Ionic uses some CustomEvents on its components and with strict TypeScript this might give you console errors like this. 

My code was the following:

doReorder(ev: CustomEvent<ItemReorderEventDetail>) {
    // Finish the reorder and position the item in the DOM based on
    // where the gesture ended. Update the items variable to the
    // new order of items
    this.items = ev.detail.complete(this.items);
}

The error I got was: 

 error TS2345: Argument of type 'Event' is not assignable to parameter of type 'Cus
tomEvent<ItemReorderEventDetail>'.

Solution

This is due to the behavior of the --strictFunctionTypes compiler flag added in TypeScript v2.6. A function of type (e: CustomEvent) => void is no longer considered to be a valid instance of EventListener, which takes an Event parameter, not a CustomEvent.

You can typecast the CustomEvent to make it go away:

  doReorder(ev: Event) {
    const customEvent = ev as CustomEvent<ItemReorderEventDetail>;
    // Finish the reorder and position the item in the DOM based on
    // where the gesture ended. This method can also be called directly
    // by the reorder group
    this.items = customEvent.detail.complete();
  }