How to define a type of class implementing an interface

[joostvdwsd]

Affected Languages

• TypeScript or Javascript
• Python
• Java
• .NET (C#, F#, ...)
• Go

General Information

• JSII Version: 1.30.0 (build adae23f), typescript 3.9.9
• Platform: macos

The Question

I like to define a type that is a subclass of class x and follows interface Y. This is a pretty standard oo pattern but I can't seem to find a way where the JSII compiler will be happy.

Attempt 1 (Works in typescript but not jsii)

export class Foo {
  bar: string
}

export interface IActivatable {
  activate();
}

export interface ActiveFoo extends Foo, IActivatable {
}

function test(activeFoo: ActiveFoo) {
}

This will give the jsii error: "Type "Foo" cannot be used as an interface"

Attempt 2 (Works in typescript but not jsii)

export class Foo {
 bar: string
}

export interface IActivatable {
  activate();
}

export type ActiveFoo = Foo & IActivatable

function test(activeFoo: ActiveFoo) {
}

The above example gives the jsii error: "Non-primitive types without a symbol cannot be processed."

Also attempt # 2 but with the type definitions in generics will give the same error.

Attempt 3 (Works but looses type enforcement)

export class Foo {
 bar: string
}

export interface IActivatable {
  activate();
}

export function isActivatable(arg: any) : arg is IActivatable {
  return (arg as IActivatable).activate !== undefined;
}

function test(activeFoo: Foo) {
  if (isActivatable(activeFoo)) {
  }
}

This last one looses the type enforcement and its not clear that it will check for the interface type.

Question

Is there any way to achieve the type enforcement in the test function for jsii?

[webratz]

Did you ever find a solution for this?

[chub-jancel]

I'd like to hear more on this topic and the other references floating around for this particular issue. Is the third option what most are going with and is there a gist of a generic approach? How did you solve it?