How to send response in mid of multiple async calls to database in NodeJS?

Question

Need to terminate POST function and send Error: something as response, without terminating program.

Sample for-loop:

for (let i = 0; i < req.body.listoftouristsbyid.length; i++) {
  client.query("SELECT tourists.id FROM tourists WHERE tourists.id = " + req.body.listoftouristsbyid[i], function(err, result) {
    done();
    if (result.rows.length === 0) {
      console.log("Tourist " + req.body.listoftouristsbyid[i] + " you want to add does not exist");
      res.status(500);
      return res.render("Bad listoftoutistbyid request, student with id " + i + " does not exist");
    }
  })
}

What should I write in place of return res.render, so POST won't work, function will terminate with error code in response, but without crashing program so I can send more requests later ?

Instead of making a lot of requests to DB I would just make one query by combining them with OR ... — Jonas Wilms
– Jonas Wilms, Commented Sep 27, 2018 at 17:52
That means that any visitor of your website might have fun with your db xkcd.com/327 — Jonas Wilms
– Jonas Wilms, Commented Sep 27, 2018 at 18:00
Well don't do a lot of queries in a loop and try to send a response after each of them returns. There can only be one response. — Bergi
– Bergi, Commented Sep 27, 2018 at 19:41

NAVIN · Accepted Answer · 2018-09-27 19:27:46Z

You can use async-await for this, as it's highly used among asynchronous call handlers like Promise and Generators.

Here's a sample code:

app.post('/APIendpoint', async (req, res) => {

    let listoftouristsbyid      = req.body.listoftouristsbyid;

    let dbResult = [];

    // Can put this whole for loop inside try too. Then success response will also be inside try after for-loop
    for (let i = 0; i < listoftouristsbyid.length; i++) {

        try {
            let result = await callDB( "SELECT tourists.id FROM tourists WHERE tourists.id = " + listoftouristsbyid[i] );
            dbResult.push(result);
        }
        catch (err) {

            console.log("Tourist " + listoftouristsbyid[i] + " you want to add does not exist");

            res.status(500);
            return res.send("Bad listoftoutistbyid request, student with id " + i + " does not exist");

        }
    }

    console.log("Successfully fetched all records ", dbResult);

    res.status(200);
    return res.send(dbResult);

});

function callDB(query) {

    return new Promise ( (resolve, reject) => {
        client.query(query, function(err, result) {

            if (result.rows.length === 0) {
                reject("not found");
            } else {
                resolve(result.rows);
            }

        })
    });
}

Dhirendra · Accepted Answer · 2018-09-27 19:07:16Z

1

Try using async.map to solve your problem. for loop is not gonna work with async functions.

async.map(
    req.body.listoftouristsbyid, 
    function(touristId, callback) {
        client.query("<query>", function(err, result) {
            if (result.rows.length === 0) {
                callback(new Error("not found"));
            }
            callback(null, result);
        });
    },
    function(err, result) {
        // send response here.
    } 
);

answered Sep 27, 2018 at 19:07

Dhirendra

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