Node.js Q Promises Multiple Parameters

Question

Trying to cleanup my callback spaghetti code using the Q promise library in my nodejs express app, but I'm having trouble translating some parts of it. Having trouble passing multiple arguments to functions and dealing with the scope.

Here's a simplified "synchronous" version to show my logic:

function updateFacebook(req, res) {
    var user = getUserFromDB(userid);
    var profile = getUserProfileFromAPI(accessToken);

    var success = updateUserDB(user, profile);
    res.json({ result: success });
}

So I convert the callback functions to return promises

function getUserFromDB(userid) {
    var deferred = Q.defer();
    // somewhere in here there's a deferred.resolve(user object);
    queryMongo()...
    return deferred.promise;
}

function getUserProfileFromAPI(accessToken) {
    var deferred = Q.defer();
    // somewhere in here there's a deferred.resolve(profile object);
    request()...
    return deferred.promise;
}

function updateUserDB(user, profile) {
    var deferred = Q.defer();
    // somewhere in here there's a deferred.resolve(updated user object);
    updateMongo()...
    return deferred.promise;
}

function handleResponse(res, user) {
    var deferred = Q.defer();

    // was thinking about putting the res.json here
    // i have no way of passing in the res
    // and res is out of scope...
    res.json({});

    return deferred.promise;
}

Now the problem is linking them up, I tried...

Q.when(getUserFromDB(userid), getUserProfileFromAPI(accessToken))
 .spread(updateUserDB)
 .done(handleResponse);

Q.all([getUserFromDB(userid), getUserProfileFromAPI(accessToken)])
 .spread(updateUserDB)
 .done(handleResponse);

Super confused. Any direction would be much appreciated.

@YarGnawh this actually looks like correct usage except for res. Are you asking about how to pass res? Also +1 for clear question and synchronous exmample. — Benjamin Gruenbaum
– Benjamin Gruenbaum, Commented Aug 25, 2014 at 19:03
@BenjaminGruenbaum yes. since deferred.resolve() can only pass one argument, spread() is the only option. i don't see how i can fulfill handleResponse(res, user) — YarGnawh
– YarGnawh, Commented Aug 25, 2014 at 19:09
slightly off-topic question... would it make sense to bind the res object to the promise at the beginning? Q.bind({ res: res }).when(....) — YarGnawh
– YarGnawh, Commented Aug 25, 2014 at 19:16

NG. · Accepted Answer · 2014-08-25 19:03:48Z

3

Looks like your handleResponse is expecting two params, but updateUserDB is only resolving a single object. You could do something like:

function getResponseHandler(res) {
    return function(user) {
        // your handleResponse code here
        // which now has access to res
    }
}

and then call it like:

Q.all([getUserFromDB(userid), getUserProfileFromAPI(accessToken)])
 .spread(updateUserDB)
 .done(getResponseHandler(res));

answered Aug 25, 2014 at 19:03

NG.

23k6 gold badges57 silver badges62 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

Benjamin Gruenbaum Over a year ago

It might be simpler to use a closure scope here.

YarGnawh Over a year ago

@NG thanks for the example. how does the user object get passed to the getResponseHandler(res) ?

NG. Over a year ago

@YarGnawh the getResponseHandler(res) returns a function that receives the user object from updateUserDB. That function also has access to the res object.

YarGnawh Over a year ago

would returning that anonymous function "reset" the stack?

Collectives™ on Stack Overflow

Node.js Q Promises Multiple Parameters

1 Answer 1

4 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

4 Comments

Your Answer

Sign up or log in

Post as a guest

Related