Const Typed Reducer

const-typed-reducer.ts

// counter.reducer.ts
import * as CounterActions from './counter.actions';

export type Action = CounterActions.All;

export function reducer(state: number = 0, action: Action): State {
  switch(action.type) {
    case CounterActions.INCREMENT: {
      return state + 1;
    }
    case CounterActions.DECREMENT: {
      return state - 1;
    }
    case CounterActions.RESET: {
      return action.payload; // typed to number
    }
    default: {
      return state;
    }
  }
}