lib.d.ts: Nullable arguments for initMouseEvent()

[vidartf]

While deprecated, the typings of MouseEvent.initMouseEvent() should still support strict null checks.

TypeScript Version: 2.0.3

Code

let event = document.createEvent('MouseEvent') as MouseEvent;

// Initialize the mouse event data.
event.initMouseEvent(
  'click', true, true, window, 0,
  0, 0,
  0, 0,
  false, false,
  false, false,
  0, null   // <--- Field `relatedTarget`. Error: null not accepted
);

Expected behavior:
MDN docs specify:

relatedTarget
the event's related EventTarget. Only used with some event types (e.g. mouseover and mouseout). In other cases, pass null.

So you would expect typing to read EventTarget | null.

Actual behavior:

Null not accepted with strict null checks on.

Not sure whether any of the other fields should accept null or undefined as well...

[vidartf]

Workaround: Pass null!.

[mhegazy]

PRs welcomed. You can find more information about contributing lib.d.ts fixes at https://github.com/Microsoft/TypeScript/blob/master/CONTRIBUTING.md#contributing-libdts-fixes.

[vidartf]

PR in microsoft/TypeScript-DOM-lib-generator#152.