an example of redux using 3 actions for a counter: increment, decrement, and reset.

counterReducer.ts

// ./counter/types.ts

export interface CounterState {
  value: number;
}

export const INCREMENT_COUNTER = "INCREMENT_COUNTER";
export const DECREMENT_COUNTER = "DECREMENT_COUNTER";
export const RESET_COUNTER = "RESET_COUNTER";

export interface IncrementCounterAction {
  type: typeof INCREMENT_COUNTER;
  step: number;
}

export interface DecrementCounterAction {
  type: typeof DECREMENT_COUNTER;
  step: number;
}

export interface ResetCounterAction {
  type: typeof RESET_COUNTER;
  value: number;
}

export type CounterActionTypes =
  | IncrementCounterAction
  | DecrementCounterAction
  | ResetCounterAction;

// ./counter/actions.ts

import {
  INCREMENT_COUNTER,
  DECREASE_COUNTER,
  RESET_COUNTER, 
  IncrementCounterAction,
  DecrementCounterAction,
  ResetCounterAction,
  CounterState
} from "./types";

export const incrementCounter = (step = 1): IncrementCounterAction => ({
  type: INCREMENT_COUNTER,
  step
});

export const decrementCounter = (step = 1): DecrementCounterAction => ({
  type: DECREMENT_COUNTER,
  step
});

export const resetCounter = (value = 0): ResetCounterAction => ({
  type: RESET_COUNTER,
  value
});

// ./counter/reducers.ts

import {
  INCREMENT_COUNTER,
  DECREASE_COUNTER,
  RESET_COUNTER, 
  IncrementCounterAction,
  DecrementCounterAction,
  ResetCounterAction,
  CounterState
} from "./types";

const initialState: CounterState = {
  value: 0
};

export const counterReducer = (
  state = initialState,
  action: CounterActionTypes
): CounterState => {
  switch (action.type) {
    case INCREMENT_COUNTER: {
      const { step } = action;

      return { ...state, value: state.value + step };
    }
    case DECREMENT_COUNTER: {
      const { step } = action;

      return { ...state, value: state.value - step };
    }
    case RESET_COUNTER: {
      const { value } = action;

      return { ...state, value };
    }
  }
};