Return in a map only when promises resolve

Question

I am using loadash map function to do some structuring of data. The data returns a set of ids for which i need to query the db and add the result to the map and then return but the data is returned later and my function returns first. How to make the return of map wait and then return in function.

let result = _.chain(value)
        .groupBy('pubId')
        .pairs()
        .map(function(currentItem) {
          let item =  _.object(_.zip(['ID', 'targetting'], currentItem));
          propsToBeDeleted.forEach(function(prop) {
            item.targetting.forEach(d => {
              item.publishername = d.RTB.name;
              item.type = d.type;
              delete d[prop];
            });
          });
          item.targetting.map(d => {
            if (d.type == 15) {
              d.name = Object.keys(JSON.parse(d.value));
              d.value = Object.values(JSON.parse(d.value))[0];
              item.dspIds = [];
            }
            if (d.type == 16) {
              let value = JSON.parse(d.value);
              d.name = Object.keys(JSON.parse(d.value)).filter(d => d != 'profileId');
              d.value = value;
              item.dspIds = d.value.profileId;
              dspAccount.find({where: {id: 139}}, (err, data) => {
                // async call wait here to get data and attach to item 
                 item.x = data ;
              });
              delete d.value.profileId;
              d.value = JSON.stringify(d.value);
            }
          });
          return item;
        })
        .value();

I also tried using promises

promises.push(new Promise((resolve, reject) => {
        if (data.id) {
          XX.upsertWithWhere({id: data.id}, payload, (err, data) => {
            if (err) {
              return reject(err);
            }
            return resolve(data);
          });
        }
      }));
    });
    Promise.all(promises).then((data) => {
      callback(null, data);
    }, err => {
      callback(err);
    });

Update i have already listed the promise .all method that doesnot work . So it cannot be called duplicate

Possible duplicate of [Best way to call an async function within map?](https://stackoverflow.com/questions/33438158/best-way-to-call-an-async-function-within-map) — Vipin Kumar, Dec 04 '17 at 17:07
@VipinKumar i have already listed the promise .all method that doesnot work . So it cannot be called duplicate — INFOSYS, Dec 04 '17 at 17:09
I think, I have said it is a possible duplicate not duplicate. Checking duplicate is first thing to do. And please don't tag anyone in your questions. — Vipin Kumar, Dec 04 '17 at 17:11
Let us [continue this discussion in chat](http://chat.stackoverflow.com/rooms/160447/discussion-between-vipin-kumar-and-infosys). — Vipin Kumar, Dec 04 '17 at 17:18

score 0 · Answer 1 · answered Dec 05 '17 at 01:31

Things are slightly tricky because you have an outer .map() and an inner .map(), containing an asynchronous element.

The asynchronous element effectively makes the whole thing asynchronous, requiring that :

the array of Promises generated by the inner .map() is aggregated with Promise.all(), and the aggregate Promise returned to the outer .map().
the array of Promises generated by the outer .map() is aggregated with Promise.all().
the results (an array of items) is available only via a chained .then().

First, to make the main code block simpler and easier to read, create a promisified version of dspAccount.findAsync().

dspAccount.findAsync = function(query) {
    return new Promise(function(resolve, reject) {
        dspAccount.find(query, (err, data) => {
            if(err) {
                reject(err);
            } else {
                resolve(data);
            }
        });
    });
}

Then the main code block can be written as follows :

let outerPromises = _.chain(value)
.groupBy('pubId')
.pairs()
.map(function(currentItem) {
    let item = _.object(_.zip(['ID', 'targetting'], currentItem));
    item.targetting.forEach(d => {
        item.publishername = d.RTB.name;
        item.type = d.type;
        propsToBeDeleted.forEach(function(prop) {
            delete d[prop];
        });
    });
    let innerPromises = item.targetting.map(d => {
        let value = JSON.parse(d.value);
        if (d.type == 15) { // synchronous - `innerPromises` receives a null
            item.dspIds = []; // 
            d.name = Object.keys(value);
            d.value = Object.values(value)[0];
            return null;
        }
        if (d.type == 16) { // asynchronous - `innerPromises` receives a Promise
            return dspAccount.findAsync({where: {id: 139}})
            .then(data => {
                d.name = Object.keys(value).filter(d => d != 'profileId');
                item.dspIds = value.profileId;
                delete value.profileId;
                item.x = data;
                d.value = JSON.stringify(value);
            });
        }
    });
    return Promise.all(innerPromises).then(() => item); // aggregate inner promises and return
})
.value(); //  returns the outerPromises array

Promise.all(outerPromises)  // aggregate outer promises
.then(items => {
    // do stuff with items here
})
.catch(error => {
    // something went wrong
    // log/display error 
    // take remedial action as required
});

I expect there's still some work to do. It doesn't look right that item should be mutated in the item.targetting.map() loop. The last element of item.targetting to deliver its result (synch or asynch) will determine the composition of item. That may not be what you want. If it is what you want, then more control will need to be exercised to ensure that the last result delivered is the result arising from the final element of item.targetting.

Return in a map only when promises resolve

1 Answers1