11-fs-async.js

const { readFile, writeFile } = require('fs');
console.log('start');
readFile('./content/first.txt', 'utf8', (err, result) => {
    if (err) {
        console.log(err);
        return;
    }
    const first = result;
    readFile('./content/second.txt', 'utf8', (err, result) => {
        if (err) {
            console.log(err);
            return;
        }
        const second = result;
        writeFile(
            './content/result-async.txt',
            `Here is the result: ${first}, ${second}`,
            { flag: 'a' }, (err, result) => {
                if(err) {
                    console.log(err);
                    return;
                }
                console.log(result);
                console.log('done with this task');
            })
    })
})
console.log('starting next task');
// will return undefined but check the actual async file to see output