Understanding the Differences: useEffect vs useLayoutEffect

The Difference Between useEffect and useLayoutEffect

React has changed front-end development significantly with hooks, making it easier for developers to manage state and side effects in functional components. Two important hooks for handling side effects are useEffect and useLayoutEffect. They might look similar initially, but knowing their differences is key to improving performance and ensuring a smooth user experience. In this post, we'll explore what these hooks do, how they differ, and when to use each one.

What is useEffect?

useEffect is a hook that lets you perform side effects in function components. It runs asynchronously and after the component has rendered to the screen. This makes it suitable for operations that don't need to block the browser from updating the screen, such as fetching data, setting up subscriptions, and manually changing the DOM.

Effects let you specify side effects that are caused by rendering itself rather than by a particular event. To use useEffect, you need to pass two arguments:

1. A Setup Function: This function contains the code that connects to an external system. It should return a cleanup function with the code that disconnects from that system.
2. A List of Dependencies: This list includes every value from your component that is used inside the setup and cleanup functions.

Here's an example of useEffect:

import React, { useEffect, useState } from 'react';

function ExampleComponent({ userId }) {
  const [userData, setUserData] = useState(null);

  useEffect(() => {
    const fetchUserData = async () => {
      const response = await fetch(`https://api.example.com/users/${userId}`);
      const data = await response.json();
      setUserData(data);
    };

    fetchUserData();

    // Cleanup function (if needed)
    return () => {
      // Any necessary cleanup code can go here
    };
  }, [userId]); // The effect runs whenever userId changes

  return (
    <div>
      {userData ? (
        <div>
          <h1>{userData.name}</h1>
          <p>{userData.email}</p>
        </div>
      ) : (
        'Loading...'
      )}
    </div>
  );
}

export default ExampleComponent;

In this example, the useEffect hook depends on the userId prop. The effect is triggered whenever the userId changes. Here’s a breakdown of what’s happening:

1. Setup Function: The fetchUserData function fetches data for a specific user from an API based on the userId prop.
2. Cleanup Function: In this case, the cleanup function is empty because there’s no ongoing subscription or external resource that needs to be cleaned up.
3. Dependencies: The dependency array contains [userId], which means the effect will re-run whenever the userId prop changes.

This ensures that the component fetches and displays the correct user data whenever the userId prop is updated.

What is useLayoutEffect?

useLayoutEffect is similar to useEffect in that it allows you to perform side effects. However, it fires synchronously after all DOM mutations but before the browser has a chance to paint. This makes it suitable for operations that need to read layout from the DOM and make visual updates synchronously. Essentially, useLayoutEffect is a version of useEffect that fires before the browser repaints the screen, making it ideal for cases where you need to measure or manipulate the DOM and ensure changes are made before the browser renders the next frame.

Example of useLayoutEffect With a Tooltip

Consider a scenario where you need to position a tooltip based on the dimensions and position of an element. Using useLayoutEffect ensures that the tooltip is positioned correctly before the browser paints the screen, preventing any visual flicker.

import React, { useLayoutEffect, useRef, useState } from 'react';

function TooltipExample() {
  const buttonRef = useRef();
  const tooltipRef = useRef();
  const [tooltipStyle, setTooltipStyle] = useState({});

  useLayoutEffect(() => {
    const buttonRect = buttonRef.current.getBoundingClientRect();
    const tooltipRect = tooltipRef.current.getBoundingClientRect();

    setTooltipStyle({
      top: buttonRect.bottom + window.scrollY,
      left: buttonRect.left + window.scrollX,
    });

    // Cleanup function (if necessary)
    return () => {
      // Any necessary cleanup code can go here
    };
  }, []); // Empty dependency array means this effect runs only once after the initial render

  return (
    <div>
      <button ref={buttonRef}>Hover over me</button>
      <div
        ref={tooltipRef}
        style={{
          position: 'absolute',
          top: `${tooltipStyle.top}px`,
          left: `${tooltipStyle.left}px`,
          background: 'lightgray',
          padding: '5px',
          borderRadius: '3px',
        }}
      >
        Tooltip text
      </div>
    </div>
  );
}

export default TooltipExample;

In this example, the useLayoutEffect hook is used to position a tooltip based on the dimensions and position of a button element. Here’s a breakdown of what’s happening:

1. Refs for DOM Elements: We use useRef to create references for the button and tooltip elements. These references allow us to access the DOM elements directly.
2. State for Tooltip Style: We use useState to manage the tooltip's position, storing it in the tooltipStyle state.
3. useLayoutEffect Hook: The useLayoutEffect hook runs synchronously after all DOM mutations but before the browser repaints the screen. This ensures that the tooltip's position is calculated and applied before the browser paints, preventing any visual flicker.
   • Calculating Position: Inside the hook, we use getBoundingClientRect to get the dimensions and positions of the button and tooltip elements.
   • Setting Tooltip Style: We update the tooltipStyle state with the calculated position, ensuring the tooltip is positioned correctly relative to the button.
4. Dependencies: The empty dependency array [] ensures the effect runs only once after the initial render.

By using useLayoutEffect, we ensure that the tooltip is positioned correctly before the browser repaints the screen, providing a smooth and flicker-free user experience.

Key Differences Between useEffect and useLayoutEffect

1. Timing:
   • useEffect runs asynchronously after the component has been rendered and painted on the screen. This means it doesn't block the painting process.
   • useLayoutEffect runs synchronously after all DOM mutations but before the browser has a chance to paint. This ensures that any layout changes happen before the next paint, making it suitable for operations that need to measure or manipulate the DOM immediately.
2. Performance Impact:
   • useEffect is non-blocking and runs in the background, allowing the browser to paint the screen without delay. This helps maintain a smooth user experience.
   • useLayoutEffect can block the painting process because it runs synchronously. If it contains heavy computations or frequent updates, it can lead to performance issues, causing the UI to feel sluggish. Therefore, it's crucial to use useLayoutEffect only when necessary to avoid impacting performance.
3. Use Cases:
   • useEffect: Ideal for side effects that don't require immediate DOM updates, such as:
     • Fetching data from an API.
     • Setting up subscriptions.
     • Managing timers.
     • Logging.
   • useLayoutEffect: Best for operations that need to be performed synchronously before the browser paints, such as:
     • Reading layout from the DOM (e.g., measuring elements' sizes and positions).
     • Making visual updates that need to be reflected immediately (e.g., repositioning elements, applying styles based on measurements).

Final Words on useEffect and useLayoutEffect

Understanding the differences between **useEffect and useLayoutEffect is crucial for optimizing your React application's performance and ensuring a smooth user experience. Here are some key takeaways:

1. UseLayoutEffect Timing: useLayoutEffect is a version of useEffect that fires before the browser repaints the screen. This ensures that any layout changes happen before the next paint, making it suitable for operations that need to measure or manipulate the DOM immediately.
2. Performance Considerations: Because useLayoutEffect runs synchronously, it can block the browser from painting, potentially leading to performance issues. Prefer useEffect whenever possible to avoid blocking visual updates and maintain a responsive UI.
3. Synchronizing With External Systems: Effects should usually be used to synchronize your components with an external system, such as fetching data, setting up subscriptions, or managing timers.
4. Internal State Management: If no external system is involved and you only want to adjust some state based on other states, you might not need an effect. Consider using state management within the component without introducing unnecessary effects in such cases.

By choosing the right hook for the job and understanding when to use each, you can ensure your applications run efficiently and provide a seamless user experience.

References:

• React Documentation: Hooks
• React Documentation: useEffect
• React Documentation: useLayoutEffect