How to get the Dropzone.js return value?

Does this help?

Dropzone.options.myDropzone = {
  init: function() {
        thisDropzone = this;
        this.on("success", function(file, responseText) {
            var responseText = file.id // or however you would point to your assigned file ID here;
            console.log(responseText); // console should show the ID you pointed to
            // do stuff with file.id ...
        });
    }
};

For example, I have mine set up to attach the server path to the image name and pass this as a value into a form field on submit. As long as you define responseText to point to the file ID you should get a return on this.

This link might be helpful as well: https://github.com/enyo/dropzone/issues/244

$("#dropzoneForm").dropzone({
    maxFiles: 2000,
    url: "../Uploader/HttpUploadHandler.ashx?param=" + result.prjID,
    success: function(file, response){
        //alert("Test1");
    }
});

Looking at the source code of dropzone.js, it seems that there is a lot of events you can listen to.

events: [
  "drop"
  "dragstart"
  "dragend"
  "dragenter"
  "dragover"
  "dragleave"
  "addedfile"
  "removedfile"
  "thumbnail"
  "error"
  "errormultiple"
  "processing"
  "processingmultiple"
  "uploadprogress"
  "totaluploadprogress"
  "sending"
  "sendingmultiple"
  "success"
  "successmultiple"
  "canceled"
  "canceledmultiple"
  "complete"
  "completemultiple"
  "reset"
  "maxfilesexceeded"
  "maxfilesreached"
]

Here the "success" event seems to be appropriate.

A good starting point would be to bind an event listener to your dropzone and see what data you get on such event.

$('#my-awesome-dropzone').on('success', function() {
  var args = Array.prototype.slice.call(arguments);

  // Look at the output in you browser console, if there is something interesting
  console.log(args);
});