Question

I’m using the async library in a class and using a fat arrow in one of the series steps causes two callbacks to be fired, where the function with the fat arrow calls the end step directly rather than the next step in the series. Why is this? Here is a simplified example.

class FakeProfileRepository

    getByEmail : (email, callback) ->
        return callback null, email

    update : (data, callback) ->
        async.series
            checkNull: (next) ->
                if data and data.uname
                    next null
                else
                    next Error("No profile to save")

            checkEmailExists: (next) =>
                @getByEmail 'test', (err, results) ->
                    if not results
                        next new Error("Could not find an existing profile to update")
                    else
                        next err

            checkProfile: (next) ->
                return next new Error('foo')


        , (err, results) ->
            console.log('series ended with error:' + err)

this causes an extra callback to fire, with checkEmailExists firing it’s callback to the final result function, as well as the checkProfile step (correctly) firing the last result function

EXPECTED:
series ended with error:foo

ACTUAL: (two callbacks fired)
series ended with error:foo
series ended with error:null

This error seems to happen if I use the fat arrow, or even if I set self= this and use the normal arrow with

        checkEmailExists: (done) ->
            self.getByEmail data.uname, (err, results) ->

Why does this error occur, and is there a better way to reference class methods and not mess up the control flow of async?

Answer 1

A couple of things to check;

Your definition of the “update” method does not use the fat arrow, so you need to make absolutely sure that the method always have the correct “this” when called. Usually, you will recognize this as a someinstance.update(…) call.

Are you absolutely sure that your code does not trigger two calls to the update method? You could add a console.log at the beginning of the method just to make sure.

Your error handling also look a bit funky, and you may want to slim the example down a bit to make absolutely sure it is behaving as you would like.