When to Use useLayoutEffect Over useEffect in React 18

Anton Ioffe - November 19th 2023 - 9 minutes read

In the ever-evolving world of React development, the nuanced mastery of its hooks can be the dividing line between average and exceptional user experiences. As React 18 ushers in new patterns and enhancements, the subtle yet critical decision of when to wield useLayoutEffect over its counterpart useEffect demands a closer look. Dive into the depths of synchronicity where milliseconds matter, and explore practical scenarios that can only be tamed by the precise touch of useLayoutEffect. Throughout this article, we navigate the intricate dance of performance, best practices, and hypothetical quandaries, arming you with the insight needed to harness this powerful hook with the finesse befitting a seasoned React developer. Prepare to elevate your UI game, as we dissect, demonstrate, and deliberate the sophisticated use of useLayoutEffect in building seamless and responsive applications.

Harnessing the Precision of useLayoutEffect in React 18

To fully grasp the complexity of side effects in React 18, it's essential to differentiate useEffect from useLayoutEffect. Both hooks are designed to handle side effects in functional components; however, they are not interchangeable owing to differences in their timing relative to the browser's painting process.

useLayoutEffect is specifically timed to execute synchronously immediately after React has updated the DOM but before the browser has had a chance to paint those changes to the screen. This hook is ideal when you need to make direct DOM measurements or updates that must be reflected visually in the same cycle, thus preventing the user from seeing any intermediary states. Amendments made within useLayoutEffect are part of a critical phase where React's virtual DOM updates must synchronize flawlessly with the actual DOM to ensure a coherent visual output on the screen.

Conversely, useEffect typically operates asynchronously and does not block browser painting. It executes after all of the DOM updates have been committed and the browser has had the opportunity to paint the screen. Nonetheless, with React 18's updates, useEffect now contains an edge case where it can behave synchronously: during discrete user interactions such as clicks or when enclosed in flushSync. Despite this synchronous invocation, the updates scheduled within useEffect remain asynchronous and do not affect the same rendering cycle's paint.

During the life cycle of a React component, the sequence is as follows: after state or prop adjustments, React manipulates the DOM and useLayoutEffect is triggered instantly to handle tasks that are visually critical. In the subsequent phase, once painting is complete, useEffect is called to manage effects that do not demand immediate reflection in the UI, such as data fetching or setting up event listeners.

useLayoutEffect is indispensable for time-sensitive DOM interaction that needs to align perfectly with the visualization sequence of updates. Its synchronous nature, while invaluable under these circumstances, should be leveraged with an understanding of potential pitfalls. For example, misuse of useLayoutEffect for operations that could be deferred to useEffect risks blocking the browser from painting, which can lead to perceptible lags, particularly on complex interfaces or when operating on large data sets.

In conclusion, useLayoutEffect should be used sparingly and only when the immediacy of state updates could critically affect the user's perception of visual continuity. By adhering to this principle, developers can harness the full potential of React's rendering lifecycle without compromising application performance.

Practical Scenarios Favoring useLayoutEffect

In certain scenarios where precise control over the layout is crucial, opting for useLayoutEffect ensures that any visual changes occur synchronously with DOM mutations. For instance, when measuring DOM elements to determine sizes or positions for dynamically rendering components such as tooltips or modals, useLayoutEffect comes into play. Instead of applying these calculations and causing potentially jarring visual shifts after the user sees the component, this hook allows us to preempt any layout thrashing by sizing and positioning elements in the same cycle as the DOM update.

Animations that require a smoothly coordinated start also benefit from useLayoutEffect. When initial dimensions or state are required before an animation can kick off, using this hook ensures that all preparatory steps are complete before the frames begin rendering. This is key to preventing animation jumps or flickers that might occur if such information were gathered after the paint.

Moreover, when dealing with third-party DOM manipulations that lack the automatic optimizations present in React, useLayoutEffect allows developers to sync these imperatives with the lifecycle of a component. Since useLayoutEffect is called right after React has updated the DOM but before the browser has painted, any changes made here will appear seamless to the user, significantly enhancing the user experience by providing a consistent and jitter-free interface.

