Asynchronous node js "For" loop with database query

Here is the simple way to do but not recommended. There is a chance it will execute res.send(response); multiple times. I suggest you learn how to use promise.

qry = "SELECT patients.*, patient_visits.visit_id,patient_visits.patient_id, patient_visits.visitdate, patient_visits.visittime FROM patients LEFT JOIN patient_visits ON patients.id = patient_visits.patient_id "+where+" GROUP BY patients.id ORDER BY patients.id  DESC LIMIT "+limit+" OFFSET "+offset;
db.all(qry, (err, results) => {
    var loopCount = 0;
    if(err){
        response.error = err;
        res.send(response);
    }else{
        response.patients = patients;
        for (var i = 0; i < patients.length; i++) {
            response.patients[i].check = "false";
            var patient = response.patients[i];
            db.each("SELECT visit_id FROM patient_visits where patient_id='"+patient.id+"' AND visitdate >='"+moment().format('YYYY-MM-DD')+"'", function(err, row) {
                if (row) {
                    response.patients[i].check = "true";
                    loopCount++;
                    if(loopCount == patients.length){
                        res.send(response);
                    }
                }
            });
        }
    }

});

Use foreach and something called promises. Since Nodejs is async, to make it wait for query results, you have to use promises.

Use Promise.all to handle multiple asynchronous requests.Write a new function to get the visit_id from the database. Something like this,

function getPatientVisitsByVisitId(visitId){
 // return a new promise.
 }
 let promises= [];
 for(var i = 0; i < patients.length; i++){
  patientVisits.push(getPatientVisitsByVisitId(response.patients[i]));
 }
 Promise.all(promises).then((results) => {
 // results will have visit id's.
 })
 .catch((error) => {
  // handle error here
  })

Is there a way to check if the query has finished?

You can use Promise.all() to know whether all the async calls were completed or not.

'use strict';

function fetchPatients(where, limit, offset) {
  let qry = "SELECT patients.*, patient_visits.visit_id,patient_visits.patient_id, patient_visits.visitdate, patient_visits.visittime FROM patients LEFT JOIN patient_visits ON patients.id = patient_visits.patient_id " + where + " GROUP BY patients.id ORDER BY patients.id  DESC LIMIT " + limit + " OFFSET " + offset;
  return new Promise((resolve, reject) => {
    db.all(qry, (err, patients) => {
      if (err) {
        return reject(err);
      }
      resolve(patients);
    });
  });
}

function queryVisitsByPatientId(patient) {
  return new Promise((resolve, reject) => {
    patient.check = "false";
    db.each("SELECT visit_id FROM patient_visits where patient_id='" + patient.id + "' AND visitdate >='" + moment().format('YYYY-MM-DD') + "'", function (err, row) {
      if (err) {
        return reject(`Failed for ${patient.id}`);
      }
      if (row) {
        patient.check = "true";
      }
      return resolve(patient);
    });
  });
}

fetchPatients(where, limit, offset).then(patients => {

  let allPatients = patients.map(patient => {
    return queryVisitsByPatientId(patient);
  });

  return Promise.all(allPatients);
}).then(patientDetails => {
  return res.send({
    patients: patientDetails
  });
}).catch(err => {
  return res.send({
    error: err
  });
});