JavaScript Promises - Creating an array of promises to be executed together

Question

I'm working with Node.js on async calls to noSQL DynamoDB. I first query to see which 'buddy list(s)' the account belongs. That may return from zero to 'n' new Primary Keys that will contain lists of all of the members of each of those buddy lists. Think of them as clubs to which a person belongs... you may have none or many; each club has several members or even one.

So far (and I am working with Promises for the first time here... though I have used callbacks on prior JA projects) I'm OK with where I am, but I know that I am assembling the array of promises incorrectly. That is, I can see in the console that the .then function executes before both promises resolve. For the record, I do expect that... it seems reasonable that any .then may be satisfied with a single promise resolving.

Anyways, Can someone offer up some tips as to how to structure? I'd like to end up with pseudo:

getBuddyLists .then(getAllTheBuddiesInTheLists) .then(function(){//doSomething});

my Node.js code:

const AWS = require('aws-sdk');
const docClient = new AWS.DynamoDB.DocumentClient({region:'us-east-1'});
var buddyGroups = [];

exports.handler = function(event, context, callback) {
    var params = {
        TableName: 'USER_INFORMATION',
        Key: {
            "DEVICE_ID": event.DEVICE_ID
        }
    };

    var getBuddyList = new Promise(function(resolve, reject){
        docClient.get(params, function(err, data){
            if(err){
                reject(err);
            }
            else{
                var dataJSON = JSON.parse(JSON.stringify(data));
                dataJSON.Item.my_buddies.values.forEach(function(value){
                    buddyGroups.push(value);
                });
                console.log('Got buddy groups: ' + buddyGroups);
                resolve(buddyGroups);
            }
        });
    });

    getBuddyList.then(function(capturedData){  // capturedData => an array of primary keys in another noSQL document
        console.log('groups: ' + capturedData);
        var myPromises = [];
        capturedData.forEach(function(value){
            var reqParams = {
                        TableName: "buddy_list",
                        Key: {"BUDDY_ID": value}
                    };
            myPromises.push(new Promise (function(resolve, reject){
                docClient.get(reqParams, function(err, data){
                    if(err){
                        //console.log(err, data);
                    reject(err);
                    }else{
                        var returnedJSON = JSON.parse(JSON.stringify(data));
                        console.log(returnedJSON);
                        resolve(returnedJSON);
                    }
                });
            }));
        });
        //
        Promise.all(myPromises); // ADDED IN EDIT <<<<<<<<<<<<<<<<<<<<<<
        // how to make sure all of myPromises are resolved?
        //
    }).then(function(){
            console.log("done");
        })
    .catch(function(err){
        console.log("error message:" + error);
    });
};

EDIT: Added location of Promise.all(myPromises);

unrelated, but you should use myPromises = capturedData.map(...) instead of a .forEach / .push loop — Alnitak
– Alnitak, Commented Jan 17, 2017 at 22:53
@Alnitak,coming from C++ it's hard for me to shed the for-loop paradigm! I suppose you offer that up due to the blocking nature of .forEach... or some other reason? — Jim from Princeton
– Jim from Princeton, Commented Jan 17, 2017 at 23:17
.forEach has no "blocking nature" ... the .map pattern is just more efficient ... Promise.all(capturedData.map(function(value) { ... return(new Promise ... - no need for the myPromises var at all — Jaromanda X
– Jaromanda X, Commented Jan 17, 2017 at 23:33
@JimfromPrinceton no, it's simpler than that - if you've got an array as input, and want an equal sized array as output (with a 1:1 mapping between input elements and output elements) then .map is simply the right function for the job. — Alnitak
– Alnitak, Commented Jan 18, 2017 at 7:16
@Alnitak Is it deemed fine to use map/filter if the Promises have side-effects (eg. POST/PATCH/etc)? — Winston Guess
– Winston Guess, Commented Nov 20, 2024 at 9:50

Jaromanda X · Accepted Answer · 2017-01-17 23:37:12Z

12

Just to clarify how you would use Promise.all, even though you accepted an answer, you edited the question, but still are not using it correctly

Also, the discussion of .map vs .forEach - this is how you would use .map

    getBuddyList.then(function(capturedData){  // capturedData => an array of primary keys in another noSQL document
        console.log('groups: ' + capturedData);
        // changes to use .map rather than .forEach
        // also, you need to RETURN a promise - your edited code did not
        return Promise.all(capturedData.map(function(value){
            return new Promise (function(resolve, reject){
                var reqParams = {
                    TableName: "buddy_list",
                    Key: {"BUDDY_ID": value}
                };
                docClient.get(reqParams, function(err, data){
                    if(err){
                        //console.log(err, data);
                    reject(err);
                    }else{
                        var returnedJSON = JSON.parse(JSON.stringify(data));
                        console.log(returnedJSON);
                        resolve(returnedJSON);
                    }
                });
            });
        }));
    }).then(function(){
            console.log("done");
        })
    .catch(function(err){
        console.log("error message:" + error);
    });

answered Jan 17, 2017 at 23:37

Jaromanda X

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1 Comment

Jim from Princeton Over a year ago

Thanks for posting the proper code... I appreciate that (along with the use of .map()

SLaks · Accepted Answer · 2017-01-17 22:51:01Z

2

You're looking for Promise.all(myPromises), which returns a single promise of an array of the results of the promises.

answered Jan 17, 2017 at 22:51

SLaks

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4 Comments

Jim from Princeton Over a year ago

So, must all of the promises contained in the .then-able context required to resolve before .then happens?

AlvMF1 Over a year ago

@JimfromPrinceton exactly! developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/…

Jim from Princeton Over a year ago

@AlvMF1 , I added as per the edit above but my log suggest that my .then happens before all the promises are resolved: START RequestId: Version: $LATEST 2017-01-17T23:23:19 Got buddy groups: 12345,ABCDE 2017-01-17T23:23:19 groups: 12345,ABCDE 2017-01-17T23:23:19 done here 2017-01-17T23:23:19 { Item: { BUDDY_ID: '12345', member: { values: [Object], type: 'String' } } } 2017-01-17T23:23:19 { Item: { BUDDY_ID: 'ABCDE', member: { values: [Object], type: 'String' } } } END

Heretic Monkey Over a year ago

return Promise.all(myPromises);

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