wait for api response before moving next - nodejs - node.js

Question

In my nodejs project, there's a function where I have two queries to get data from. First query is dependent on a flag, if flag is true, then I've to run that query and pass its data to second query. Otherwise I've to set some hard-coded data for second query. But it does not wait for first query response and run second query. I've added callbacks to it but nothing worked for me. Here is my code snippet;
getData(req, callback) {
if(!req.flag){
lstData.push('d');// hardcoded data
return Reader.open(filename).then(reader => {
let code = reader.data;
if(!!code ){
return dao.getFirstData(code , (err, results) => {
if (err) return callback(err);
if(results && results.length > 0){
return lstData = results;
}
});
}
});
}
else{
lstData.push('d');// hardcoded data
}
let asyncFuncs = [
(callback) => {
dao.getSecondData(lstData, (err, results) => {
if (err) return callback(err);
return callback(null, { amounts: results });
});
}
];
asyncFuncs.push(callback => {
dao.thirdFunction(id, callback);
});
async.parallel(asyncFuncs, (err, results) => {
if (err) return callback(err);
let data1= results[0].amount;
let data2= results[1];
// some calculations with data1 & data2
return callback(err, finalResult);
});
}
No matter flag is true or false, getSecondData always returns data against d (hard-coded value). I am a newbie to nodejs, so please let me know what am I doing wrong.
SO I updated my code to add promise in it...here is updated code;
getData(req, callback) {
if(!req.flag){
lstData.push('d');// hardcoded data
Reader.open(filename).then(reader => {
let code = reader.data;
if(!!code ){
var prom = new Promise((resolve, reject) => {
dao.getFirstData(code , (err, results) => {
if (err) return callback(err);
if(results && results.length > 0){
let lstData = results;
return resolve(lstData);
}
});
});
prom.then((result) => {
return result;
});
}
});
}
else{
lstData.push('d');// hardcoded data
}
let asyncFuncs = [
(callback) => {
dao.getSecondData(lstData, (err, results) => {
if (err) return callback(err);
return callback(null, { amounts: results });
});
}
];
asyncFuncs.push(callback => {
dao.thirdFunction(id, callback);
});
async.parallel(asyncFuncs, (err, results) => {
if (err) return callback(err);
let data1= results[0].amount;
let data2= results[1];
// some calculations with data1 & data2
return callback(err, finalResult);
});
}
But still same response. It is not waiting for promise result.

Answer 1

There plenty of ways to achieve this but I would prefer http://async.io/ to perform multi function operations.
By using promise you can do,
function f1(argument){
return new Promise((resolve, reject) => {
try {
data = // do your thing
resolve(data)
}
catch(err){
reject(err)
}
})
}
and then use await to achieve this
async function caller(){
await f1()
}
Few other links to help you [here][2]
[2]: https://stackoverflow.com/questions/22442321/callback-function-example/48333938#:~:text=A%20callback%20function%2C%20is%20a,or%20executed)%20inside%20the%20otherFunction.&text=Without%20thinking%20too%20much%2C%20see%20the%20following%20example.&text=Generally%2C%20JavaScript%20allows%20function%20as%20a%20parameter.

Answer 2

You can use async/await for asynchronous calls or you can uses promises. This will wait at the asynchronous calls and once you get the response your code will execute the dependent subsequent calls.
getData(req, callback) {
if(!req.flag){
lstData.push('d');// hardcoded data --> this might be an issue as you are already pushing data.
// these are async calls where you can ask your code to wait
return Reader.open(filename).then(reader => {
let code = reader.data;
if(!!code ){
var promise = dao.getFirstData(code , (err, results) => {
if (err) reject(err);
if(results && results.length > 0){
lstData = results;
resolve(lstData);
}
});
return promise --> you can subscribe this using then
}
});
}
else{
lstData.push('d');// hardcoded data
}
let asyncFuncs = [
(callback) => {
dao.getSecondData(lstData, (err, results) => {
if (err) return callback(err);
return callback(null, { amounts: results });
});
}
];
}
To get hands on this you can check the ways of using async/await
https://javascript.info/async-await