page renders before getting all the values sorted - node.js

Question

I think the rendering takes place before the searching of the string on the files, i have tried different methods but don't seems to get this working. any help will be appreciated. im a noob on to the nodejs. im trying to get the id of the user and query and get all the data and there after see if he is in any of the lists given and finally render the page.
const j = [];
let name = '';
const filename = [];
var ext = '';
module.exports = function(app, express) {
app.use(bodyParser.urlencoded({ extended: false }));
app.use(bodyParser.json());
app.post('/cusdetails', isLoggedIn, function (req, res) {
var cusid=req.body.cusid;
var insertQuerys = "SELECT * FROM customer WHERE cusid=? ORDER BY rowid DESC LIMIT 1";
connection.query(insertQuerys,[cusid],
function(err, rows){
rows.forEach( (row) => {
name=row.fncus;
});
fs.readdir('./views/iplist', function(err, files) {
if (err)
throw err;
for (var index in files) {
j.push(files[index])
}
j.forEach(function(value) {
var k = require('path').resolve(__dirname, '../views/iplist/',value);
fs.exists(k, function(fileok){
if(fileok) {
fs.readFile(k, function(err, content) {
if (err) throw err;
if (content.indexOf(name) > -1) {
ext = path.extname(k);
filename.push(path.basename(k, ext));
}
});
}
else {
console.log(" FileNotExist ");
}
});
});
});
console.log(filename);
res.render('cusdetails.ejs', {rows: rows, user:req.user , aml: filename });
});
})

Answer 1

You can create simple Promise wrapper and then use it inside async/await function to pause execution until resolved.
// use mysql2 package as it provides promise, less work to write promise wrappers
const mysql = require('mysql2/promise');
// create the connection to database
const connection = mysql.createConnection({
host: 'localhost',
user: 'root',
database: 'test'
});
// sample wrapper
function some(k) {
// more advisable to have local variables, why do you need this to be array?
var filename = [];
return new Promise((resolve, reject) => {
// doing this is also not recommended check nodejs documentation **fs.exists** for more info
fs.exists(k, function(fileok){
if(fileok) {
fs.readFile(k, function(err, content) {
if (err) reject(err);
if (content.indexOf(name) > -1) {
ext = path.extname(k);
filename.push(path.basename(k, ext));
resolve(filename)
}
});
}
else {
// reject(new Error("FileNotExist"))
console.log(" FileNotExist ");
}
});
})
}
// note the use of async
app.post('/cusdetails', isLoggedIn, async function (req, res) {
var cusid=req.body.cusid;
var insertQuerys = "SELECT * FROM customer WHERE cusid=? ORDER BY rowid DESC LIMIT 1";
// using await to pause excution, waits till query is finished
const [rows] = await connection.query(insertQuerys,[cusid])
rows.forEach( (row) => {
name=row.fncus;
});
// then you can
var result = await some(k)
...
Note however this way you loose the advantage of concurrent execution, as it's kindoff blocking. If the result of one call is not used in another, you can execute in parallel and await for result to achieve sequencing like
const [rows] = connection.query(insertQuerys,[cusid])
var result = some(k)
console.log(await rows) // do something
console.log(await result) // do something

Answer 2

JavaScript is asynchronous. This means that if you have a function with a callback (i.e. your query), the callback will be called asynchronously, at an unknown time, while the other code executes.
You need to look up some tutorials how to deal with callbacks, to get a proper understanding of it. Another method is using async/await and/or promises.
Basically, if you take the following code:
console.log("this will print first");
setTimeout(function () {
console.log("this will print last");
}, 1000);
console.log("this will print second");
If you run the code above, the top level is executed synchronously, so, it first calls console.log, then it executes setTimeout, which is synchronous. It sets a timeout, then says "I'm ready", and the code continues to the other console.log. After 1 second (1000 milliseconds), the callback in the setTimeout function is executed, and only then that console.log is called. You can not make the rest of the code wait this way, you need to restructure your code or read into promises.