event_handler_callback.js

const fs = require("fs");

// this is the higher-order function that takes a callback function and passes it as an event-handler for concerned events
function readFile(filename, callback) {
    let data = '';
    let readStream = fs.createReadStream(filename, { encoding: 'UTF8' });

    readStream.on('data', function (chunk) {
        data += chunk;
    });

    readStream.on('end', function(){
        callback(null, data)
    });

    readStream.on('error', function(err) {
        callback(err)
    });
}


readFile('input.txt', function(err, data){
    // this is the function that is being passed to readFile as a callback.
    // It prints the contents of 'input.txt' once it has been read
    if(err)
        console.log(err.stack);
    // Or, in case an error occurs, it prints the error
    else
        console.log(data);
})