[lylepstein]

Tagged unions are indeed pretty helpful for Redux reducers. One thing to note is that you can actually skip some code here as well. Specifically, the constant definitions:

  export type INCREMENT_COUNTER = 'App/INCREMENT_COUNTER';
  export const INCREMENT_COUNTER : INCREMENT_COUNTER = 'App/INCREMENT_COUNTER';

These aren't necessary, you can use those type strings directly:

  export interface IncrementCounterAction = {
      type: 'App/INCREMENT_COUNTER',
      by: number
  };

In vanilla, untyped Redux the constants are a good idea because they give you some safety and IDE-ability. With Typescript they're just a waste of code, since TS is going to check those type strings for you. Cut out the middleman!

[WayneBro]

Could you then do the case statements in the reducer like this?

    switch (action.type) {
    case IncrementCounterAction.type:
        return state.update(...);
    case DeccrementCounterAction.type:
        return state.update(...);
    ....
    }

[lylepstein]

I believe you can, though I haven't tried it. Typescript will actually give you completion support for plain strings inside that switch statement, which is normally what I go with.