What is the best way to explain to Typescript how to interpret a factory function that generates variable class definitions?

In my current project, I am utilizing Angular v16 and ngxs v18.0 simultaneously. The project consists of multiple grids that require a similar set of actions. To streamline this process, we attempted to create a factory function that would generate action class definitions with unique type properties. The function simply outputs an anonymous object with a specified interface, where each object property represents a class definition. Although the code functions correctly, I am struggling to make typescript recognize that the class definitions are properties of the object rather than a globally scoped namespace.

Below is the code snippet for the factory module:

export interface GridActions {
  Initialize: ActionDef,
  Fetch: ActionDef,
  FetchSuccess: ActionDef,
  FetchFailed: ActionDef,
  UpdatePagingParams: ActionDef,
  ToggleRowExpanded: ActionDef,
  ExpandRow: ActionDef,
  CollapseRow: ActionDef,
  ToggleExpandAll: ActionDef,
  UpdateFilter: ActionDef,
  UpdateSort: ActionDef
}

export class GridActionFactory {
  cache: {[key:string]:GridActions} = {};
  private static instance: GridActionFactory;

  static getActions(gridName: string): GridActions {
    // Code implementation here
  }
}

Furthermore, there is an ExampleActions.ts file, which includes the following code:

export const ExampleActions = {
  ActionableGrid: GridActionFactory.getActions('actionable-grid'),
  PendingGrid: GridActionFactory.getActions('pending-grid'),
  HistoricalGrid: GridActionFactory.getActions('historical-grid')
}

Subsequently, the usage of this code is as follows:

import { ExampleActions } from './example.actions';

const AtgActions = ExampleActions.ActionableGrid;
const HtgActions = ExampleActions.HistoricalGrid;

export const EXAMPLE_STATE_TOKEN = new StateToken<ExampleStateModel>('example');

@State({
  name: EXAMPLE_STATE_TOKEN,
  defaults: exampleStateModelDefaults,
})
@Injectable()
export class ExampleState {
  // Code implementation here
}

Answer №1

The typescript parser is throwing a compilation error for the action param of the action handler, stating that it cannot locate the namespace "AtgActions"

Indeed, because AtgActions is a value, not a type. It can be confusing as classes are both:

class Foo {}
const foo: Foo = new Foo();

Classes, besides their runtime value, define a type that represents the return type of their constructor (i.e. the type of instances created using the class).

However, this is not the case for objects, even if they only map names to classes and are returned by a static method of another class. Your factory does not return a class, but an object whose values are classes. To get its type, you would have to use typeof.

It will be necessary to change the method signature, but perhaps

(typeof AtgActions)['UpdatePagingParams']
could be a better substitution for AtgActions.UpdatePagingParams:

updateActionableGridPagingParams(ctx: StateContext<ExampleStateModel>, action: (typeof AtgActions)['UpdatePagingParams']) {

I have also attempted to update the GridActions interface to this

You will need to do that in any case if you want them to be type-safe. Referencing the definition of ActionDef, those type parameters default to any, so your original version is just filled with numerous anys.

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