Redux-thunk вложенные функции

Question

Почему функция Redux-thunk вложена с функциями (next) и (action)

function createThunkMiddleware(extraArgument) {
      return ({ dispatch, getState }) => (next) => (action) => {
        if (typeof action === 'function') {
          return action(dispatch, getState, extraArgument);
        }

        return next(action);
      };
    }

    const thunk = createThunkMiddleware();
    thunk.withExtraArgument = createThunkMiddleware;

    export default thunk;