Type safe reducer

Counter.tsx

type Increment = {
  type: 'increment';
  incrementStep: number;
};
type Decrement = {
  type: 'decrement';
  decrementStep: number;
};
type Actions = Increment | Decrement;

const reducer = (state: State, action: Actions): State => {
  switch (action.type) {
    case 'increment':
      return { count: state.count + action.incrementStep };
    case 'decrement':
      return { count: state.count - action.decrementStep };
  }
};