How to refactor this code to use Promises for asyncronous functionality

JavaScript
1

So I did last week the challenges of ES6 and I saw how promises work. Until now I have worked a lot with Javascript but the only thing I understood was how setTimeOut works.

Now I have the following code which first downloads a file, and then, converts that file into a json one.

Right now I have it working by setting a setTimeOut to the transform() function so the downloadCSV() function can finish before attempting to transform but that is so crappy, I wanted to use something professional to do this.

The problem is If i create a promise then the code inside doesn´t really get executed and I dont know how to pass a function to that promise (if it can be done). Also i imagine is not the best clean way to do this.

So my question is, how would you make this code asynchrous so the transform() function doesn´t get called until the file is download it? I dont care much if it´s not done with promises i just want to know what is the correct/clean way to do it.

// 1. Download the CSV:
const downloadCSV = (csv) => {

    console.log('downloading csv file')

    const fetchPage = (urlF, callback) => {
        https.get(urlF, (response) => {
            let buff = ''
            response.on('data', (chunk) => {
                buff += chunk
            })
            response.on('end', () => {
                callback(null, buff)
            })
        }).on('error', (error) => {
            console.error(`Got error: ${error.message}`)
            callback(error)
        })
    }

    var folderNameCSV = uuidv1()
    folderNameCSV = 'CSV_TO_JSON'
    fs.mkdirSync(folderNameCSV)
    fetchPage(csv, (error, data) => {

        if (error) return console.log(error)
        fs.writeFileSync(path.join(__dirname, folderNameCSV, 'file.xml'), data)

    })
}


function transform(){
    csvtojsonV2().fromFile('./CSV_TO_JSON/file.xml')
    .then((jsonObj) => {

        var folderNameJSON = uuidv1()
        folderNameJSON = 'JSON_CONVERTED'

        fs.mkdirSync(folderNameJSON)
        jsonObj = JSON.stringify(jsonObj)
        fs.writeFileSync(path.join(__dirname, folderNameJSON, 'archivo.json'), jsonObj)

    })
}

My crappy approach :sweat_smile::joy::

downloadCSV(csv)
setTimeout(transform, 3000);

Obviously i cannot do a setTimeout for that code because if the csv is too big or conection is too slow the code would break. Is a terrible approach.

2

There are a number of options. Among them, you could use promises like you suggested, eg in downloadCSV

return new Promise((resolve, reject) => {
// call resolve() when you're done
// or call reject() when something fails.
});

Perhaps read mdn docs on Promise if you’re unfamiliar.

You can also use an async version of writeFile and pass an appropriate callback (your transform). Node docs on fs.writeFile