Redux Toolkit's createReducer: Building Reducers for Real-Time Data Streams

Anton Ioffe - January 11th 2024 - 11 minutes read

In the ever-evolving landscape of modern web applications, managing state with agility and precision has become indispensable, especially when grappling with the frenetic pace of real-time data streams. Within this article, we'll journey through the heart of the Redux Toolkit's createReducer function, an ingenious tool that elegantly crafts efficiency into the art of reducer design. By charting a course from establishing the fundamentals to dissecting advanced scenarios, we invite seasoned developers to refine their reducible prowess, transform challenges into triumphs, and reimagine what's possible in the conductor's seat of dynamic state management. Prepare to unravel coding narratives brimming with robust examples, performance tunings, and thought experiments that will equip you to harness the true power of createReducer in the theater of real-time updates.

The Redux Toolkit and Real-Time Data: Mastering createReducer for Dynamic State

Managing state for modern web applications often entails handling rapidly changing data, such as real-time streams that update the UI incessantly. This poses a challenge in ensuring that state updates are handled efficiently and reliably. Redux Toolkit's createReducer function stands out as a key player in simplifying this process. It uses the Immer library to enable seemingly mutable logic while safely producing immutable state updates under the hood. This immutability is vital for real-time applications where shared state is frequently updated from various sources.

The design of createReducer allows developers to map actions to state changes explicitly. For instance, when dealing with a stream of data, such as live messages or stock prices, developers can dispatch actions for every new data point received. Using createReducer, the handling of such actions becomes intuitive as it accepts an initial state and an object where keys are action types and values are reducer functions. This approach reduces complexity and increases readability, leading to more maintainable code when dealing with complex real-time data flows.

One of the main benefits of using createReducer for managing real-time data streams is the significant reduction in boilerplate code. Traditional Redux requires the manual construction of immutable state updates, which can become error-prone as the number of actions grows. createReducer abstracts this aspect, allowing developers to focus on the actual state transformations. The syntax is straightforward, mapping action types to reducer functions that describe how the state changes. This results in a clean and declarative way of updating the state that aligns well with the needs of applications requiring high-throughput and low-latency state management.

However, while the abstraction provided by createReducer is powerful, it's essential for developers to understand common mistakes that can arise. One such mistake is handling side effects within reducers. Despite the temptation to perform asynchronous operations or impure actions within a reducer function, it's crucial to remember that reducers must remain pure functions. To adhere to this principle, side effects should be managed in middleware like Redux Thunk, keeping reducers solely for computing the new state.

When thinking about the usage of createReducer with real-time data, consider scenarios where state updates are frequent and need to be reflected immediately in the UI. How would createReducer enhance the process of managing this state, in contrast to traditional reducer constructions? Consider the implications on performance, especially with regards to the reconciliation process in React, and how efficiently the state updates are batched and rendered. The seamless integration with Redux Toolkit paves the way for more efficient real-time state management, especially when paired with carefully thought-out action dispatching strategies and middleware utilization.

Scaffolding Reducers for Real-Time Updates

Writing efficient reducers for real-time data streams in JavaScript applications requires a mindful approach to ensure updates are handled swiftly without compromising performance and memory efficiency. Real-time applications like chat platforms or live stock tickers necessitate reducers to deal with high-frequency updates which may often occur in bursts or continuous flows. This high velocity and volume of state changes pose unique challenges; hence, a reducer must be well-architected to efficiently integrate these updates into the application state.

To manage the complexity inherent to these scenarios, modular design within the reducers becomes crucial. Modularity in this context speaks to the practice of breaking down the reducer logic into smaller, manageable functions that can be independently tested and reused. This design pattern also allows for better scalability, as each module can be isolated, maintained, and optimized without affecting other parts of the state management system.

When crafting reducers for real-time updates, one common pitfall to avoid is the direct mutation of the state. This practice can lead to unpredictable application behavior and bugs that are difficult to trace. Instead, a safer approach involves treating the state as immutable and making use of helper functions that facilitate creating the new state based on the incoming actions. Memory performance is also a consideration; it's necessary to avoid unnecessary object creations within the reducer to keep the memory footprint minimal and prevent Garbage Collection spikes that could lead to jitters in the real-time data presentation.

Below is an example of a reducer designed to handle streaming data. It leverages modularity and ensures performance by minimizing redundant computations and preserving immutability:

// Modular function to handle individual updates
function updateItemInArray(array, itemId, updateItemCallback) {
    const updatedItems = => {
        if ( !== itemId) {
            // Since we only need to update one item, keep the rest unchanged
            return item;

        // Use the provided callback to create an updated item
        const updatedItem = updateItemCallback(item);
        return updatedItem;

    return updatedItems;

// Reducer function to handle real-time data stream
function realTimeDataReducer(state = [], action) {
    switch (action.type) {
        case 'ADD_UPDATE_DATA': {
            // Add new data or update existing data in real-time
            return updateItemInArray(state,, item => {
                // Perform the actual update
                return { ...item, ...action.payload.update };
        case 'REMOVE_DATA': {
            // Filter out the data that needs to be removed
            return state.filter(item => !==;
            // In the default case, return the previous state without changes
            return state;

In this example, updateItemInArray is a utility function that encapsulates the logic for updating an item within an array, showcasing modularity. Additionally, the use of spread syntax (...) when constructing the updatedItem enforces immutability. It's also performance-conscious since it iterates over the array only once per action dispatch, rather than recreating the entire array unnecessarily.

