What is the difference between `mapStateToProps()` and `mapDispatchToProps()`?

mapStateToProps() is a utility which helps your component get updated state (which is updated by some other components):

const mapStateToProps = (state) => {
  return {
    todos: getVisibleTodos(state.todos, state.visibilityFilter),
  };
};

mapDispatchToProps() is a utility which will help your component to fire an action event (dispatching action which may cause change of application state):

const mapDispatchToProps = (dispatch) => {
  return {
    onTodoClick: (id) => {
      dispatch(toggleTodo(id));
    },
  };
};

Recommend always using the “object shorthand” form for the mapDispatchToProps

Redux wrap it in another function that looks like (…args) => dispatch(onTodoClick(…args)), and pass that wrapper function as a prop to your component.

const mapDispatchToProps = {
  onTodoClick,
};