Array push gives empty result node js - node.js

Question

I am creating an API for listing trip data with image and pdf base url,
All things are working fine but I can not access the last result array data_to_send out of for loop.
app.js
app.get("/getChallanList/:userId/:role", (req, res) => {
const userData = req.params;
let site_source = "";
let site_destination = "";
var site_from_name = "";
const data_to_send = [];
if (userData.role == "D") {
db.select("trip", "*", `driver_id = '${req.params.userId}'`, (data) => {
for (let i = 0; i < data.data.length; i++) {
site_source = data.data[i].site_from;
site_destination = data.data[i].site_to;
db.select(
"site",
"*",
`id in ('${site_source}','${site_destination}')`,
(data_site) => {
data.data[i].site_from = data_site.data[0].name;
data.data[i].site_to = data_site.data[1].name;
if (data.data[i].truck_challan_pdf != "") {
data.data[i].truck_challan_pdf =
base_url + "truckchallan/" + data.data[i].truck_challan_pdf;
}
if (data.data[i].driver_challan_pdf != "") {
data.data[i].driver_challan_pdf =
base_url + "driverchallan/" + data.data[i].driver_challan_pdf;
}
if (data.data[i].preparer_img != "") {
data.data[i].preparer_img = base_url + data.data[i].preparer_img;
}
if (data.data[i].driver_img != "") {
data.data[i].driver_img = base_url + data.data[i].driver_img;
}
data_to_send.push(data.data);
// console.log(data_to_send); // working
}
);
}
console.log(data_to_send); // empty
});
}
}
db.select
let select = (table, column, condition, callback) => {
try {
let sql = "SELECT " + column + " FROM " + table + " WHERE " + condition;
conn.query(sql, (err, results) => {
if (err) {
let data = {
status: 0,
data: sql,
message: "Something went wrong!",
};
callback(data);
} else {
let data = {
status: 1,
data: results,
message: "Success",
};
callback(data);
}
});
} catch (err) {
let data = {
status: 0,
data: err,
message: "In catch",
};
callback(data);
}
};
async await
app.get("/getChallanList/:userId/:role", async (req, res) => {
const userData = req.params;
let site_source = "";
let site_destination = "";
var site_from_name = "";
const data_to_send = [];
if (userData.role == "D") {
await db.select(
"trip",
"*",
`driver_id = '${req.params.userId}'`,
async (data) => {
// const data_to_send_ = [];
for (let i = 0; i < data.data.length; i++) {
site_source = data.data[i].site_from;
site_destination = data.data[i].site_to;
await db.select(
"site",
"*",
`id in ('${site_source}','${site_destination}')`,
(data_site) => {
data.data[i].site_from = data_site.data[0].name;
data.data[i].site_to = data_site.data[1].name;
if (data.data[i].truck_challan_pdf != "") {
data.data[i].truck_challan_pdf =
base_url + "truckchallan/" + data.data[i].truck_challan_pdf;
}
if (data.data[i].driver_challan_pdf != "") {
data.data[i].driver_challan_pdf =
base_url + "driverchallan/" + data.data[i].driver_challan_pdf;
}
if (data.data[i].preparer_img != "") {
data.data[i].preparer_img =
base_url + data.data[i].preparer_img;
}
if (data.data[i].driver_img != "") {
data.data[i].driver_img = base_url + data.data[i].driver_img;
}
data_to_send.push(data.data);
// console.log(data_to_send); // working
}
);
// data_to_send_.push(data_to_send);
}
console.log(data_to_send); // empty
}
);
}
}

Answer 1

this is because of the asynchronous behavior of NodeJs, so you have to plan things accordingly i.e
console.log(1)
db.select(
"trip",
"*",
`driver_id = '${req.params.userId}'`,
async (data) => {
console.log(2)
})
console.log(3)
The output of the above code would be 1 then 3 and then 2 and this is how NodeJs works it does not wait for I/O calls i.e DB query in your case.
Please check how promises work in NodeJs for more details.
Here is how you can accomplish your task:
const challanList = (userData) => {
return new Promise((resolve, reject) => {
const data_to_send = [];
db.select("trip", "*", `driver_id = '${req.params.userId}'`, data => {
for (let i = 0; i < data.data.length; i++) {
const site_source = data.data[i].site_from;
const site_destination = data.data[i].site_to;
db.select("site", "*", `id in ('${site_source}','${site_destination}')`, data_site => {
data.data[i].site_from = data_site.data[0].name;
data.data[i].site_to = data_site.data[1].name;
if (data.data[i].truck_challan_pdf != "") {
data.data[i].truck_challan_pdf = base_url + "truckchallan/" + data.data[i].truck_challan_pdf;
}
if (data.data[i].driver_challan_pdf != "") {
data.data[i].driver_challan_pdf = base_url + "driverchallan/" + data.data[i].driver_challan_pdf;
}
if (data.data[i].preparer_img != "") {
data.data[i].preparer_img = base_url + data.data[i].preparer_img;
}
if (data.data[i].driver_img != "") {
data.data[i].driver_img = base_url + data.data[i].driver_img;
}
data_to_send.push(data.data);
// console.log(data_to_send); // working
});
}
resolve(data_to_send);
});
});
};
app.get("/getChallanList/:userId/:role", async (req, res) => {
const userData = req.params;
const challanListResult =await challanList(userData);
console.log(challanListResult);
resp.json(challanListResult);
});

Answer 2

Without knowing what database or ORM you are using it is difficult to answer, but my suspicion is that db.select is an asynchronous method, i.e. it is returning a Promise. If so, the second console log is still seeing the "old" data_to_send.
Try adding an await in front of the first db.select call. (Don't forget the async in front of the callback in second argument of app.get.

Answer 3

Your database is asynchronous so console.log(data_to_send) gets called before the query finished executing. Try adding async before (req, res) in line 1 then await before db.select.

Answer 4

This works for me
app.get("/getChallanList/:userId/:role", async (req, res) => {
const userData = req.params;
let site_source = "";
let site_destination = "";
var site_from_name = "";
const data_to_send = [];
if (userData.role == "D") {
const data = await db.query(
`SELECT * FROM trip WHERE driver_id = '${req.params.userId}'`
);
// console.log(data.length);
// const data_to_send_ = [];
for (let i = 0; i < data.length; i++) {
site_source = data[i].site_from;
site_destination = data[i].site_to;
// cons
const site_data = await db.query(
`SELECT * FROM site WHERE id in ('${site_source}','${site_destination}')`
);
// console.log(site_data);
db.select(
"site",
"*",
`id in ('${site_source}','${site_destination}')`,
(data_site) => {
data[i].site_from = data_site.data[0].name;
data[i].site_to = data_site.data[1].name;
if (data[i].truck_challan_pdf != "") {
data[i].truck_challan_pdf =
base_url + "truckchallan/" + data[i].truck_challan_pdf;
}
if (data[i].driver_challan_pdf != "") {
data[i].driver_challan_pdf =
base_url + "driverchallan/" + data[i].driver_challan_pdf;
}
if (data[i].preparer_img != "") {
data[i].preparer_img = base_url + data[i].preparer_img;
}
if (data[i].driver_img != "") {
data[i].driver_img = base_url + data[i].driver_img;
}
data_to_send.push(data);
// console.log(data.data);
// console.log(data_to_send); // working
}
);
// data_to_send_.push(data_to_send);
}
// console.log(data_to_send);
// console.log(data_to_send);
res.send({ success: 1, data: data, message: "" });
}