11-fs-async.js

const {readFile,writeFile} =require('fs')

console.log('start')

readFile('./content/first.txt','utf-8',(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}`,
        (err,result)=>{
            if (err){
                console.log(err)
                return
            }
            console.log(result)
        }
        )
    })
})

console.log('end')