log-enhancer.js

const logEnhancer = (createStore) => (reducer, initialState, enhancer) => {
  // Do stuff like wrap the reducer in a higher-order function.
  const reducerWithConsoleLogs = (previousState, action) => {
    const nextState = reducer(previousState, action);
    console.log({ action, previousState, nextState });
    return nextState;
  };

  return createStore(reducerWithConsoleLogs, initialState, enhancer);
};

monitor-middleware.js

const monitorMiddleware = (store) => (next) => (action) => {
  const start = performance.now();
  next(action);
  const end = performance.now();
  const diff = round(end - start);

  console.log("reducer process time:", diff);
};