node promise for loop wait for result

Question

How can I wait for the for loop to go to next loop?

How can I make sure that the userdetails is executed and realresult.playername is assigned to the new variable before passing it on ?

code:

return new Promise( function (resolve, reject) {

    arr = [];


    messages.find({convid: convid}).then(function(result) {

        for(var a in result) {
            realresult = result[a];

            userData.userDetails(realresult.userid).then(function (result) {
                realresult.playername = result.username;
            });
            userData.usercashinfo(realresult.userid).then(function (resulttwo) {
                realresult.playerdetails = resulttwo;
            });

            arr.push(realresult);

        }

        return resolve(arr);
    });
});

use `Promise.all` https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise/all — Daniel Lizik, Sep 12 '16 at 12:38
@Daniel_L i dont know how i would remake my function to make it work with that — maria, Sep 12 '16 at 12:39
Possible duplicate of [Javascript for loop Promises](http://stackoverflow.com/questions/32701957/javascript-for-loop-promises) — roberrrt-s, Sep 12 '16 at 12:46

yoz · Answer 1 · 2016-09-12T18:11:07.440

You're trying to write a promise that, when it resolves, returns the array of resolved promise values. There are 2 things you should do here:

Use return to await the completion of a promise. In a promise chain, nothing actually resolves the promise value unless it is returned. (You should also use this for message.find() since it's also giving you a promise.)
Construct the array as an array of promises, rather than iterating and having the promises push into the array when they resolve.
Use Promise.all() to collect the promise objects - all() returns only after all of the promises passed to it have resolved.
(optional) Don't reuse the result variable name to mean multiple things. It's not wrong here, but it can cause unnecessary confusion.

Put together, this would look something like:

return messages.find(...).then(function(result) {
  var promiseArr = [];
  for (var a in result) {
    promiseArr.push(
      promise.all([userData.userDetails(...), userData.usercashinfo(...)])
      .then(function(detailinfo) {
        var realresult = result[a];
        var details = detailinfo[0];
        var cashinfo = detailinfo[1];
        realresult.playername = details.username;
        realresult.playerdetails = cashinfo;
        return realresult;
    });
  }
  return Promise.all(promiseArr);
});

hey @yoz , i updated my post, could you make some edits to make that work? — maria, Sep 12 '16 at 14:55

naortor · Answer 2 · 2016-09-12T17:34:42.470

0

so I thought it is a good idea trying solving it with the combination of generators and promises, I'm not sure that its the best way to go, but Its a good practice for the combination of the two so I gave it a try.

The idea is that the generator function is waiting for the callback function before starting a new iteration and the callback function is waiting for both promises to complete before telling the generator function to go on iterating.

here is a more simple example that I first tried on codepen

return new Promise( function (resolve, reject) {
    arr = [];
    messages.find({convid: convid}).then(function(result) {
        function* iterations() {
            for (var i = 0; i < result.length; i++) {
                realresult = result[i];
                try {
                    var cbResult = yield cb(result[i].userid);
                    realresult.playername = cbResult.playername;
                    realresult.playerdetails = cbResult.playerdetails;
                    arr.push(realresult);
                } catch (e) { reject(e) }
            }
            return arr.value;
        }

        var it = iterations();
        it.next();

        function cb(userid) {
            Promise.all([
                       userData.userDetails(userid),
                       userData.usercashinfo(userid)
                       ]).then(
                           function(values) {
                               var realresult = {playername : values[0], playerdetails : values[1]}
                               returnValue = it.next(realresult);
                               if (returnValue.done) {
                                   resolve(returnValue);
                               }
                           },
                           function(e) { reject(e) }
                       );
        }
    });
});

edited Sep 12 '16 at 17:34

answered Sep 12 '16 at 16:04

naortor

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Avoid the [`Promise` constructor antipattern](http://stackoverflow.com/q/23803743/1048572)! – Bergi Sep 12 '16 at 16:07
This code is quite convoluted and doesn't handle any errors. Instead of using generators you should just go for `async`/`await`. – Bergi Sep 12 '16 at 16:09
http://prntscr.com/ch6i49 i did what you did @naortor, but then it wont return the array in plain response? as [0] and [1] ? – maria Sep 12 '16 at 16:18
@Bergi , what would the best in this case be? – maria Sep 12 '16 at 16:22
@maria I edited the answer so it will return the array as you want. but its more of a POC - as Bergi said it does't handle errors yet - you should wrap the yield call in a try / catch block and reject out errors from the promise – naortor Sep 12 '16 at 16:23
could we take a chat @naortor ? – maria Sep 12 '16 at 16:26
This task can be easily handle by async module – abdulbarik Sep 12 '16 at 16:30
sure, send me an invite cuz i'm not sure how to do it. – naortor Sep 12 '16 at 16:32
@abdulbarik The `async` module is of no help for promise-returning methods – Bergi Sep 12 '16 at 16:32

node promise for loop wait for result

2 Answers2