Pass additional parameters to Javascript request 3 argument callback

Question

I am trying to nest several callbacks with the goal of programmatically navigating a page using POST requests. I have a list of different files I am trying to download, and want to assign each of them a different filename so that I can make asynchronous download calls and perform OCR on them.

The callback that actually downloads the PDF is here:

filename = "doodle" + Math.floor((Math.random() * 100) + 1) + ".pdf";    
request.post(
                {
                    url:'https://ecorp.sos.ga.gov/BusinessSearch/DownloadFile',
                    form: {
                        'documentId': body
                    },
                    headers: {
                        'Referer': 'https://ecorp.sos.ga.gov/BusinessSearch/BusinessFilings'
                    }
                }, getPDF).pipe(fs.createWriteStream(filename));

Notice that the request sends the resulting page to a getPdf function, which actually opens the PDF for OCR after the request has been made. Currently I have a filename hardcoded in the OCR method, because I do not know how to pass an additional variable to the callback function. I would like to be able to write to a randomly generated file, and in the callback method retrieve that filename.

function getPDF (error, response, body) {
    if(!error){
        console.log(body);


        filename = "/doodle" + Math.floor((Math.random() * 100) + 1) + ".pdf";
        //console.log(__dirname + filename);

        /*pdfText(__dirname + "/doodle.pdf", function(err, chunks) {
            //console.log(chunks);
        });*/



        var pdf_path = __dirname + filename;

        //option to extract text from page 0 to 10 
        var option = {from: 0, to: 10};
        var pdf_body;

        pdfUtil.pdfToText(pdf_path, option, function(err, data) {
            pdf_body = data;  
            var result;

            try{
                var namez = /AUTHORIZED SIGNATURE:\s+(.+?)\s{2}/.exec(data)[1];
                var emailz = /Email: (.+?)\s{2}/.exec(data)[1];
                result = {query:query, info:{name: namez, email: emailz}};

            } catch (err){
                //result = {query:query, error:"non-selectable text in PDF"};
            }

            results.push(result);

            //console.log(JSON.stringify(result));
        });


    }
}

the library I am using is called request

and the documentation shows the following:

The callback argument gets 3 arguments:

 An error when applicable (usually from http.ClientRequest object) An http.IncomingMessage object The third is the response body (String or Buffer, or JSON object if the json option is supplied) 

Answer 1

The most straight forward approach would be to simply change the signature of getPDF to

function getPDF (filename, error, response, body) {...}

then you'd call it from within an anonymous function as the callback handler of request.post:

request.post({...}, function(error, response, body){
    getPDF( 'filename', error, response, body);
});

E6 notation makes this even nicer:

request.post({...}, (error, response, body)=>{
    getPDF( 'filename', ...arguments);
});

You can accomplish this using a more sophisticated method by partially applying the additional argument and then calling it in point-free fashion:

function getPDF (filename, error, response, body, ) {...}
getPdf2 = getPDF.bind(null, "filename"); 
request.post({...}, getPdf2);

The approaches amount to to the same thing and I might use either depending on circumstances. Arguably, the intent of the second approach is easier to read. It's certainly more fashionable.