Reviewing such reducers requires a balance between maintaining high throughput and ensuring minimal memory impact. Keeping the functions pure—free from side effects—and offloading any computational heavy-lifting to auxiliary utilities helps maintain this balance. It is imperative to continuously profile performance under load to identify and rectify any bottlenecks that could arise from the reducer's implementation in a real-time context.

By fostering thoughtful structural design, maintaining immutable state updates, and optimizing memory usage, a reducer becomes an effective and robust conduit for handling real-time data streams in JavaScript applications. These principles not only facilitate the crafting of performant reducers but also prevent common coding mistakes, such as unintentional state mutations, which undermine application reliability.

Implementing Reducers with createReducer: Performance and Pitfalls

Using createReducer from Redux Toolkit has distinct performance implications for managing states updated by real-time data streams. When reducers are improperly implemented, they can introduce inefficiencies like deep cloning of state objects or unnecessary re-render cycles in the front end. These inefficiencies become increasingly costly as the update frequency scales.

For instance, consider a scenario where a real-time messaging app mishandles the update of individual messages — in such cases, developers might lean towards spreading the entire state to update a single message:

(state, action) => {
    return {
        messages: =>
   === ? { ...message, ...action.payload.updates } : message

This naive approach traverses the entire messages array, even if only one message requires an update, leading to performance bottlenecks when dealing with large state objects with frequent changes.

A more performant solution with createReducer takes advantage of Immer, allowing developers to directly "mutate" the state:

createReducer(initialState, {
    [updateMessage.type]: (state, action) => {
        const index = state.messages.findIndex(message => ===;
        if (index !== -1) state.messages[index] = {...state.messages[index], ...action.payload.updates};

This method updates the targeted message without unnecessary traversal of unaffected elements, minimizing the impact on performance.

Another common pitfall occurs when managing state for applications with high-throughput data, like stock tickers. Here, a reducer can become a choke point if it handles each state update as a discrete action, causing massive performance degradation. Example of an unoptimized reducer handling frequent updates:

(state, action) => {
    switch (action.type) {
        case 'UPDATE_TICKER':
            return {
                tickers: {
                    []: action.payload.value
        // other cases

To circumvent this, developers should aggregate updates where practical and use batch actions, reducing the number of times a reducer is run and leveraging createReducer to make these updates succinct:

const updateTickers = (tickers, updates) => {
    updates.forEach(update => {
        const ticker = tickers[];
        if(ticker) tickers[] = {...ticker, ...update.value};

createReducer(initialState, {
    [batchUpdateTickers.type]: (state, action) => {
        updateTickers(state.tickers, action.payload);

This pattern of aggregating updates reduces the computational load on the reducer, making it akin to throttling, which is essential in real-time applications for maintaining responsiveness without sacrificing up-to-date information.

When handling streams of data, developers must consider the timing and granularity of state updates. Are the most efficient data structures being used? Does each action require an immediate update, or can updates be batched? These are critical questions that can drastically affect a reducer's performance. Remember, while createReducer provides useful abstractions, it is incumbent upon developers to apply them thoughtfully, ensuring they are a boon — not a bottleneck — to application performance.

State Modularity and Reusability with createReducer

When leveraging createReducer within Redux Toolkit for real-time data streams, the modularity and reusability of state logic become paramount. createReducer facilitates a pattern where developers can construct reducers that readily adapt to the ever-changing demands of data-intensive applications. Unlike traditional switch statements, createReducer leans on an object map approach, which enables individual slices of state to be encapsulated within discrete functions. These functions, focused on specific state transitions, can be composed and reused across different parts of the application. This leads to a modular structure where the handling of similar data patterns—say, adding a message to a conversation—is encapsulated once and referenced wherever needed, avoiding redundancy and making the overall state management easier to maintain.

One best practice when developing for real-time systems is to create fine-grained reducers that can be combined to fashion the complete state shape. This composability aids in handling streams that constantly push updates; it ensures that the reducer logic remains manageable and can be scaled without rewriting existing code structures. By isolating functionality, developers can build upon existing logic without inadvertently introducing regressions or duplications. For instance, if a module processes incoming stock tickers, it should be decoupled from the UI components displaying them, thus promoting reuse and preventing tight coupling between state management and presentation logic.

A common design flaw arises when developers create oversized reducers that attempt to handle too much logic, making them rigid and cumbersome to update as application requirements evolve. The use of createReducer encourages the segmentation of reducers into smaller, function-focused pieces that can be independently updated—one avoids monolithic structures that would otherwise become unmanageable as complexity grows. For example, a single reducer that aggregates and updates user profiles, messages, and settings can be split into separate managing functions, each responding to domain-specific actions.

