How to trigger an event with a custom event (specifically a custom dataTransfer property)?

Question

I'm currently attempting to test some code that uses drag-and-drop. I found some other questions that were kinda related to this, but they were way too specific to help me, or not related enough.

This being a test, I'm struggling on trying to automatically execute code inside a .on('drop',function(e){....} event. The main issue is not that I can't run the code inside, but it's that I can't transfer the dataTransfer property, and I can't seem to fake it because it's read-only. Is there anyway to fake the dataTransfer property or otherwise get around it?

I came up with this JSFiddle that serves as a template of what I'm trying to do: https://jsfiddle.net/gnq50hsp/53/

Essentially if you are able to explain to me (if this is at all possible) how I can possibly fake the dataTransfer property, I should be all set.

Side notes:

• I'm totally open to other ways of somehow getting inside that code, like for example, maybe its possible to trigger the event and pass in a fake event object with a fake dataTransfer object.

• To see the drag-drop behavior, change the JavaScript load type from no-wrap head to on-Load, then you should see what I'm trying to simulate.

• Important to note that I cannot modify any of the code inside the event handlers, only inside the outside function

• Using Karma/Jasmine so use of those tools are also possible like spies

• Also, I'm using Chrome.

Thanks in advance, and let me know for any questions/clarifications!

Answer 1

You should be able to override pretty much everything you want using Object.defineProperty. Depending on what you want to test it can be very simple or very complex. Faking the dataTransfer can be a bit tricky, since there's a lot of restrictions and behaviors linked to it, but if you simply want to test the drop function, it's fairly easy.

Here's a way, this should give you some ideas as to how to fake some events and data:

//Event stuff
var target = $('#target');
var test = $('#test');

test.on('dragstart', function(e) {
  e.originalEvent.dataTransfer.setData("text/plain", "test");
});
target.on('dragover', function(e) {

  //e.dataTransfer.setData('test');
  e.preventDefault();
  e.stopPropagation();
});
target.on('dragenter', function(e) {
  e.preventDefault();
  e.stopPropagation();
});

//What I want to simulate:
target.on('drop', function(e) {
  console.log(e)
    //Issue is that I can't properly override the dataTransfer property, since its read-only
  document.getElementById('dataTransferDisplay').innerHTML = e.originalEvent.dataTransfer.getData("text");
});

function simulateDrop() {

  // You'll need the original event
  var fakeOriginalEvent = new DragEvent('drop');

  // Using defineProperty you can override dataTransfer property. 
  // The original property works with a getter and a setter,
  // so assigning it won't work. You need Object.defineProperty.
  Object.defineProperty(fakeOriginalEvent.constructor.prototype, 'dataTransfer', {
    value: {}
  });

  // Once dataTransfer is overridden, you can define getData.
  fakeOriginalEvent.dataTransfer.getData = function() {
    return 'test'
  };
  // TO have the same behavior, you need a jquery Event with an original event
  var fakeJqueryEvent = $.Event('drop', {
    originalEvent: fakeOriginalEvent
  });
  target.trigger(fakeJqueryEvent)
}

https://jsfiddle.net/0tbp4wmk/1/