Develop Reference

How to read and Write multiple files using node js?

In a array I have filenames; I want to first read one file and perform some operation then store result in a separate file. Then read 2nd file, perform operation again and save result in new 2nd file. Do the same procedure for all files. Below I have written code to read and write files.
TextReader.js
var fs = require('fs');
const readline= require('readline');
var headerIndex = [];
var isFirstLine = true;
var finalList = [];
module.exports={
readTextFile: (filename)=>{
console.log('inside textreader')
readline.createInterface({
input: fs.createReadStream(`./s3/${filename}`)
}).on('line', function(line) {
console.log(line);
console.log("-----------------------------");
if (isFirstLine) {
headerIndex = line.split('|');
}
else if (!isFirstLine){
let rowValues = line.split('|');
let valueIndex = 0;
var singlePerson = {};
headerIndex.forEach(currentval => {
singlePerson[currentval] = rowValues[valueIndex];
valueIndex++;
});
finalList.push(singlePerson);
}
isFirstLine = false;
}).on('close',function(){
//console.log(finalList);
var data='';
var header= "Employee ID"+'\t'+headerIndex[0]+'\t'+headerIndex[2]+'\t'+headerIndex[1]+'\t'+headerIndex[4]
+'\t'+headerIndex[3]+'\t'+headerIndex[5]+'\n';
for (var i = 0; i < finalList.length; i++) {
function split(name){
var conv=name.split(' ');
var result=[conv.slice(0, -1).join(' '),conv.slice(-1)[0]].join(conv.length < 2 ? '' : ',');
return result;
}
split(finalList[i].UserName);
data=data+finalList[i].LoginID+'\t'+split(finalList[i].UserName)+'\t'+finalList[i].Email+'\t'
+finalList[i].LoginID+'\t'+'A&G Professional'+'\t'+finalList[i].Title+'\t'+finalList[i].State+'\n';
}
var newFilename= filename.substr(0, filename.lastIndexOf("."))
var alldata= header + data;
//console.log(alldata)
fs.appendFile(`./s3/${filename}.xlsx`,alldata, (err) => {
if (err) throw err;
console.log('File created');
});
});
}
}
I am calling readTextFile(); from another file.
demo.js
const { readTextFile } = require("./textReader");
var array=['UserRoleDetails_12102021063206.txt',
'UserRoleDetails_12102021064706 (1).txt',
'UserRoleDetails_12102021064706.txt',
'UserRoleDetails_12102021070206.txt']
array.forEach(function(currentItem){
readTextFile(currentItem);
})
The problem i am facing is that all files are processed at the same time and all the datas of all files are stored together.

first, this node js is not work in sequential as you mention here
and second, array.forEach is not useful here to do the sequential operation
you need to use
const { readTextFile } = require("./textReader");
var array=['UserRoleDetails_12102021063206.txt',
'UserRoleDetails_12102021064706 (1).txt',
'UserRoleDetails_12102021064706.txt',
'UserRoleDetails_12102021070206.txt']
for (const element of array) {
readTextFile(currentItem);
}
NOTE:- readTextFile(currentItem) your this function is not async so maybe you need to make it async
if you are not clear then raise your hand

Iterating with callback/anonymous functions

I'm new to Node.JS and advanced Javascript in general, but I'm trying to build a schedule manager application on my own and I faced a problem (I will detail it later) when trying to execute the following code:
router.get('/', function (req, res) {
var day = new Date(req.query.day);
Location.getLocations(function (err, locations) {
if (locations.length > 0) {
var i;
for (i = 0; i < locations.length; i++) {
var location = locations[i];
Appointment.getAppointments(day, location, function (err, appointments) {
if (err) throw err;
if (appointments.length == 0) {
// CREATE APPOINTMENTS
for (var j = location.available_time_start; j <= location.available_time_end; j += location.appointment_duration) {
var newAppointment = new Appointment();
newAppointment.start_date = new Date(day.getFullYear(), day.getMonth() + 1, day.getDate(), j);
newAppointment.appointment_duration = location.appointment_duration;
newAppointment.location = location.id;
newAppointment.booked = false;
newAppointment.locked = false;
Appointment.createAppointment(newAppointment, function (err, appointment) {
if (err) throw err;
console.log(appointment.location + ' - ' + appointment.start_date);
});
}
}
});
}
} else {
// THERE ARE NO LOCATIONS
}
res.render('appointments', { locations: locations });
});
The problem is:
When I try to iterate the locations object and then execute the getAppointments function the code isn't executed at this exact moment. Later, when it's executed, location object is always the same (the iteration doesn't work), resulting on a unexpected result (all appointments with the same/last location).
I tried using IIFE (Immediately-invoked function expression) to execute the code instantly, but when I did this I couldn't get the appointments callback object and my logic is broken too.
Thanks in advance!

