constructor(type, eventInitDict) {
    this.type = type.toString();
    Object.assign(this, eventInitDict);
  }
Event object passed to the `onopen`, `onclose`, `onmessage`, `onerror`
callbacks of `WebSocket`.

The `type` property is "open", "close", "message", "error" respectively.

In case of "message", the `data` property contains the incoming data.