However, the benefits of using useLayoutEffect do come with potential downsides where performance is concerned. Particularly in complex applications with numerous components and state updates, unnecessarily leveraging useLayoutEffect could introduce jank, as it could delay the browser’s paint, leading to perceptible lag in the user interface. It's important to assess the trade-offs, ensuring that this hook is invoked only when synchronous updates are strictly necessary for maintaining the integrity of the user experience.

When deliberating between useEffect and useLayoutEffect, keep in mind that resorting to the latter should be for scenarios where useEffect would lead to a user-perceivable inconsistency. Engage useLayoutEffect judiciously to manipulate the DOM in-sync with React's updates, such as for moment-critical entrances and exits, where stalling the screen paint is preferable over presenting quick, successive visual states that might be distracting or confusing for the user.

Analyzing useLayoutEffect Performance Considerations

When considering the impact of useLayoutEffect on performance, one must be mindful of the synchronous nature of this hook. Unlike useEffect, which permits the browser to defer effect execution until after painting, useLayoutEffect executes immediately after DOM updates and before the paint cycle. This ensures that any layout readjustments or data gathering from the DOM occurs within the same frame as the update. The direct implication of this synchronicity is an increased CPU cost, as the main thread must execute the effect's code before any visual rendering can proceed. Consequently, the utilization of this hook is optimal only when necessary to maintain a seamless user interface, such as when calculating layout dimensions for direct DOM manipulation.

The cost of executing useLayoutEffect extends beyond CPU cycles. Reflow and repaint operations are potentially expensive operations that the browser undertakes in response to DOM mutations. By forcing synchronous execution, useLayoutEffect may instigate additional reflow or repaint cycles if the effect modifies the layout. This is particularly concerning for high-complexity applications running on less powerful devices, where the weight of these additional calculations could lead to noticeable performance bottlenecks.

Despite the potential drawbacks, in specific scenarios, useLayoutEffect provides a significant benefit to perceived performance. By eliminating visual flickers which might occur with asynchronous updates, useLayoutEffect offers a more polished user experience. For instance, if an item's position in a list needs to be recalculated due to dynamic content size, using useLayoutEffect can ensure that the list layout updates in congruence with the rest of the DOM, providing an uninterrupted flow of content to the user. Selective use in such instances mitigates the performance cost by improving the user's perceptual experience of the application's reactivity.

In evaluating the justification for useLayoutEffect, developers must weigh the necessity of immediate DOM updates against the cost imparted on the application's responsiveness. This trade-off often comes down to the specific use case and user interaction metrics, like the responsiveness to user inputs or the smoothness of animations. The heavier workload on the main thread should be justified by a tangible improvement in the immediate user experience, such as preventing content shifting or preserving the consistency of visual transitions which demand a synchronous update cycle.

In conclusion, useLayoutEffect should not be employed without a thorough consideration of its repercussions. While it provides a mechanism for real-time DOM interaction, its use must be scrutinized and benchmarked against the critical path of rendering to avoid unnecessary jank and degraded performance. Developers should carefully profile their applications, considering both the depth of component trees and the complexity of effects, to ensure that the benefits of using useLayoutEffect for maintaining visual harmony do in fact outweigh the additional burdens placed upon the browser engine.

Code Excellence: Best Practices with useLayoutEffect

When writing high-quality, maintainable React code, the implementation of useLayoutEffect must be attentive to the subtleties of its operation. It is paramount to ensure your useLayoutEffect functions encompass clean-up processes, much akin to their useEffect counterparts. Neglecting to return a clean-up function can result in memory leaks and erratic behavior, particularly when dealing with listeners or subscriptions that are persistent across renders. Observe the following proper use which includes a clean-up routine:

import { useLayoutEffect } from 'react';

function useCustomHook() {
    useLayoutEffect(() => {
        // Setup logic here
        const subscription = someLibrary.subscribe();

        // Clean-up function returned to clear the subscription
        return function cleanup() {
    }, []); // Dependency array ensures the effect runs once

Conversely, a common pitfall is an improper handling of the dependencies array; either overpopulating it, triggering unnecessary re-runs, or leaving it empty when dependencies do exist. As such, it's critical to analyze which variables effectively influence the effect and include them in the dependencies array to avoid unintended side-effects and re-renderings:

import { useLayoutEffect, useState } from 'react';

function ResponsiveComponent({ width }) {
    const [height, setHeight] = useState(0);

    useLayoutEffect(() => {
        function adjustHeight() {
            // Adjust height based on width change
            setHeight(width * 0.75);

        window.addEventListener('resize', adjustHeight);

