Correctly chaining Promises

Question

I want to bind promises sequentially, inside a loop. I need this to user accounts, where the result of one operation depends on another. I am trying to write a flat version - all code in one place, using bind. That's at least what I wanted. I wrapped promises around two create methods, as below:

function create(myApi, record) { return new Promise(function (resolve, reject) { myApi.create(record, function (err, result) { if (err) reject(err); else resolve(result); }); }); } function createUser(myApi, record) { return new Promise(function (resolve, reject) { myApi.createUser(record, function (err, result) { if (err) reject(err); else resolve(result); }); }); }

Now, I want to create users in a loop as:

for ( var i = 0; i < dummyData.accounts.length; i++) { var cursorUser = dummyData.accounts[i]; var auth0User = { email: cursorUser.email, password: cursorUser.password, connection: 'Username-Password-Authentication' }; createUser(api, auth0User) .then( function(auth0Info) { console.log("Auth0 userInfo: ", auth0Info); cursorUser.authProfile = auth0Info; create(accountsAPIService, cursorUser) .then(function (account) { console.log("created account:", account); }) .catch(function (err) { console.log('count not create account for user, error: ', err, '\nfor: ', auth0User); }); }) .catch(function (err) { console.log('could not create auth0 user, error: ', err, '\nfor: ', auth0User); }); }

Since the two method are asynchronous, it is of course not working correctly. Calls are not executed sequentially. I want to chain promises so that create does not run until a call from createUser returned. Tried using bind, but it did not work for me. It is how one should do the sequential chaining? I bind on .then of the createUser? Please advise.

Hey is ES6 an option?if so moving to generators would make your life so much easier — itai
– itai, Commented Jan 4, 2016 at 14:02
@itai Thank you for noticing my question. ES6 might be an option. Regardless, I would love to know how this could be done with ES6, if nothing else. — Moshe Shmukler
– Moshe Shmukler, Commented Jan 4, 2016 at 14:17
@itai Yes, thank you. I believe that I figured out how to use bind, but it is definitely nice to know how things could work under es6. At work, my colleagues do lots of typescript->es6->es5 with babel. It is too advanced for me at the moment, but I will certainly try to understand your example. — Moshe Shmukler
– Moshe Shmukler, Commented Jan 4, 2016 at 17:35

Joseph · Accepted Answer · 2016-01-04 14:22:45Z

When you return a promise from a then, the then chained after will resolve/reject with that promise instead of the original promise.

createUser(api, auth0User).then(function(auth0Info) { cursorUser.authProfile = auth0Info; // Return create's promise return create(accountsAPIService, cursorUser); }, function (err) { console.log('could not create auth0 user, error: ', err, '\nfor: ', auth0User); }) // This will wait for create's promise instead of createUser's promise .then(function (account) { console.log("created account:", account); }, function (err) { console.log('count not create account for user, error: ', err, '\nfor: ', auth0User); })

Thank you for the answer. I am not positive that you are correct, here. At least my data looks wrong. I might be messing it up myself. From my tests, it does not look like the loop is waiting.
I am pretty sure that these promises are being executed in parallel and not sequentially. Yet, I know nothing about this and still don't understand the subject. I might be wrong.
you were nearly on the spot. First then needs to be replaced for bind.

itai · Accepted Answer · 2016-01-04 15:00:20Z

Using ES6 you can use generators which allows you to do write async task as they were async. In this example i am using bluebird but ofc there are others great valid options.

var CreateUserGenerator = BPromise.coroutine(function * (arg) { for ( var i = 0; i < dummyData.accounts.length; i++) { var cursorUser = dummyData.accounts[i]; var auth0User = { email: cursorUser.email, password: cursorUser.password, connection: 'Username-Password-Authentication' }; var auth0Info = yield createUser(api, auth0User); console.log("Auth0 userInfo: ", auth0Info); cursorUser.authProfile = auth0Info; var account = yield create(accountsAPIService, cursorUser) console.log("created account:", account); } } function generateorWithCatch (argument) { creatLoopUser(arg) .catch(function (err) { console.log('could not create auth0 user, error: ', err, '\nfor: ', auth0User); }); }

In this solution i assume your create and createUser functions are written and return value correctly

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