Custom Middleware

Custom Middleware Overview

At its core, custom middleware is about doing one thing well in Redux. Once you understand the pattern, you can apply it consistently across projects and teams.

Good custom middleware pays off across the whole project: fewer surprises, easier collaboration, and smoother onboarding. The snippet below is a solid starting point.

import { createListenerMiddleware } from '@reduxjs/toolkit';
import { increment } from './counterSlice';

export const listener = createListenerMiddleware();

listener.startListening({
  actionCreator: increment,
  effect: async (action, api) => {
    console.log('incremented', api.getState());
  },
});

Listener middleware reacts to dispatched actions to run side effects, replacing many custom thunks.

Custom Middleware Example

const slice = createSlice({ name, initialState, reducers });
const store = configureStore({ reducer: { key: slice.reducer } });
// dispatch(slice.actions.something())

• Start from a minimal Custom Middleware example and grow it only as needed.
• Keep things explicit so Custom Middleware behaves the same for everyone on the team.
• Name things clearly so teammates understand your Custom Middleware at a glance.
• Verify Custom Middleware works as expected before relying on it in important work.

Redux Cheatsheet

Quick Redux Toolkit reference related to custom middleware.

How Custom Middleware Works in Redux

Custom Middleware follows Redux's predictable data flow: components dispatch actions, reducers compute the next state from the previous state and the action, and subscribed components re-render.

Listener middleware reacts to dispatched actions to run side effects, replacing many custom thunks.

• State lives in a single, read-only store.
• Actions are the only way to describe a change.
• Reducers are pure functions that return the next state.
• Redux Toolkit removes most boilerplate with slices and thunks.

Practical Guidance for Custom Middleware

In modern apps, custom middleware should use Redux Toolkit rather than hand-written Redux. Keep state minimal, colocate logic in slices, and use RTK Query for server data.

Common Mistakes

• Skipping edge cases and error handling when using custom middleware.
• Not verifying the result of custom middleware before moving on.
• Over-complicating custom middleware before you actually need the extra flexibility.
• Ignoring documentation, which makes custom middleware hard for the next person to follow.

Key Takeaways

• Custom Middleware is a core part of working effectively with Redux.
• Start small and keep custom middleware focused on a single goal.
• Apply consistent patterns so custom middleware scales across your project.
• Practise and document custom middleware to keep your workflow maintainable.