        // Preemptively adjust the height

        return () => {
            window.removeEventListener('resize', adjustHeight);
    }, [width]); // Correctly depends on the 'width' prop

Moreover, it's critical to reserve the employment of useLayoutEffect for scenarios where manipulating the DOM or layout measurements must occur before the browser's paint to ensure visual coherence. Integrating it in situations devoid of this necessity could lead to performance degradations. A best practice example is adjusting the position of a DOM element in reaction to external changes:

import { useLayoutEffect, useRef } from 'react';

function PositionedElement({ position }) {
    const elementRef = useRef(null);

    useLayoutEffect(() => {
        if (elementRef.current) {
            elementRef.current.style.transform = `translate(${position.x}px, ${position.y}px)`;
    }, [position]); // Depends on 'position'

    return <div ref={elementRef}>I'm a moving element</div>;

Within the realm of useLayoutEffect, one must approach the inclusion of complex logic judiciously. Prioritize brevity and directness in your hooks to prevent expensive operations from constricting the main thread and causing notable layout thrashing. Simplify where you can, and question the necessity of every line of code within your effect function.

Finally, spark curiosity about whether a given use case indeed warrants useLayoutEffect: Is there a definitive layout manipulation that users will perceive before the next paint? If not, re-evaluate whether the logic could be positioned elsewhere or within a regular useEffect without diminishing the user experience. It's this level of discernment in employing useLayoutEffect that shapes a React application's excellence in both performance and maintainability.

Thought-Experimenting with useLayoutEffect

Imagine a scenario where you're dealing with a complex form that integrates with a third-party canvas drawing library. In real-time collaboration apps, like a shared whiteboard, user expectations lean heavily towards immediate visual feedback. Applying your understanding of hooks, how would you ensure that as one user draws on the canvas, all other collaborating users see the drawings without perceptible delay or awkward jumps? If you jump to useLayoutEffect to synchronize the drawn state, what considerations around performance are crucial to keep in mind given the potentially high frequency of updates?

Consider another case where you're building an interactive game using React. The game has numerous animations and UI state changes that must feel instantaneous to provide an engaging experience. As the game scales in complexity, how would your use of useLayoutEffect vary compared to a simple application? What strategies would you employ to manage the synchronous execution of state updates while maintaining fluid animation frames and avoiding janking?

Let's delve into a third hypothetical: a data visualization dashboard with charts that dynamically update based on user interaction. Here, layout shifts are not merely an inconvenience but could lead to misinterpretation of data if elements move unexpectedly. To forestall confusion, how might useLayoutEffect be used to measure and render these updates? Furthermore, what might be the repercussions on the responsiveness of the dashboard with intensive DOM measurements?

Imagine developing an application with a complex, nested component structure where layout calculations are needed at multiple levels. How would you prevent the cascading use of useLayoutEffect from causing a domino effect of layout recalculations? Is there a strategy you can formulate to localize the use of useLayoutEffect to the subcomponents that explicitly need them, thus preserving the performance of the overall application?

Lastly, ponder upon a scenario with an accessibility feature such as a focus trap for modals. Accessibility features often require precise timings to ensure that focus behavior appears seamless to the user. If your solution involves useLayoutEffect, what potential issues might arise from focus management during rapid state changes? How would you mitigate these problems to ensure that the feature enhances usability rather than detracts from it due to faulty execution timings?


The article "When to Use useLayoutEffect Over useEffect in React 18" explores the nuanced differences between useLayoutEffect and useEffect hooks in React 18 and provides guidance on when to use each. It emphasizes that useLayoutEffect should be used sparingly and only when immediate visual updates are critical for maintaining a seamless user experience. The article also highlights practical scenarios where useLayoutEffect is favored, performance considerations, and best practices. To challenge the reader, the article poses hypothetical scenarios related to complex forms, interactive games, data visualization dashboards, and accessibility features, prompting them to think about how they would utilize useLayoutEffect in these situations and address potential issues.

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