Upload Twitter Video using its API

twitter-controller.js

// Retrieve user access token someway, get the twitter text and pass the file in req.file 
var doPublishVideoTweet = function(access_token, text, file) {

  var stats = fs.statSync(file.path);

  var formData = {
    command: 'INIT',
    media_type: file.mimetype,
    total_bytes: stats.size
  };

  // First step, we send the size of the video
  request.post({ url: twitterUtils.getUserUploadMediaURL(), oauth: twitterUtils.getOauthParam(access_token),
    form: formData, json: true }, function(err, response, body) {
    if (!err) {
      // With its response, start transfering the video
      transferProcess(0, body.media_id_string, file, stats.size, access_token, function(err) {
        if (!err) {

          var formData = {
            command: 'FINALIZE',
            media_id: body.media_id_string
          };

          // Once the transfer is done, we publish the video
          request.post({ url: twitterUtils.getUserUploadMediaURL(), oauth: twitterUtils.getOauthParam(access_token),
            form: formData, json: true }, function(err, response, body) {
            if (!err && !body.error) {
              doPublishTweet(access_token, text, body.media_id_string, null);
            }
          });
        }
      });
    }
  });
};

// Transfers each part of the video until the end
var transferProcess = function(index, mediaId, file, fileSize, access_token, callback) {

  // We open the file
  var fd = fs.openSync(file.path, 'r');

  var bytesRead, data, bufferLength = 268435456;
  var buffer = new Buffer(1000000000);

  var startOffset = index * bufferLength;
  var length = startOffset + bufferLength > fileSize ? fileSize - startOffset : bufferLength;

  // We read the amount of bytes specified in startOffset until length
  bytesRead = fs.readSync(fd, buffer, startOffset, length, null);

  // This boolean tells us that the last part is being transferred
  var completed  = bytesRead < bufferLength;
  data = completed ? buffer.slice(0, bytesRead) : buffer;

  // We generate a chunk file with the readed data
  var chunkFileName = copyFileName + '-chunked-' + index;
  // And save that file so we can read it and send it
  fs.writeFile(chunkFileName, data, function(err) {
    if (err) {
      callback(err);
    }
    else {

      var formData = {
        command: 'APPEND',
        media_id: mediaId,
        segment_index: index
      };

      formData.media = fs.createReadStream(chunkFileName);

      var oauth = {
        consumer_key: 'TWITTER_KEY',
        consumer_secret: 'TWITTER_SECRET',
        token: access_token.token,
        token_secret: access_token.token_secret
      };
      
      // Once the file has been written, we upload it
      request.post({ url: 'https://upload.twitter.com/1.1/media/upload.json', oauth: oauth,
        formData: formData, json: true }, function (err, response) {
        if (err) {
          callback(err);
        }
        else if (completed) {
          callback(null);
        }
        // We keep uploading increasing the index until the end
        else {
          transferProcess(index + 1, mediaId, file, fileSize, access_token, callback);
        }
      });
    }
  });
};

// Finally we publish the Tweet with the associated mediaId
var doPublishTweet = function(access_token, text, mediaId, callback) {

  var qs = {};
  if (text) {
    qs.status = text;
  }
  if (mediaId) {
    qs.media_ids = mediaId;
  }
  var url = twitterUtils.getUserPublishContentURL();

  var oauth = {
    consumer_key: 'TWITTER_KEY',
    consumer_secret: 'TWITTER_SECRET',
    token: access_token.token,
    token_secret: access_token.token_secret
  };
  
  request.post({ url: 'https://api.twitter.com/1.1/statuses/update.json', oauth: oauth, qs: qs, json: true }, function(err, response) {
    callback(null);
  });
};

user-routes.js

// Import multer library
var multer  = require('multer');

// Craete an upload object
var upload = multer({ limits: { fileSize: 50 * 1024 * 1024 }, storage: multer.diskStorage({}) });

// In an Express router, add the upload object as a middleware
userRoutes.post('/:user_id/media', upload.single('file'), twitterController.publishVideo);

Why does one need to use 'multer'?