How can I create an event with a dataTransfer property?

Question

I'm trying to write a test that checks that my dragstart event listener sets properties on e.dataTransfer.

I can manually create and dispatch a MouseEvent:

var event = new MouseEvent('dragstart');
html[0].dispatchEvent(event);

but event.dataTransfer is null so I'm getting null reference exceptions.

I've tried manually setting the property:

event.dataTransfer = {};

and I've tried setting the property using a property descriptor:

var event = new MouseEvent('dragstart');
Object.defineProperty(event, "dataTransfer", {
    configurable: true,
    writable: true,
    value : {}
});

but neither has worked.

Answer 1

This worked for me.

var dataTransferObj = { setData: function (val) { this.data = val }}
var event = new CustomEvent('dragover');
event.dataTransfer=dataTransferObj
target.dispatchEvent(event);