Promise in loop, code after loop is not executed

Question

I'm using a loop to query events, but the code after the loop is not executed. Why is this the case and how do I solve this problem?

router.post("/components/Pages/BrokerAPage/queryAll", (req, res) => {
  var requestedResult;

  for (i = 0; i < EventIDList.length; i++) {
      queryEvent(EventIDList[i])
          .then((result) => {
              if (result) {
                  requestedResult += result;
              }
          })
          .catch(err =>
              res.send(JSON.stringify({ status: 'error', message: err.message})));
    }
    console.log("*************TEST************")
    console.log("*************"+requestedResult+"*************")
    res.json(JSON.stringify(requestedResult));
});

This is an async issue. You have to wait until all calls to queryEvent are finished. Otherwise you might get to your last three lines of code before requestedResult holds the expected output. — Luis Serrano
– Luis Serrano, Commented Mar 6, 2018 at 11:14
Your for loop is not synchronous. After promise is return, print requestedResult. — Shubham
– Shubham, Commented Mar 6, 2018 at 11:14
Its empty cause promises are resolve async. When requestedResult is outputted the queryEvent promises are not done. You should store queryEvent in array an use Promise.all() — enno.void
– enno.void, Commented Mar 6, 2018 at 11:17

ponury-kostek · Accepted Answer · 2018-03-06 13:57:59Z

1

router.post("/components/Pages/BrokerAPage/queryAll", (req, res) => {
    var requestedResult;
    var promises = [];
    EventIDList.forEach((EventID) => {
        // gather all Promises
        promises.push(queryEvent(EventID)
            .then((result) => {
                if (result) {
                    requestedResult += result;
                }
            }));
    });
    // wait till all promises are settled, then log result 
    Promise.all(promises).then(() => {
        console.log("*************TEST************");
        console.log("*************" + requestedResult + "*************");
        res.json(JSON.stringify(requestedResult));
    }).catch(err => res.send(JSON.stringify({
        status: "error",
        message: err.message
    })));
});

answered Mar 6, 2018 at 13:57

ponury-kostek

8,0835 gold badges27 silver badges33 bronze badges

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

Comments

Vikum Dheemantha · Accepted Answer · 2018-03-06 12:03:48Z

0

You can use async npm module to make the loop synchronous.

router.post("/components/Pages/BrokerAPage/queryAll", (req, res) => {
  var requestedResult;
async.eachOf(EventIDList, function(EventId, index, acb) {
queryEvent(EventId)
          .then((result) => {
              if (result) {
                  requestedResult += result;
                  return acb(null);
              }
          })
          .catch(err =>
              res.send(JSON.stringify({ status: 'error', message: err.message})));
           return acb(null);
    }
});

answered Mar 6, 2018 at 12:03

Vikum Dheemantha

8559 silver badges27 bronze badges

Collectives™ on Stack Overflow

Promise in loop, code after loop is not executed

2 Answers 2

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related