Generalising the Event interface

[rtpg]

Been having an issue with this snippet of code:

var reader = new FileReader();
var debug = {hello: "world"};
var blob = new Blob([JSON.stringify(debug, null, 2)], {type : 'application/json'});

reader.onload = function(evt){
  console.log(evt.target.result);
}
reader.readAsText(blob);

So when running this as plain JS, I end up logging {hello:"world"}. But this doesn't typecheck right now.

So currently the interface structures involved in lib.d.ts are something like (trimmed for space):

interface Event {
    target:EventTarget;
}

interface FileReader  extends EventTarget{
    onload:(ev:Event)=>any;
    result: any;
}

The interface signature makes it clear that FileReader is meant to be the target, but because of how the interfaces are structured, this doesn't bubble down properly.

In a vacuum, the simple solution seems to be to make Event be generic on its target:

interface Event<T extends EventTarget>{
    target:T
}

interface FileReader extends EventTarget{
    onload:(ev:Event<FileReader>)=>any;
    result: any;
}

But two things cause me to hesitate at this patch:

• The Event interface is defined in the W3C DOM spec .I'm not quite sure what the policy is upstream with Edge on this, but introducing generics into the typing might be too much deviation from this.
• Event touches a lot of stuff, so there's a lot of collateral damage. I think it would require rewriting almost everything related to Events (kind of defeating the purpose of generating lib.d.ts from Edge).

As a side note, I've noticed that there's a lot of onX(ev:Event<Self>)=>any-type event listeners a bit all over the place (where Self is the containing interface). I run into this quite a bit in my own code, I wonder if there's value in having a special type variable to refer to the containing class.

Anyways, I wanted to get some feedback on this before moving forward.

[mhegazy]

looks like a duplicate of #299