"Property does not exist" error on union type even though all union members implement interface

[distinctdan]

TypeScript Version: 3.0.1

Search Terms: Property does not exist union type implements interface

Code

interface IGameObject {
    onEvent?: (event) => void;
}

class Player implements IGameObject {}
class Enemy implements IGameObject {}

type MovingObjects = Player | Enemy;

const movingObjs: MovingObjects[] = [];

for (const obj of movingObjs) {
    // Error: "Property 'onEvent' does not exist on type 'MovingObjects[]'.""
    if (movingObjs.onEvent) {
        movingObjs.onEvent('someEvent');
    }
}

Expected behavior:
I would expect to be able to check if the method onEvent is present on the objects because they implement the interface, even though none of them implement the method yet. Some of them may implement it in the near future, so I want this check there to be safe. In my actual code, the interface also has non-optional properties that the classes do implement.

Actual behavior:
The compiler throws an error for "Property 'onEvent' does not exist on type 'MovingObjects[]'".

Playground Link:
https://www.typescriptlang.org/play/#src=interface%20IGameObject%20%7B%0D%0A%20%20%20%20onEvent%3F%3A%20(event)%20%3D%3E%20void%3B%0D%0A%7D%0D%0A%0D%0Aclass%20Player%20implements%20IGameObject%20%7B%7D%0D%0Aclass%20Enemy%20implements%20IGameObject%20%7B%7D%0D%0A%0D%0Atype%20MovingObjects%20%3D%20Player%20%7C%20Enemy%3B%0D%0A%0D%0Aconst%20movingObjs%3A%20MovingObjects%5B%5D%20%3D%20%5B%5D%3B%0D%0A%0D%0Afor%20(const%20obj%20of%20movingObjs)%20%7B%0D%0A%20%20%20%20%2F%2F%20Error%3A%20%22Property%20'onEvent'%20does%20not%20exist%20on%20type%20'MovingObjects%5B%5D'.%22%22%0D%0A%20%20%20%20if%20(movingObjs.onEvent)%20%7B%0D%0A%20%20%20%20%20%20%20%20movingObjs.onEvent('someEvent')%3B%0D%0A%20%20%20%20%7D%0D%0A%7D%0D%0A

Related Issues:

[DanielRosenwasser]

Unfortunately, implementing an interface doesn't actually declare those members in the class. You'll need to explicitly declare onEvent (even as an optional member) in Player and Enemy.

[typescript-bot]

This issue has been marked 'Working as Intended' and has seen no recent activity. It has been automatically closed for house-keeping purposes.