, args);
          debug('- %s - promise result %j', sharedMethod.name, result);
          cb(null, result);
        },
        cb // error handler
      );
    }
    return retval;
  } catch (err) {
    debug('error caught during the invocation of %s', this.name);
    return cb(err);
  }
};

What it does here - it calls your function and if it is an async function - it will return a promise (retval.then === 'function' will be true). In this case loopback will handle your result correctly, as a promise. It also do the error check for you, so you no longer try/catch blocks in your code anymore.

So, in your own code you just need to use it like below:

Airport.nearbyAirports = async (location) => {
    let airports = await new Promise((resolve, reject) => {
        Airport.find({
            // code
        }, (err, results) => {
            if (err)
                reject(err) // Reject here
            else
                resolve(results)
        })
    });

    if (!airports.empty)
        return airports;
    }
    else  {
        return {}; // not sure what you would like to return here as it wan not handled in your sample...
    }
}

Note, you do not need to use callback (cb) at all here.

user_26197280
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