Nodejs read multiple files in async approach and show reading status

Well, that’s a bit difficult question and there a lot of packages in Nodejs to read multiple files in async approach. When you want to read the multiple files or remote URLs in the asynchronous method by storing the file content in objects. When the number files are more than 100 or 1000 then you need to observe the status that how many files have been read and how much files in pending and possibly this situation occurs in reading large files. In other words, when you will get to know reading all files has been finished.

Nodejs read multiple files in async approach:
In nodejs, you can read the file in both asynchronous and synchronous approach using the file server fs. The file server is an inbuilt feature. When you go with the asynchronous approach it always has a callback. The last argument will be considered as callback and the first argument in the callback is specially reserved for exception handling. Here look at an example.

const fileReader = require('fs');
let fileContent = {};
fs.readFile('file.txt', 'utf-8', (err, data) => 
{

    if (err) throw err
    fileContent = data;

});

How to process all multiple file content as the final result?
Understanding when reading all the files has been finished is a bit complicated with a callback. I have written a code for reading the multiple file or URL by using the request method. Let you store all the files path or URLs in the text files in the below format.

//fileList.txt
https://urlnameorpathname1.com
https://urlnameorpathname2.com
https://urlnameorpathname3.com

Solution 1 :
Create a function called as readUrl for reading the file with promise
Read the content of fileList.txt file as a string.
Break the string into an array of URLs.
For loop the array of URL
Get the current URL path and call the readUrl bypassing URL.
Now readUrl function returns the status with help of promise.
Now process the response object and store the content in finalResult
Display the currently reading URL and what has been read.

// File reader
var fs = require('fs');

// Request package
var request = require("request");

// Variable to store all final urls results
var urlResponseObject = [];

// List of urls to be processed
var urlList = [];

console.log('Initializing the file reading request....');
console.log('..................................\n');

//Read the file content and convert to array
var fileData = fs.readFileSync('list_of_urls.txt', 'utf8');
if (fileData.length > 5) {
    console.log('File content...\n' + fileData);
    var urlListTemp = fileData.split("\n");
    for (let ui = 0; ui < urlListTemp.length; ui++) {
        var urlString = urlListTemp[ui];
        urlList[ui] = urlString.replace(/\n|\r/g, "");
    }
    console.info(urlList);
} else {
    console.log('File is empty....');
    return false;
}


// Function to read the url content and return response
function readUrl(url) {
    // Setting URL and headers for request
    var options = {
        url: url,
        headers: {
            'User-Agent': 'request'
        }
    };
    // Return new promise 
    return new Promise(function(resolve, reject) {
        // Do async job
        request.get(options, function(err, resp, body) {
            if (err) {
                resolve(404);
            } else {
                resolve((body));
            }
        })
    })
}

// An object to store the urls properties
var urlObject = function(url) {
    this._url = url;
    this.updateSucess = updateSucess;
    this.updatePending = updatePending;
    this.updateFailure = updateFailure;

}

// Update success status of the url
function updateSucess() {
    this._status = 'Success';
    this._statusCode = 200;
}

// Update pending status of the url
function updatePending() {
    this._status = 'Pending';
}

// Update failure status of the url
function updateFailure() {
    this._status = 'Failed';
    this._error = 'Address not found';
}

// Process all the urls and store response in object
function runApp() {
    console.log('.........Initialing for loop........\n');
    for (let i = 0; i < urlList.length; i++) {
        let curentUrl = urlList[i];
        
        // Create new object of url
        let currentUrlObject = new urlObject(curentUrl);
        currentUrlObject.updatePending();

        // call the promise to return url response
        let initializePromise = readUrl(curentUrl);
        initializePromise.then(function(result) {
                console.log('Processing the url => ' + JSON.stringify(curentUrl));
                if (result === 404) {

                    // console.log(result);
                    currentUrlObject.updateFailure();
                    console.log('Current reading url => ' + JSON.stringify(currentUrlObject));
                    console.log('Reading status => Failed');
                    console.log('Completed reading for url => ' + JSON.stringify(curentUrl));
                    urlResponseObject[i] = currentUrlObject;
                    let finalResult = JSON.stringify(urlResponseObject);
                    console.log('Total reading completed urls : ' + finalResult);
                    console.log('..................................\n');
                } else {
                    // console.log(result);
                    let sourceCode=result;
                    currentUrlObject.updateSucess();
                    console.log('Current reading url : ' + JSON.stringify(currentUrlObject));
                    console.log('Reading status => Success');
                    // console.log('Source code og the url  => ' +sourceCode);
                    console.log('Completed reading for url => ' + JSON.stringify(curentUrl));
                    urlResponseObject[i] = currentUrlObject;
                    let finalResult = JSON.stringify(urlResponseObject);
                    console.log('Total reading completed urls => ' + finalResult);
                    console.log('..................................\n');
                }
            },
            function(err) {
                console.log('Error => ' + error);
            })
    }

}

// Run the main app
runApp();

// Show processed urls for every 2 seconds
var showProcessedUrls = setInterval(function () { 

            console.log('...............Total processed result...................'); 
            console.log(JSON.stringify(urlResponseObject));
            var originalArray = Object.size(urlList);
            var processedArray = Object.size(urlResponseObject);

            // Stope if all urls has been processed
            if(originalArray==processedArray)
            {
                clearInterval(showProcessedUrls);
            }
    }, 2000);

// Get the size of the objects
Object.size = function(obj) {
    var size = 0, key;
    for (key in obj) {
        if (obj.hasOwnProperty(key)) size++;
    }
    return size;
};

Also read, What is inheritance in OOP?