Understanding Redux Middleware: Concepts, Implementation, and Custom Middleware Creation

Redux middleware is a pluggable mechanism that allows interception and processing of actions before they reach the reducer, enabling enhancements such as logging, async operations, and authentication.

What is Middleware: Middleware is a higher‑order function that receives store , returns a function that receives next , which in turn returns a function that receives action . It can execute logic before and after calling next(action) .

Usage: To apply middleware, use applyMiddleware when creating the store: import { createStore, applyMiddleware } from 'redux'; import rootReducer from './reducers'; import middleware1 from './middleware/middleware1'; import middleware2 from './middleware/middleware2'; const store = createStore(rootReducer, applyMiddleware(middleware1, middleware2));

Core mechanics: applyMiddleware curries each middleware with middlewareAPI (providing getState and dispatch ), then uses compose to chain them together, ultimately enhancing the store’s dispatch method. The simplified compose implementation: export default function compose(...funcs) { if (funcs.length === 0) return arg => arg; if (funcs.length === 1) return funcs[0]; return funcs.reduce((a, b) => (...args) => a(b(...args))); }

Custom middleware example – a simple logger: const loggerMiddleware = storeAPI => next => action => { console.log('dispatching', action); let result = next(action); console.log('next state', storeAPI.getState()); return result; };

By wrapping dispatch with composed middleware, Redux enables flexible, predictable state updates while preserving its core principles of a single immutable state tree and pure reducer functions.