Express JS + Amazon S3 + DropzoneJS Code Snippet

express-controller.js

// Requires https://www.npmjs.org/package/s3-policy and https://www.npmjs.org/package/node-uuid
var config = require('env/' + process.env.NODE_ENV),
  policy = require('s3-policy'),
  uuid = require('node-uuid');

// SETUP S3
const AWS_ACCESS_KEY = config.s3.accessKey;
const AWS_SECRET_KEY = config.s3.secret;
const S3_BUCKET = config.s3.bucket;

// PRIVATE METHODS
function _getExtension(filename) {
  var i = filename.lastIndexOf('.');
  return (i < 0) ? '' : filename.substr(i);
}

// PUBLIC METHODS
exports.signS3 = function signS3(req, res) {
  var fileExtension = _getExtension(req.body.name),
    filename = uuid.v4() + fileExtension,
    acl = 'public-read',
    p = policy({
      acl: acl,
      secret: AWS_SECRET_KEY,
      length: 5000000, // in bytes?
      bucket: S3_BUCKET,
      key: filename,
      expires: new Date(Date.now() + 60000),
    }),
    result = {
      'AWSAccessKeyId': AWS_ACCESS_KEY,
      'key': filename,
      'policy': p.policy,
      'signature': p.signature
    };
  res.write(JSON.stringify(result));
  res.end();
};

frontend-app.js

var app = {};
app.signS3RequestURL = '/admin/S3/sign';
app.S3_BUCKET = 'http://your-bucket-name.s3.amazonaws.com/';
app._dropzoneAcceptCallback = function _dropzoneAcceptCallback(file, done) {
  file.postData = [];
  $.ajax({
    url: app.signS3RequestURL,
    data: {
      name: file.name,
      type: file.type,
      size: file.size,
      _csrf: $('#__createPostToken').val() // this is not needed to make Dropzone work, it's express related
    },
    type: 'POST',
    success: function jQAjaxSuccess(response) {
      response = JSON.parse(response);
      file.custom_status = 'ready';
      file.postData = response;
      file.s3 = response.key;
      $(file.previewTemplate).addClass('uploading');
      done();
    },
    error: function(response) {
      file.custom_status = 'rejected';
      if (response.responseText) {
        response = JSON.parse(response.responseText);
      }
      if (response.message) {
        done(response.message);
        return;
      }
      done('error preparing the upload');
    }
  });
};

app._dropzoneSendingCallback = function(file, xhr, formData) {
  $.each(file.postData, function(k, v) {
    formData.append(k, v);
  });
  formData.append('Content-type', '');
  formData.append('Content-length', '');
  formData.append('acl', 'public-read');
};

app._dropzoneCompleteCallback = function(file) {
  var inputHidden = '<input type="hidden" name="attachments[]" value="';
  var json = {
    url: app.S3_BUCKET + file.postData.key,
    originalFilename: file.name
  };
  console.log(json, JSON.stringify(json), JSON.stringify(json).replace('"', '\"'));
  inputHidden += window.btoa(JSON.stringify(json)) + '" />';
  $('form#createPost').append(inputHidden);
}

app.setupDropzone = function setupDropzone() {
  if ($('div#dropzone').length === 0) {
    return;
  }
  Dropzone.autoDiscover = false;
  app.dropzone = new Dropzone("div#dropzone", {
    url: app.S3_BUCKET,
    method: "post",
    autoProcessQueue: true,
    clickable: true,
    maxfiles: 5,
    parallelUploads: 3,
    maxFilesize: 10, // in mb
    maxThumbnailFilesize: 8, // 3MB
    thumbnailWidth: 150,
    thumbnailHeight: 150,
    acceptedMimeTypes: "image/bmp,image/gif,image/jpg,image/jpeg,image/png",
    accept: app._dropzoneAcceptCallback,
    sending: app._dropzoneSendingCallback,
    complete: app._dropzoneCompleteCallback
  });
};

app.startup = function startup() {
  app.setupDropzone();
}
$(document).ready(app.startup);

Thank you guys! I never saw these notifications.
In case anyone stumbles upon this, I wrote a more detailed explanation here

what exactly is the file.custom_status used for?

Wow this was SO useful!

best example I've ever seen