Complex patterns, such as those needed to manage a real-time feed, can be simplified by using the modular reducers pattern that createReducer supports. Continuous data streams like live comments can be managed without overwhelming the state structure by employing separate reducers for adding, updating, and removing entries, which can then be composed into a higher-order reducer. With this approach, updating the feed becomes a matter of dispatching the corresponding action to the appropriate reducer, which handles the state transition in isolation.

Refactoring existing reducers with createReducer can significantly improve the reusability and readability of state management code. Developers should audit their current reducer logic for repeated patterns and extract these into standalone functions. By doing so, they can identify opportunities to simplify and optimize their state management. A practical example is extracting common update operations that are often replicated, like adjusting a timestamp or toggling a flag across multiple entities. With these operations isolated into reusable handlers, they can be easily integrated into new or existing reducers, leading to a more cohesive and malleable state architecture that's well-suited for the demands of real-time data.

Advanced Usage and Thought Experiments

Real-time data streams in a JavaScript application pose a unique set of challenges, especially when we strive to maintain a high-performance, easily maintainable state architecture. With createReducer from Redux Toolkit, we're equipped with a powerful tool to handle such intricate scenarios. To address the influx of actions from diverse streams without causing a negative performance impact, we can design reducers that selectively update state segments, thereby minimizing the amount of work done on each action received.

For example, consider a scenario where we're receiving a mixture of actions from user interactions and live data updates:

const realTimeReducer = createReducer(initialState, (builder) => {
        .addCase(userAction, (state, action) => {
            // Handle user actions, typically low frequency
        .addCase(liveUpdateAction, (state, action) => {
            // Responsively integrate high-frequency updates without re-creating state
            state.liveData = action.payload;

This minimal snippet showcases the separation of concerns where different actions lead to specific state transitions without aggressively re-creating entire state objects.

In managing interdependent data, structuring the state to reflect the nature of these dependencies becomes a critical skill. We can utilize createReducer to fluidly adapt our state shape to the needs of the application. Let's take an IoT sensor application, where the state needs to reflect not only the sensor data but also user preferences for notifications:

const notificationsReducer = createReducer(initialNotificationsState, (builder) => {
        .addCase(updateThreshold, (state, action) => {
            // Modify the thresholds for notifications

const sensorDataReducer = createReducer(initialSensorDataState, (builder) => {
        .addCase(sensorDataReceived, (state, action) => {
            // Store or update sensor data 

These reducers keep separate concerns isolated while still enabling the construction of an interconnected state when combined in a rootReducer.

When tackling memory optimizations, createReducer comes into play by allowing fine-grained control over how state is updated. Rather than naive deep copying, developers should focus on making targeted updates that are cognizant of performance implications. For example, when updating a list of items, we would avoid re-creating the entire list if only one item changes:

const updateItemById = (items, itemId, updateFunction) => {
    const index = items.findIndex(item => === itemId);
    if (index >= 0) {
        const item = items[index];
        const updatedItem = updateFunction(item);
        return [
            ...items.slice(0, index),
            ...items.slice(index + 1),
    return items;

const itemsReducer = createReducer(initialItemsState, (builder) => {
        .addCase(updateItem, (state, action) => {
            state.items = updateItemById(state.items,, existingItem => ({

This code example optimizes memory usage by avoiding unnecessary array creation, only targeting the item that requires an update.

Lastly, the concept of 'smart' reducers introduces a level of dynamism to the typically static nature of Redux reducers. With createReducer, we could potentially implement logic that responds to varying application contexts:

const dynamicGranularityReducer = createReducer(initialState, (builder) => {
        .addCase(dataReceived, (state, action) => {
            const updateDetail = determineUpdateDetailBasedOnContext();
            applyDataWithGranularity(state, action.payload, updateDetail);

function determineUpdateDetailBasedOnContext() {
    // Returns the level of detail needed for the update
    // based on the user's current focus or other application contexts

function applyDataWithGranularity(state, data, granularity) {
    // Applies the incoming data to the state with a level of detail
    // defined by the granularity variable

This strategy allows state updates that dynamically adjust based on actual use patterns, further enhancing the responsiveness and efficiency of our application.

In contemplating these advanced concepts and implementations, it's evident that createReducer is a versatile tool in the Redux Toolkit. By fostering thoughtful conversation about how we architect our reducers, we can pave the way for innovative, high-performance state management in real-time applications.


This article explores the use of Redux Toolkit's createReducer function in building reducers for real-time data streams. It discusses the benefits of using createReducer for handling rapidly changing data and provides examples of efficient reducer design. The article also highlights common mistakes and pitfalls to avoid when using createReducer. The key takeaways from the article are the importance of modularity and immutability in reducer design, the performance implications of different approaches, and the use of createReducer to handle complex scenarios. As a challenging task, readers are encouraged to refactor their existing reducers using createReducer and optimize their state management for real-time data.

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