Migration Checklist: Upgrading to React 18

Anton Ioffe - November 18th 2023 - 9 minutes read

As we stand on the cusp of React 18, the ecosystem is buzzing with the promise of enhanced performance and advanced concurrency features that are set to revolutionize the way we build web applications. This article serves as your definitive migration checklist, designed to arm you, the seasoned JavaScript developer, with the strategies and insights needed to seamlessly transition your codebase to React 18's frontier. We'll dissect the architectural innovations, render engine upgrades, and thorough testing methodologies that will not only prepare your applications for the future but will also sharpen their current edge. Step into this journey with us as we navigate the pivotal modifications and the strategic adaptations necessary to thrive in the era of React 18.

Architecting Your Codebase for React 18

React 18 introduces significant optimizations such as automatic batching and concurrent rendering, paving the way for developers to rethink their codebase architecture. To ensure your application draws maximum benefit from these enhancements, focus on structuring code around function components and Hooks. Adopting function components across the board simplifies the application logic, allowing React to better manage the rendering process.

When designing components, prefer smaller, composable units that encapsulate their own state and behaviors, thus achieving a more modular and maintainable codebase. This approach not only enhances readability but also aligns well with concurrent features, as the encapsulated state lends itself to fewer side effects on render. The granular nature of these components allows for targeted updates, which reduces the likelihood of performance bottlenecks, especially in large-scale applications.

State management in React 18 should favor integrated patterns that allow for finer control over updates. Utilize useReducer or Context API judiciously, reserving such global state management tools for truly app-wide states. This adheres to React 18's philosophy of minimizing render cycles and leveraging automatic batching. When possible, defer to local component state managed by useState to contain updates to the components affected by them.

In concert with React 18's automatic batching, consider deferment techniques for non-critical state updates. Employ startTransition for lower-priority updates, differentiating between urgent and non-urgent state changes, to keep the UI responsive. This allows you to control the rendering pace, which can dramatically improve user experience by prioritizing essential UI updates while relegating others to the background.

Finally, for global state that can change frequently, invest in libraries compatible with concurrent rendering, being attentive to their latest updates for React 18 support. This ensures that complex state management aligns with React’s performance strategies, avoiding rendering pitfalls and state inconsistency. Centralizing and optimizing fetch-intensive operations will also benefit from React 18’s improved server-side rendering capabilities. Adhering to these modularization and design patterns will poise your codebase to take full advantage of the significant performance uplift that React 18 delivers.

Upgrading Rendering Engines: Root API Considerations

One of the most significant changes developers will encounter during the migration to React 18 is the transition from ReactDOM.render to the new createRoot API. The difference here is more than syntactical; it marks a divergence into a new paradigm of rendering in React. Previously, the render method was used to mount a React element to a DOM node as follows:

// Before
import { render } from 'react-dom';
const container = document.getElementById('app');
render(<App />, container);

However, with the introduction of React 18, this approach is deprecated in favor of creating a root:

// After
import { createRoot } from 'react-dom/client';
const container = document.getElementById('app');
const root = createRoot(container);
root.render(<App />);

This refactoring is a pivotal point for React applications to harness the efficiencies of the new rendering engine. The createRoot method encapsulates the internal handling of the rendering process, enabling React to optimize for concurrent features and providing a foundation for future enhancements. Therefore, while the API changes are minimalist, the impact on performance and app behavior is profound.

It's important to note the shift in lifecycle controlled by the root instance created with createRoot. This object is now the orchestrator of updates to the rendered tree, thus all operations like rendering and unmounting are controlled directly from the root, adding a layer of abstraction that centralizes rendering control. For example, the unmountComponentAtNode method in the legacy API now translates to a unmount call on the root:

// Unmounting in React 17
import { unmountComponentAtNode } from 'react-dom';
const container = document.getElementById('app');

// Unmounting in React 18
const root = createRoot(container);

The broader impact of adopting createRoot is observed in the app's performance envelope. By enabling features like concurrent rendering and unlocking patterns that were less practical or impossible in the past, developers can better optimize their applications. Reactive state updates and complex UI interactions gain responsiveness, ultimately offering a smoother user experience.

Yet, developers must tread carefully — introducing createRoot without considering concurrent features and new React 18 behavior might result in unintuitive bugs or performance regressions. A meticulous review of component lifecycle methods and state management logic is advisable to leverage the full benefits of React 18's rendering pipeline without adverse side effects. The transition to createRoot represents not just a code modification, but a step towards embracing the evolutionary enhancements of React's engine.

Embracing the Concurrency Model with Strict Mode

React 18's enhanced Strict Mode sets the stage for future functionality by preparing your application for concurrent rendering. In contrast to its predecessor, which just mounted components and established effects once, the new Strict Mode behavior intentionally simulates a component unmounting and remounting sequence during the initial mounting phase. This exercise is designed to identify components that might not be robust against multiple mount and unmount cycles—an essential resilience for components to possess in a concurrent environment.

function UserProfile({ userId }) {
    const [user, setUser] = useState(null);

    useEffect(() => {
        async function fetchUser() {
            const response = await fetch(`/api/user/${userId}`);
            const userData = await response.json();

        // The cleanup function now plays a critical role
        return () => {
            // Resetting or canceling any outstanding requests
            // will prevent leaks and unwanted behavior
            /* ... */
    }, [userId]);

    // rendering logic
    return <div>User: {user ? user.name : 'Loading...'}</div>;

In the above example, the cleanup function has become crucial under Strict Mode's new regime. Previously, omitting this might have been benign, but with Strict Mode mimicking a remount, any lingering effects, such as undeleted subscriptions or non-canceled requests, can lead to duplicated side effects, leaks, or inconsistent UI states.

