Adapting to the Updated ESM/CJS Packaging in Redux v5.0.0

Anton Ioffe - January 10th 2024 - 10 minutes read

In the rapidly evolving landscape of JavaScript development, Redux v5.0.0 emerges as a beacon of transformation, foreshadowing the future of state management with its bold leap into ECMAScript modules and TypeScript. This article is your compass through the significant terrain Redux has charted, from the reimagined ESM/CJS packaging to the adoption of advanced TypeScript typings. We'll decode the intricate blueprint of codebase modernization, navigate through the potential pitfalls of module interplay, and equip you with strategies for refactoring that not only streamline your current projects but also fortify them against the tides of change. Gear up for a deep dive into the architectural decisions that are reshaping Redux, forging a path towards more robust, maintainable, and cutting-edge applications.

Unveiling the ESM/CJS Convergence in Redux v5.0.0

Redux v5.0.0 marks a decisive transition in module management, opting for the ECMAScript Modules (ESM) format as its primary artifact with redux.mjs leading the charge. This metamorphosis signifies more than just a change in file extension; it encompasses a re-envisioning of Redux's packaging approach. By adopting ESM, Redux taps into the benefits of static analysis and treeshaking, which are instrumental for minimizing bundle size, thereby optimizing performance. The shift embraces modern JavaScript's trajectory, ensuring Redux remains a staple in contemporary web development where module efficiency is paramount.

Simultaneously, Redux v5.0.0 maintains a commitment to backward compatibility by supporting the CommonJS (CJS) format. This dual-module strategy ensures that developers locked into legacy systems aren't left adrift. However, the implications are twofold: developers must now be more deliberate in configuring their build systems to correctly resolve the appropriate module format, as automatic resolution may be less reliable. The traditional require() calls syncing with .js files will serve CJS modules, while new code leveraging import statements expects the .mjs ESM variant.

The Redux team's implementation leverages the exports field in package.json, clearly delineating the resolution paths for different module formats. This small, yet powerful, addition to the package descriptor plays a critical role. It guarantees that the correct entry points are selected depending on the consuming environment. This explicit definition harmonizes the consumption across varying setups—be it a modern environment beckoning ESM or a stable, older system reliant on CJS—reducing the ambiguity that often leads to module resolution errors.

The integration of ECMAScript 2020 features into Redux's codebase serves as a testament to the library's forward-leaning posture. Use of advanced language constructs like optional chaining and object spreads exemplifies Redux's adaptation to the latest JavaScript standards. By nurturing this modern syntax, Redux not only hones its edge in performance but also accentuates the developer experience by offering concise, powerful, and expressive code patterns.

In the balancing act between lean module design and comprehensive support, Redux v5.0.0 dispenses with Universal Module Definition (UMD) builds, emphasizing the evolution towards a dual ESM/CJS consumable library. This thoughtful omission acknowledges the shifting trends in module consumption, foreseeing a drop in UMD's relevance as both Node.js and browser environments progress. Redux thus streamlines its offering, providing a redux.browser.mjs targeted at direct consumption through script tags, complementing the core ESM-centric thrust while catering to those utilizing HTTP/2's efficient loading capabilities.

Embracing TypeScript: Redux's Type-Safe Evolution

With the advent of Redux v5.0.0, developers are introduced to a paradigm where the type safety and maintainability of state management are bolstered by TypeScript’s rigorous typing system. Emphasizing compile-time error checking, TypeScript's integration with Redux encourages proactive error prevention and promotes codebase sustainability. The driving principle in this transition is that enhanced type annotations result in more predictable behavior and more straightforward state flow analysis, aligning with modern development expectations for dependable applications.

The age of the AnyAction type in Redux, which offered generic flexibility at the expense of type safety, is giving way to a more precise and structured approach. The previously favored AnyAction is now deprecated in preference for UnknownAction, which compels developers to define explicit type guards. This change is aligned with TypeScript's intent of equipping developers to author applications where actions are robustly defined, narrowing the potential for type-related errors and fortifying the code against unpredictable behaviors.

Consider the approach familiar to Redux developers when handling actions within reducers. Typically, one would pattern the action handling like this:

import { AnyAction } from 'redux';

function rootReducer(state: StateType, action: AnyAction): StateType {
    if (action.type === 'SPECIFIC_ACTION') {
        // Pre TypeScript, accessing any property was permitted
        return { ...state, data: action.payload };
    return state;

To align with the stringent typing of Redux v5.0.0, the code must mature into using type definitions and guards for actions:

import { UnknownAction, Action } from 'redux';
import { StateType, SpecificAction } from './types'; // Hypothetical imports

function isSpecificAction(action: UnknownAction): action is SpecificAction {
    return action.type === 'SPECIFIC_ACTION';

function rootReducer(state: StateType, action: Action): StateType {
    if (isSpecificAction(action)) {
        // Action is verified and typed as SpecificAction
        console.log(action.payload); // Types are confidently asserted
        return { ...state, data: action.payload };
    return state;

