ErrorEvent sent to WebSocket.onError doesn't have "message" on Firefox

[jypma]

When the onerror callback of the following code gets invoked:

def onerror(e: ErrorEvent): Unit = {
  global.console.log(e)
  val msg:String = e.message
}
val ws = new WebSocket(someURLthatDoesntWork)
ws.onerror = onerror _

The e.message expression fails on Firefox at runtime, since its ErrorEvent (in this case, apparently) doesn't have a message member: (from the firefox console)

error { target: WebSocket, isTrusted: true, currentTarget: WebSocket, eventPhase: 2, 
bubbles: false, cancelable: false, defaultPrevented: false, composed: false, 
timeStamp: 2195.899715, cancelBubble: false, originalTarget: WebSocket }

It results in an uncaught exception: scala.scalajs.runtime.UndefinedBehaviorError: An undefined behavior was detected: undefined is not an instance of java.lang.String, which however somehow doesn't even get printed until the page is reloaded.

I'm unsure whether ErrorEvent itself is modelled incorrectly, whether WebSocket.onerror should have a different type, or whether coercing undefined to a String should have a bit better semantics that having to throw js.isUndefined around all over the place...

[jypma]

I think the type of WebSocket.onerror might be wrong: read API spec here