The problem was solved by using let instead of var as suggested by #JaromandaX.

Your code seems to be saving appointments but doesn't do anything with the saved appointments (are you mutating locations?).
When saving an appointment goes wrong the requestor doesn't know about it because createAppointment is asynchronous and by the time the callback is called back res.render('appointments', { locations: locations }); is already executed.
You could try converting your callback based functions to promises:
const asPromise = (fn,...args) =>
new Promise(
(resolve,reject)=>
fn.apply(undefined,
args.concat(//assuming one value to resole
(err,result)=>(err)?reject(err):resolve(result)
)
)
);
const savedAppointmentsForLocation = (day,location,appointments) => {
const savedAppointments = [];
if (appointments.length == 0) {
// CREATE APPOINTMENTS
for (var j = location.available_time_start; j <= location.available_time_end; j += location.appointment_duration) {
var newAppointment = new Appointment();
newAppointment.start_date = new Date(day.getFullYear(), day.getMonth() + 1, day.getDate(), j);
newAppointment.appointment_duration = location.appointment_duration;
newAppointment.location = location.id;
newAppointment.booked = false;
newAppointment.locked = false;
savedAppointments.push(
asPromise(
Appointment.createAppointment.bind(Appointment),
newAppointment
)
);
}
}
//you are not doing anything with the result of the saved appointment
// I'll save it as promise to see if something went wrong to indicate
// to the requestor of the api that something went wrong
return Promise.all(savedAppointments);
}
router.get('/', function (req, res) {
var day = new Date(req.query.day);
asPromise(Location.getLocations.bind(Location))
.then(
locations=>
promise.all(
locations.map(
location=>
asPromise(Appointment.getAppointments.bind(Appointment),[day,location])
.then(appointments=>[location,appointments])
)
)
)
.then(
results=>//results should be [ [location,[appointment,appointment]],...]
Promise.all(
results.map(
([location,appointments])=>
savedAppointmentsForLocation(day,location,appointments)
.then(ignoredSavedAppointment=>location)
)
)
)
.then(locations=>res.render('appointments', { locations: locations }))
.catch(
error=>{
console.log("something went wrong:",error);
res.status(500).send("Error in code");
}
)
});

Nodejs get result from callback

i'm tryna get my array of results out of the callback method, the array is containing all of those things i want in but i can use it outside of the functions always keep undefined result. I was looking on many website for helping me to improve it but no results.
var DOMParser = require('xmldom').DOMParser;
var fs = require('fs');
var Initializer = require('./Initializer');
var StaticRoomList = Initializer.getStaticRoomList(fs, DOMParser);
console.log(StaticRoomList);
openRoomListXML = function(fs, DOMParser, callback){
try{
fs.readFile('./XML/RoomList.xml', function(e, data){
if(e){
callback(e);
}
else{
var BuffertoXMLString = String(data);
var XMLOutput = new DOMParser().parseFromString(BuffertoXMLString, "text/xml");
var XMLDocument = XMLOutput.documentElement;
callback(null, XMLDocument);
}
});
}
catch(fsException){
console.log(fsException);
}
};
getStaticRoomList = function(fs, DOMParser){
openRoomListXML(fs, DOMParser, function readList(e, XMLDocument){
if(e){
console.log(e);
}
else{
var nodeList = XMLDocument.getElementsByTagName("room");
var arrayRoomList = [];
for(i = 0; i < nodeList.length; i++){
arrayRoomList.push(nodeList[i].childNodes[0].nodeValue);
}
return arrayRoomList;
}
});
};
exports.getStaticRoomList = getStaticRoomList;

getStaticRoomList doesn't return anything since the code inside the callback function is executed long after the function is terminated. You should define another callback function for getStaticRoomListtoo.
So in the end, to obtain the array you will do something like
var StaticRoomList;
Initializer.getStaticRoomList(fs, DOMParser,function(result){
StaticRoomList = result;
});
And in the callback inside getStaticRoomList, instead of trying to return the result, you simply call the callback function of getStaticRoomList

Functions in JS are async.
You have to pass a callback function as a parameter.
Below there is an example:
getStaticRoomList = function(fs, DOMParser, callBack){
openRoomListXML(fs, DOMParser, function readList(e, XMLDocument){
if(e){
console.log(e);
}
else{
var nodeList = XMLDocument.getElementsByTagName("room");
var arrayRoomList = [];
for(i = 0; i < nodeList.length; i++){
arrayRoomList.push(nodeList[i].childNodes[0].nodeValue);
}
callBack(arrayRoomList);
}
});
getStaticRoomList(fs, DOMParser, function(error, result){
if(error != null){
//Deal with resul
}
})

