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MixinTarget type

A replacement for typeof Target to be used in mixin class definitions. This is a workaround for TypeScript limitation described in - https://github.com/microsoft/TypeScript/issues/17293 - https://github.com/microsoft/TypeScript/issues/17744 - https://github.com/microsoft/TypeScript/issues/36060

Signature:

export declare type MixinTarget<T extends object> = Constructor<{
    [P in keyof T]: T[P];
}>;

Example

export function MyMixin<T extends MixinTarget<Application>>(superClass: T) {
  return class extends superClass {
    // contribute new class members
  }
};

TypeScript does not allow class mixins to access protected members from the base class. You can use the following approach as a workaround:

// @ts-ignore
(this as unknown as {YourBaseClass}).protectedMember

The directive @ts-ignore suppresses compiler error about accessing a protected member from outside. Unfortunately, it also disables other compile-time checks (e.g. to verify that a protected method was invoked with correct arguments, and so on). This is the same behavior you would get by using Constructor<any> instead of MixinTarget<Application>. The major improvement is that TypeScript can still infer the return type of the protected member, therefore any is NOT introduced to subsequent code.

TypeScript also does not allow mixin class to overwrite a method inherited from a mapped type, see https://github.com/microsoft/TypeScript/issues/38496 As a workaround, use @ts-ignore to disable the error.

export function RepositoryMixin<T extends MixinTarget<Application>>(
superClass: T,
) {
  return class extends superClass {
   // @ts-ignore
   public component<C extends Component = Component>(
     componentCtor: Constructor<C>,
     nameOrOptions?: string | BindingFromClassOptions,
   ) {
     const binding = super.component(componentCtor, nameOrOptions);
     // ...
     return binding;
   }
}