Document authoring mixins in TypeScript

[web-padawan]

I have been looking for a small example of how to create a mixin, but there wasn't one.

So far I was able to come up with the following snippet:

import { LitElement, property } from 'lit-element';

type Constructor<T = object> = {
  new (...args: any[]): T;
  prototype: T;
};

export interface ReadonlyInterface {
  readonly: boolean;
}

export const ReadonlyMixin = <T extends Constructor<LitElement>>(
  base: T
): T & Constructor<ReadonlyInterface> => {
  class ReadonlyMixin extends base implements ReadonlyInterface {
    @property({ type: Boolean }) readonly: boolean = false;
  }
  return ReadonlyMixin;
};

Some things that I learned before I was able to make it work:

• I started by tweaking the code from model-viewer (which has mixins) 🤓
• I found this article useful to understand the Constructor type 🔖
• I stumbled into Decorators not allowed classes expressions microsoft/TypeScript#7342 and False error TS4094: ... exported class expression may not be private or protected. microsoft/TypeScript#30355 (as well as related PR at Revert mixin type inference google/model-viewer#761) 🤔
• Finally, I found a working example at stackoverflow 🙂

Overall it was a bit challenging to figure that out 😅 So the example would help a lot.

@justinfagnani @cdata can you please confirm if the code snippet above is correct? Also, do you have any other guidelines about how mixins should be written in TypeScript?

[tpluscode]

here's how I have been defining my mixing:

import { LitElement, property } from 'lit-element';

// or HTMLElement or other base class
type Constructor<T> = new (...args: any[]) => LitElement

interface ReadonlyInterface {
  readonly: boolean;
}

type ReturnConstructor = new (...args: any[]) => LitElement & ReadonlyInterface

export function ReadonlyMixin<B extends Constructor> (Base: B): B & ReturnConstructor {
    class Mixin extends Base implements ReadonlyInterface {
        @property({ type: Boolean }) readonly: boolean = false;
    }

    return Mixin
}

Looks similar but I wonder if the differences are significant. 🤔

[balloob]

TypeScript has added support for the mixin pattern. However, it does not work with the constructor type that is exported by LitElement (and which is also the one referenced in the first post of this issue).

The TS compatible constructor type is

export type Constructor<T = {}> = new (...args: any[]) => T;

If you use that compatible constructor, you don't need to specify an output type, as TypeScript will be able to derive it.

For an example of this pattern, see https://github.com/home-assistant/home-assistant-polymer/blob/dev/src/mixins/subscribe-mixin.ts

@justinfagnani, do you know why the constructor type of LitElement is the way it is?

[abdonrd]

I'm also interested on this! Right now I have:

type Constructor<T> = new (...args: any[]) => T;

export const myMixin = () => <T extends Constructor<HTMLElement>>(
  baseElement: T
) => {
  class MyMixin extends baseElement {
    @property({ type: Object })
    public data: any;

    // ...
  }

  return MyMixin;
};

[web-padawan]

A follow-up question: how do we annotate protected methods defined on the mixin, in a way that would make them overridable by the class extending that mixin?

So far I only found the workaround with a dumb clas: microsoft/TypeScript#25163 (comment)

Here is the code example illustrating what I want to accomplish:
vaadin/component-mixins@852bce2#diff-0838769eda184397106cb15d225e1bdbR8-R21

@justinfagnani @rictic I would really appreciate any advice on how to handle this case.

[cdata]

@balloob I'm interested to know if you are able to build with "declarations": true in your tsconfig.json.

For <model-viewer>, we briefly landed a change that moved from specifying a separate interface to relying on similar inference, but we ran into a wall because it would not compile if you configure the compiler to generate declaration files. See google/model-viewer#761 for context.

[balloob]

@cdata correct, unable to build if we set declarations to true 😞. We're building an app and not a lib, so don't need declarations.

[web-padawan]

Regarding the above question, I submitted my research regarding available workarounds for TS mixins & protected methods to microsoft/TypeScript#17744 (comment) so we can discuss it there.

[trusktr]

It should be documented that if we use mixin classes, we must forgo the ability to publish them in a library with declaration files. microsoft/TypeScript#35822

[web-padawan]

The new docs website has a dedicated page:
https://lit.dev/docs/composition/mixins/#mixins-in-typescript

Closing as fixed.