In middleware, where the type safety of next and action was previously unspecified, Redux v5.0.0 ushers in a necessary evolution. Adhering to TypeScript, middleware now treats these parameters as of unknown type, requiring explicit type checks or guards:

import { MiddlewareAPI, Dispatch, UnknownAction } from 'redux';

const exampleMiddleware = (api: MiddlewareAPI) => (next: Dispatch) => (action: UnknownAction) => {
  if (isSpecificAction(action)) {
    // Action must be validated before usage
    console.log('Dispatching specific action:', action);
  return next(action);

This diligent approach mitigates risk in the middleware chain by using TypeScript's stringent typing to affirm the integrity of dispatched actions.

As Redux charts its course into a future governed by TypeScript's type safety, developers need to engage in introspection regarding these adjustments. How do stricter type definitions shape the longevity and adaptability of a Redux codebase? Does the reduction of runtime errors via strict type enforcement compensate for the increased initial development overhead? Such questions challenge seasoned developers to balance the trade-offs inherent in adopting this forward-looking, type-centric paradigm, which promises to elevate standards of state management in JavaScript applications.

Codebase Modernization: New Patterns and Practices

Adopting the latest ES2020 constructs in Redux v5.0.0 stands as a testimonial to the deliberate progression towards a more concise and expressive syntax conducive to the modern JavaScript landscape. Take, for instance, the optional chaining operator (?.), which streamlines the process of accessing deeply nested object properties. This operator eliminates the need for verbose and abundant null checks, making the code more readable and less prone to errors of inadvertently accessing undefined. Concretely, in a Redux scenario where selectors might deal with complex state shapes, optional chaining enhances code legibility while ensuring robustness.

// Redux selector without optional chaining
function selectUserDetails(state) {
    if (state && state.user && state.user.details) {
        return state.user.details;
    return null;

// Redux selector with optional chaining
function selectUserDetails(state) {
    return state?.user?.details ?? null;

Parallel to this, the introduction of the object spread operator lends exceptional clarity to the mutation of state within reducers. Pre-ES2020 code often required object assignments or utility functions like Object.assign() to create copies of the state. With the object spread operator, we now have a more intuitive and shorter syntax that explicates the intent of creating new state objects without mutating the original state.

// Pre-ES2020 Redux reducer
function userReducer(state = {}, action) {
    switch (action.type) {
        case 'UPDATE_USER':
            return Object.assign({}, state, { name: });
            return state;

// Redux reducer using object spread
function userReducer(state = {}, action) {
    switch (action.type) {
        case 'UPDATE_USER':
            return {...state, name:};
            return state;

However, while the richness of the ES2020 syntax introduces elegance and brevity, it brings about considerations for performance. JavaScript engines efficiently optimize these newer constructs, yet the impact on performance is nuanced and context-dependent. For instance, excessive reliance on optional chaining might incur a penalty when used in performance-critical code paths, as the runtime checks could outweigh the benefits. Therefore, developers should wield these features judiciously, harmonizing the gains in legibility with the potential for performance bottlenecks.

Furthermore, it is imperative to acknowledge how these syntactic enhancements intersect with modularity and reusability. While they empower developers to write more declarative Redux reducers and selectors, misplaced reliance on them may lead to an over-abundance of terse but cryptic code literate only to those deeply familiar with Redux patterns. When a balance is struck, such code becomes a testament to both maintainability and developer ergonomics.

The enhancements within Redux v5.0.0 invite thoughtful deliberation on the architectural cadence of our applications. In what ways can the introduction of contemporary JavaScript features boost the expressiveness of your state management? How might they stimulate or, inversely, impede the reusability and modularity of your Redux slices? These are the contemplations that not only navigate the course of a single codebase's evolution but concurrently steer the trajectories of development teams riding the crest of JavaScript's unceasing innovation.

Pitfalls and Best Practices in ESM/CJS Integration

When integrating ESM and CJS modules within Redux v5.0.0, developers may encounter pitfalls if default and named imports are confused. Misinterpreting module syntax could lead to errors in binding module exports. For instance, attempting to destructure a default export from a CommonJS module as a named import is incorrect:

// Incorrect: Treating a CommonJS default export incorrectly as a named import
import { createStore } from 'redux/cjs/redux.js';

function configureStore() {
    // createStore is not a named export and may be undefined

The correct usage involves importing the default export from the CommonJS module before destructuring:

// Correct: Importing the CommonJS default export
import redux from 'redux/cjs/redux.js';

function configureStore() {
    const { createStore } = redux;
    return createStore( /* ... */ );

Directly importing CommonJS middleware into an ESM-configured Redux can lead to runtime issues:

// Incorrect: Direct use of a CommonJS middleware in an ESM setup
import thunkMiddleware from 'redux-thunk/cjs/redux-thunk.js';