Javascript push object into array not working

im using node mirc to retrieve data from mysql
then i want convert data to array, using code below :
function getQuestion (arr{
var obj = {};
connection.connect();
connection.query("SELECT * FROM quiz", function (err, rows, fields) {
if (err) throw err;
for (var i = 0; i < rows.length; i++) {
var row = rows[i];
obj = {"id":row.id, "question":row.question, "answers":row.answers.split(", ")};
f100.push(obj);
};
});
connection.end();
}
f100 = [];
getQuestion();
console.log(f100);
but, its only print []

Due to .query()'s async behavior its taking some time to execute, but you're executing the log just after calling the function. You need to put the log withing callback of .query() function.
function getQuestion (arr{
var obj = {};
connection.connect();
connection.query("SELECT * FROM quiz", function (err, rows, fields) {
if (err) throw err;
for (var i = 0; i < rows.length; i++) {
var row = rows[i];
obj = {"id":row.id, "question":row.question, "answers":row.answers.split(", ")};
f100.push(obj);
};
// do log here
console.log(f100);
});
connection.end();
}
f100 = [];
getQuestion();

connection.query is asynchronous, so i'd recommend consoling out f100 inside the connection.query callback. If you want to do something with f100 after it's finished populating, you'd need to pass it into another function from inside the connection query callback. Example:
connection.query("SELECT * FROM quiz", function (err, rows, fields) {
if (err) throw err;
for (var i = 0; i < rows.length; i++) {
var row = rows[i];
obj = {"id":row.id, "question":row.question, "answers":row.answers.split(", ")};
f100.push(obj);
};
console.log(f100); // should console out correctly here
handleArray(f100); // passes in f100 to a new function
});

'Juggling Async' - Why does my solution not return anything at all?

After asking a question and getting a very helpful answer on what the 'Async Juggling' assignment in learnyounode was asking me to do, I set out to implement it myself.
The problem is, my setup isn't having any success! Even though I've referred to other solutions out there, my setup simply isn't returning any results when I do a learnyounode verify myscript.js.
GIST: jugglingAsync.js
var http = require('http');
var app = (function () {
// Private variables...
var responsesRemaining,
urls = [],
responses = [];
var displayResponses = function() {
for(var iterator in responses) {
console.log(responses[iterator]);
}
};
// Public scope...
var pub = {};
pub.main = function (args) {
responsesRemaining = args.length - 2;
// For every argument, push a URL and prep a response.
for(var i = 2; i < args.length; i++) {
urls.push(args[i]);
responses.push('');
}
// For every URL, set off an async request.
for(var iterator in urls) {
var i = iterator;
var url = urls[i];
http.get(url, function(response) {
response.setEncoding('utf8');
response.on('data', function(data) {
if(response.headers.host == url)
responses[i] += data;
});
response.on('end', function() {
if(--responsesRemaining == 0)
displayResponses();
});
});
}
};
return pub;
})();
app.main(process.argv);
Question: What am I doing wrong?

This line
for(var iterator in urls) {
doesn't do what you think it does. It actually loops over the properties of urls (see https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/for...in). Instead, you have to do something like
for(var i = 0; i < urls.length; i++) {
var url = urls[i];
...
}
or
urls.forEach(function(url, index) {
...
});

In addition to not properly looping through the arrays inside the app module, I was also not properly concatenating data returned from the response.on('data') event. Originally I was doing...
responses[index] += data;
Instead, the correct thing to do was:
responses[index] = responses[index] + data;
Changing that, as well as the things noted by #arghbleargh got the 'Async Juggling' to fully verify!

I have tested my code and it all worked:
~ $ node juggling_async.js site1 site2 site3 site4 ...
The JS code does not limit only to three sites.
var http = require('http');
// Process all the site-names from the arguments and store them in sites[].
// This way does not limit the count to only 3 sites.
var sites = [];
(function loadSites() {
for(var i = 2, len = process.argv.length; i < len; ++i) {
var site = process.argv[i];
if(site.substr(0, 6) != 'http://') site = 'http://' + site;
sites.push(site);
}
})();
var home_pages = [];
var count = 0;
function httpGet(index) {
var home_page = '';
var site = sites[index];
http.get(site, function(res) {
res.setEncoding('utf8');
res.on('data', function(data) {
home_page += data;
});
res.on('end', function() {
++count;
home_pages[index] = home_page;
if(count == sites.length) {
// Yahoo! We have reached the last one.
for(var i = 0; i < sites.length; ++i) {
console.log('\n############ Site #' + (+i+1) + ': ' + sites[i]);
console.log(home_pages[i]);
console.log('============================================\n');
}
}
});
})
.on('error', function(e) {
console.log('Error at loop index ' + inddex + ': ' + e.message);
})
;
}
for(var i = 0; i < sites.length; ++i) {
httpGet(i);
}