Docs: Mixins with static properties

[ackvf]

Forgive me for writing an issue instead of filing a Pull Request. I don't have enough experience with TS classes and Declaration merging to write a viable solution.

Page URL:
https://www.typescriptlang.org/docs/handbook/mixins.html
https://github.com/microsoft/TypeScript-Website/blob/v2/packages/handbook-v1/en/Mixins.md

Issue:

• Inheritance via Mixins doesn't respect static properties on base class.
• An interface which extends from class that defines static properties will ignore those s.p. since interface cannot define s.p. and in this case, the class is being treated as an interface, which without any warning exhibits this behavior.

  This treats the classes as interfaces

While the mixin function can be easily fixed, this doesn't hold true for the interface extending a class, which in turn renders the solution impractical when there are static properties.

Recommended Fix:

• Mention these shortages on Mixin doc page.
• Possible solution with implements? - class SmartObject implements Disposable, Activatable {}

And I have one follow-up question, which I would be happy to file an issue in TS repo if you deem it worthy: Would it be possible and does it make sense to explicitly warn when class that is being treated as interface foregoes its static properties?

[orta]

Yeah, I think the mixin page probably needs a list of caveats at the end and a few more non-trivial examples. There are a few people who have a lot of experience with using mixins in TS, and maybe I can round them up to try get that all documented

The static properties issue sounds like a bug, and I couldn't find an issue on it (maybe I missed it, give it a look before making something with a sample )

[ackvf]

Actually from what I read when trying to make the inheritance work in various ways, interfaces don't support static properties and when class is being treated as an interface which is the case in the example in mixins doc page, the static properties are ignored.

It is indeed an unexpected behavior without prior knowledge and can cause hours of debugging.

I would still like to get notified in the IDE about this though, because this is rather subtle feature.

[basarat]

Because it only copies members from prototype it only copies methods.

It also misses initialized properties:

import { applyMixins } from "./applyMixins";

class Disposable {
  isDisposed: boolean = false;
  dispose() {
    this.isDisposed = true;
  }
}

class Activatable {
  isActive: boolean = false;
  activate() {
    this.isActive = true;
  }
  deactivate() {
    this.isActive = false;
  }
}

class SmartObject {
  // ...
}

interface SmartObject extends Disposable, Activatable { }
applyMixins(SmartObject, [Disposable, Activatable]);

console.log(new SmartObject().isActive); // `undefined`. Expected `false`

And would also miss anything initialised in the constructors.

Also strict issue: #682

Thoughts

I feel like the caveat should be "don't use mixins by prototype mutation" 😂 🌹

[orta]

Alright, the mixins docs got a rewrite in #719 👍