Instead, interoperation is achieved through dynamic import which reconciles the ESM and CJS divide:

// Correct: Adapting a CommonJS middleware for ESM
let thunkMiddleware;

(async () => {
    thunkMiddleware = (await import('redux-thunk/cjs/redux-thunk.js')).default;

Tree shaking is less effective when Redux selectors are grouped inside an object due to ESM's static nature:

// Incorrect: Grouping selectors hinders tree shaking
export const selectors = {
  selectUser: state => state.user

Exporting selectors individually facilitates better tree shaking:

// Correct: Facilitating tree shaking with individual exports
export const selectUser = state => state.user;

Mixing ESM and CJS without a resolution strategy can result in module collisions and interoperability issues:

// Incorrect: Combining module syntax without resolution causes conflicts
const createStore = require('redux').createStore;
const reducer = require('./reducer'); // Assuming CJS
const thunkMiddleware = require('redux-thunk').default; // Also CJS

export const store = createStore(reducer, applyMiddleware(thunkMiddleware));

Employing a consistent approach for remixing syntax can avoid such issues:

// Correct: Unifying import statements for harmony between module systems
import { createStore, applyMiddleware } from 'redux';
import reducer from './reducer-cjs-adapter.js'; // Adapted CJS
import getThunkMiddleware from './redux-thunk-esm-wrapper.js'; // CJS wrapped for ESM

export const store = createStore(reducer, applyMiddleware(getThunkMiddleware()));

Ensure each module integration preserves the modular structure that underlines Redux's architecture, being particularly mindful when channelling between ESM and CJS to maintain integrity and compatibility.

Strategic Refactoring for Legacy and Future-Proofing

As the Redux library evolves, aligning with its current direction treats not only today's problems but prepares your codebase for the future. With Redux v5.0.0, the once-standard createStore is now deprecated in favor of configureStore from Redux Toolkit. This transition is a significant step towards leveraging the efficiencies of modern Redux, as configureStore streamlines setup and prescribes best practices. Critical refactoring should replace:

import { createStore, combineReducers } from 'redux';
import rootReducer from './reducers';

const store = createStore(
    // Possible middleware or enhancer

with the updated Redux Toolkit approach:

import { configureStore } from '@reduxjs/toolkit';
import rootReducer from './reducers';

const store = configureStore({
    reducer: rootReducer,
    // Middleware and enhancers are handled automatically

The shift to configureStore simplifies the initialization logic, empowering developers to focus on building features rather than configuring the store. The automatic provision of essential middleware, dev-tools extension integration, and out-of-the-box performance enhancements are key at driving a uniform, conducive development environment across varied projects.

Refactoring your Redux implementation to align with v5.0.0 isn't just about adopting new API methods; it’s also about setting a course for easier maintainability and scalability. Consider the shift as shedding outdated methodologies to embrace practices that are scalable and modular. Critically assessing each piece of the Redux integration, such as reducer composition, state selection, and middleware implementation, should now be seen through the lens of the new capabilities and APIs provided by Redux Toolkit.

While simplifying and future-proofing are apparent benefits, it's essential to understand that with these refactorings, the performance of your application may also be indirectly influenced. Redux Toolkit's defaults are designed for broad use-cases and include what's generally considered beneficial middleware, such as redux-thunk for async logic and redux-immutable-state-invariant for catching accidental mutations in development mode. However, you should evaluate whether these defaults align with your application's specific needs and adjust accordingly.

Finally, staying up-to-date with Redux's latest offerings must be balanced with the unique requirements of your existing codebase. Refactor incrementally, methodically testing changes to ensure they yield positive improvements. It's not purely about being modern; it’s about strategically placing your project to benefit from current and future advancements in the rapidly progressing JavaScript ecosystem. Thoughtful refactoring can transform an outdated codebase into a contemporary model of JavaScript development, poised to adapt and thrive amidst the continuous evolution of the web landscape.

Approaching the refactor with a strategic eye for both immediate improvements and long-term benefits sets a precedent for continuous learning and adaptation within your development team. How will this transition influence team practices, and what new opportunities for learning and growth will arise? Keeping an anticipatory stance in the face of change not only maintains the vitality of your codebase but also enriches the collective expertise of your team.


The article explores the updated ESM/CJS packaging in Redux v5.0.0 and its implications for JavaScript developers. It highlights the benefits of adopting ECMAScript modules for performance optimization and the integration of TypeScript for enhanced type safety. The article also discusses new patterns and practices in codebase modernization, as well as potential pitfalls and best practices for ESM/CJS integration. A key takeaway is the importance of strategic refactoring to future-proof codebases, with a challenging task for readers to refactor their Redux implementation using the updated configureStore method from Redux Toolkit. This task encourages developers to adapt to the latest Redux version and leverage its efficiencies for simpler, more maintainable, and scalable code.

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