How to read file in nodejs Server sent from Web Client

formidable is a very handy library for working with forms.

The following code is a fully functional example node app which I took from formidable's github and slightly modified. It simply displays a form on GET, and handles the upload from the form on POST, reading the file and echoing its contents:

var formidable = require('formidable'),
    http = require('http'),
    util = require('util'),
    fs = require('fs');

http.createServer(function(req, res) {
  if (req.url == '/upload' && req.method.toLowerCase() == 'post') {
    // parse a file upload
    var form = new formidable.IncomingForm();

    form.parse(req, function(err, fields, files) {
      res.writeHead(200, {'content-type': 'text/plain'});

      // The next function call, and the require of 'fs' above, are the only
      // changes I made from the sample code on the formidable github
      // 
      // This simply reads the file from the tempfile path and echoes back
      // the contents to the response.
      fs.readFile(files.upload.path, function (err, data) {
        res.end(data);
      });
    });

    return;
  }

  // show a file upload form
  res.writeHead(200, {'content-type': 'text/html'});
  res.end(
    '<form action="/upload" enctype="multipart/form-data" method="post">'+
    '<input type="text" name="title"><br>'+
    '<input type="file" name="upload" multiple="multiple"><br>'+
    '<input type="submit" value="Upload">'+
    '</form>'
  );
}).listen(8080);

This is obviously a very simple example, but formidable is great for working with large files too. It gives you access to a read stream of the parsed form data as it's handled. This allows you to work with the data as it's being uploaded, or pipe it directly into another stream.

// As opposed to above, where the form is parsed fully into files and fields,
// this is how you might handle form data yourself, while it's being parsed
form.onPart = function(part) {
  part.addListener('data', function(data) {
    // do something with data
  });
}

form.parse();

You will need to parse the body of the http request which can contain a file from an HTML file input. For instance, when using the express web framework with node, you can send a POST request via an HTML form and access any file data via req.body.files. If you're just using node, have a look at the 'net' module to assist in parsing http requests.