How Redux Implements Custom Middleware

In Redux, middleware is a powerful mechanism that allows developers to insert custom logic before an action is dispatched to the reducer. Creating a custom Redux middleware involves writing a function that returns another function adhering to the Redux middleware API specification.

I will demonstrate how to implement a simple logger middleware that logs information to the console when an action is dispatched.

Here are the basic steps to create a custom Redux middleware:

1. Write a function that receives the dispatch and getState methods from the store.
2. This function returns another function that receives the next function for the subsequent middleware.
3. The returned function then returns another function that receives the action.
4. Within the innermost function, you can execute custom logic and then call next(action) to pass the action to the next middleware or reducer in the chain.

Here is an example of a custom logger middleware:

javascript
// Custom logger middleware
const loggerMiddleware = store => next => action => {
  // Custom logic: Log the action before it is processed
  console.log('dispatching', action);

  // Call the next middleware or reducer in the chain
  let result = next(action);

  // Custom logic: Log the new state after the action is processed
  console.log('next state', store.getState());

  // Return result, as the middleware chain requires the return value from next(action)
  return result;
};

export default loggerMiddleware;

In the above middleware code:

• store: Redux store instance, which contains the dispatch and getState methods.
• next: is a function that passes the action to the next handler in the chain (middleware or reducer).
• action: is the current action object being processed.

The typical way to use this middleware is by applying it when creating the Redux store:

javascript
import { createStore, applyMiddleware } from 'redux';
import rootReducer from './reducers';
import loggerMiddleware from './middleware/loggerMiddleware';

// Apply applyMiddleware to enhance the store with the custom loggerMiddleware
const store = createStore(
  rootReducer,
  applyMiddleware(loggerMiddleware)
);

export default store;

In this example, any action dispatched to the store will first pass through the loggerMiddleware, where its information is logged to the console, and then continue through the middleware chain until it is finally processed by the reducer.

This is a simple example of custom middleware, but you can implement more complex logic within the middleware as needed, such as asynchronous operations, route navigation, or any other custom behavior you desire.