Moving forward, the lifecycle of components will witness a significant shift. Developers must rigorously check that functions inside useEffect and other lifecycle hooks are not reliant on the assumption that they will run only once upon mounting and unmounting. The Strict Mode intervention reveals any idempotency issues—situations where multiple executions lead to adverse side effects, which might hinder optimal concurrent operations.

Reviewing components for potential side effects during development is now more straightforward with the removal of console log suppression in React 18 Strict Mode. Developers can spot suspect double-invocations with precision thanks to React DevTools, which show these duplicitous logs in grey, with an option to suppress if desired. Moreover, improved memory usage is on offer as React 18 ensures better cleanup of internal fields on unmount, safeguarding your application against the implications of unresolved memory leaks.

React developers must therefore consider Strict Mode no longer just as a best practice, but as a necessary part of the development process to ensure their applications are ready for the concurrency model. Although this feature may introduce some initial adjustment overhead, it is a valuable tool for highlighting which components need to be adjusted for the new concurrent reality. This proactive approach future-proofs applications and aligns them with React's evolving capabilities, harnessing the full power of concurrent rendering for more fluid user experiences.

Handling Deprecated Features and Breaking Changes

React 18 marks a transition away from several longstanding features and ushers in a series of breaking changes. The traditional methods such as ReactDOM.render and ReactDOM.hydrate have not been fully deprecated but will operate in React 17 compatibility mode, complete with warning notices, while developers are encouraged to adopt the new rendering API.

// React 17
import ReactDOM from 'react-dom';

const container = document.getElementById('app');
ReactDOM.render(<App />, container);

// React 18 using createRoot
import { createRoot } from 'react-dom/client';

const container = document.getElementById('app');
const root = createRoot(container);
root.render(<App />);

With React 18, timing for useEffect is now more consistent. Effects triggered during discrete events such as clicks or keypresses are flushed synchronously to prevent unexpected behavior. This may require developers to reevaluate and possibly alter their effect's logic.

// React 17 timing for useEffect could lead to unpredictable behavior
useEffect(() => {
    // Effect logic...

// React 18 guarantees synchronization of effects post discrete user inputs
useEffect(() => {
    // New effect logic with synchronous assumption

The introduction of the new JSX Transform in React 18 eliminates the need to import React into every file using JSX, which can result in more optimized build sizes and simplification of version management by automatically including the necessary JSX functions.

// React 17 required an explicit React import for JSX
import React from 'react';

function MyComponent() {
    return <div>Hello, React 17</div>;

// React 18 with the New JSX Transform no longer requires React import
function MyComponent() {
    return <div>Hello, React 18</div>;

Server-side rendering approaches have evolved; React 18 deprecates ReactDOMServer.renderToNodeStream() in favor of new methods that align with its streaming and concurrent capabilities. Understanding these new methods is crucial for an effective migration.

The discontinuation of Internet Explorer support is another significant change with React 18, which illustrates a move towards modernized practices in web development. Migrating to React 18 is not just about adapting to deprecations and breaking changes but also seizing the opportunity to fully leverage the advances offered by this framework for enhanced web experiences.

Testing Strategy Overhaul for React 18

When migrating to React 18, developers will find that the testing strategy requires significant attention. React 18 introduces enhancements that affect the testing environment due to its new rendering mechanism. Adapting tests to work reliably in this context is paramount. Many developers have reported that they had to modify a substantial number of their tests to use asynchronous queries within React Testing Library to ensure elements are found accurately, which is likely due to React 18's rendering engine.

The updated act utility is a cornerstone of React 18 testing. While its purpose—to prepare a piece of code to be asserted in a way that reflects the user's experience—remains the same, its usage patterns may differ. Previously synchronous operations may now need to be considered in an asynchronous manner to appropriately simulate user interactions. This implies that some test cases may require reimplementation to handle promises and ensure that state updates and component renders are fully completed before assertions are made.

Integrating with third-party testing libraries might require developers to wait for or contribute to updates that ensure compatibility with React 18. Fortunately, many popular libraries like React Testing Library are being updated in anticipation of React 18's new features. These libraries accommodate concurrent features and provide built-in support for the new behaviors without substantial configuration changes. Thus, developers are advised to keep abreast with the latest versions of their chosen testing tools and apply necessary upgrades to leverage these enhancements.

Evaluating the performance of test suites post-upgrade is crucial. With the shift to support concurrent features, developers might observe different performance characteristics in their tests. Test suites could potentially run slower due to the need for additional asynchronous wait operations, which could prompt the developers to optimize their tests for the new rendering mechanism. It is also imperative to confirm that the performance cost does not lead to a substantial increase in test suite execution time or flakiness.

Finally, it's prudent to assess how the new architecture handles test suite memory usage. With React 18's concurrent features possibly leading to more complex component trees in memory, it's necessary to ensure that tests clean up properly to prevent memory leaks. The use of proper teardown methods and validators becomes more significant to avoid side effects that can skew the results of subsequent tests or affect their reliability. Developers should inspect their testing strategies to certify thorough cleanup after each test case, thus maintaining test suite integrity with the adoption of React 18.


In this article, the author provides a migration checklist for upgrading to React 18 in modern web development. Key takeaways include the importance of architecting codebase for React 18, handling the transition to the new rendering engine, embracing the concurrency model with Strict Mode, and adapting the testing strategy for React 18. The challenging technical task for the reader is to review their components for potential side effects during development, ensuring they are not reliant on assumptions that they will run only once upon mounting and unmounting, in order to optimize their applications for the new concurrent reality.

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