Unable to resolve promise rejection and send array as response

Question

I am trying to handle two queries within each other.

exports.get_users = (req, res) => {

  SubscriptionPlan.find()
    .then((result) => {
      if (!result) {
        return res.status(400).json({ message: "unable to process" });
      }
      let modifiedData = [];
      result.forEach(async (data) => {
        if (data.processStatus === "active") {
          await Users.findById(data.userId).then(
            (response) => {
              console.log(response)
              modifiedData.push(response);
              res.json(modifiedData)
            }
          );
        }
      });
    })
    .catch((err) => console.log(err));
};

My issue goes, if I follow this approach, then I get the error for promise rejection:

node:15036) UnhandledPromiseRejectionWarning: Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client
    at ServerResponse.setHeader (_http_outgoing.js:470:11)

and also, the response of modifiedData array I receive in postman, only a single object out of array of length 3. The statement console.log(response), returns 3 objects that I need to push to new array.

If I change my code like this to resolve the rejection,

exports.get_users = (req, res) => {

  SubscriptionPlan.find()
    .then((result) => {
      if (!result) {
        return res.status(400).json({ message: "unable to process" });
      }
      let modifiedData = [];
      result.forEach(async (data) => {
        if (data.processStatus === "active") {
          await Users.findById(data.userId).then(
            (response) => {
              console.log(response)
              modifiedData.push(response);
            }
          );
        }
      });
      res.json(modifiedData)
    })
    .catch((err) => console.log(err));
};

it returns me an emtpy array as reponse in the postman. I am aware that I am stuck between async nature of JS, but unable to resolve this issue.

PS. Using async-await is not helpful here for me.

Any early help will be appreciated.

Like here: https://stackoverflow.com/questions/32264225/how-to-get-multiple-document-using-array-of-mongodb-id — Anurag Srivastava, Apr 28 '20 at 09:14
@AnuragSrivastava but using simple findById()is also returning me multiple documents. Console prints them all as per the condition I am passing. But it is not sending it in array of objects — program_bumble_bee, Apr 28 '20 at 09:16

Ashh · Accepted Answer · 2020-04-28T09:29:40.977

2

result.forEach returns array of promises. You need to promisify all at once using Promise.all([])

exports.get_users = (req, res) => {
  SubscriptionPlan.find().then(async (result) => {
    if (!result) {
      return res.status(400).json({ message: "unable to process" });
    }
    let modifiedData = [];
    await Promise.all(
      result.map(async(data) => {
        if (data.processStatus === "active") {
          const response = await Users.findById(data.userId);
          modifiedData.push(response);
        }
      })
    );
    return res.json(modifiedData);
  }).catch((err) => console.log(err));
};

Or can find at once

exports.get_users = async (req, res) => {
  try {
    const result = await SubscriptionPlan.find({ processStatus: "active" });
    if (!result) {
      return res.status(400).json({ message: "unable to process" });
    }
    const ids = result.map(({ userId }) => userId);
    const response = await Users.find({ userId: { $in: ids } });
    return res.json(response);
  } catch (err) {
    console.log(err)
    return res.status(400).json({ message: "unable to process" });
  }
};

edited Apr 28 '20 at 09:29

answered Apr 28 '20 at 09:19

Ashh

44,693
14
105
132

I tried both of your approaches. The first one is not sending any reponse back. While the second one, returns the empty array – program_bumble_bee Apr 28 '20 at 09:29
@bubble-cord Try it again. And please be sure and catch the errors. – Ashh Apr 28 '20 at 09:33
The first one worked. Can you help me understand how changing from forEach to map marked the difference? – program_bumble_bee Apr 28 '20 at 09:36
1

@bubble-cord https://stackoverflow.com/questions/37576685/using-async-await-with-a-foreach-loop – Ashh Apr 28 '20 at 09:39

Unable to resolve promise rejection and send array as response

1 Answers1

Linked

Related