Argument of type '(e: CustomEvent) => void' is not assignable to parameter of type 'EventListenerOrEventListenerObject'

Question

I have this custom event setup, and it works with TypeScript 2.5.3, but when I updated to 2.6.1 I get an error

window.addEventListener(         


        

Answer 1

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.

So one way to fix it is to turn off --strictFunctionTypes.

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Another way is to pass in a function that takes an Event and then narrows to CustomEvent via a type guard:

function isCustomEvent(event: Event): event is CustomEvent {
  return 'detail' in event;
}

window.addEventListener('OnRewards', (e: Event) => {
  if (!isCustomEvent(e))
    throw new Error('not a custom event');
  // e is now narrowed to CustomEvent ...
  // my code here 
})

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A third way is to use the other overload of addEventListener():

addEventListener(type: K, listener: (this: Window, ev: WindowEventMap[K]) => any, useCapture?: boolean): void;

If the type parameter is the name of a known event type (K extends keyof WindowEventMap) like "onclick", then the listener function will expect its parameter to be of that narrowed event type (WindowEventMap[K]). The problem is that "OnRewards" is not a known event type... unless you use declaration merging to make it known:

// merge into WindowEventMap
interface WindowEventMap {
    OnRewards: CustomEvent
}

Or, if you're inside a module (anything with export in it), use global augmentation:

// merge into WindowEventMap
declare global {
  interface WindowEventMap {
    OnRewards: CustomEvent
  }
}

Then use your code as before:

// no error!
window.addEventListener('OnRewards', (e: CustomEvent) => {
    // my code here
})

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So, those are your options. Which one you want to choose is up to you. Hope that helps; good luck!