useEffect vs useLayoutEffect in React: Understanding When to Use Each

React provides two primary hooks for side effects: useEffect and useLayoutEffect. While they may seem similar at first glance, they have distinct behaviors and different use cases. Understanding the differences will help you optimize performance and prevent unexpected bugs in your React applications.

1. useEffect: The Standard for Side Effects

When it Runs

• useEffect runs asynchronously after the browser has painted the changes to the screen.

• It does not block the browser from rendering the component.

• Ideal for tasks that don't need to block visual updates, such as:
• Fetching data
• Subscribing to event listeners (e.g., scroll, resize)
• Logging to the console
• Interacting with external APIs or systems that don’t impact layout

Behavior

• The browser paints the screen first, then React runs the useEffect after the UI has been updated. This allows for a non-blocking user experience.

• Because it runs asynchronously after the component has rendered, it doesn't affect the layout of the component.

Example Usage:

import { useEffect } from 'react';

function ExampleComponent() {
  useEffect(() => {
    console.log('Component rendered');
    // Fetch data or set up subscriptions here
  }, []); // Empty dependency array means this runs only once on mount

  return <div>Hello World!</div>;
}

When to Use useEffect

• Data fetching: Fetch data after the component has been painted on the screen.

• Non-visual updates: Logging, subscriptions, or interacting with APIs that don’t impact the layout.

• Performance-sensitive operations: Since useEffect is non-blocking, it’s great for tasks that can happen after the render phase without delaying UI updates.

2. useLayoutEffect: For Layout-Sensitive Operations

When it Runs

• useLayoutEffect runs synchronously after all DOM mutations but before the browser paints.

• This means that useLayoutEffect blocks the browser from painting until the effect has run and any DOM measurements or changes are applied.

Behavior

• Since useLayoutEffect runs before the browser repaints the UI, it’s useful when you need to measure DOM elements or synchronize layout changes.

• Any updates made in useLayoutEffect are applied immediately, before the browser performs a visual update, ensuring that changes are reflected immediately.

Example Usage:

import { useLayoutEffect, useRef } from 'react';

function LayoutEffectExample() {
  const divRef = useRef();

  useLayoutEffect(() => {
    const rect = divRef.current.getBoundingClientRect();
    console.log('Width and Height:', rect.width, rect.height);
    // Manipulate layout or measure DOM elements here
  }, []);

  return <div ref={divRef}>Hello World!</div>;
}

When to Use useLayoutEffect

• DOM measurements: Use it when you need to read the layout (e.g., getBoundingClientRect(), scroll positions) before the browser repaints.

• Synchronous UI updates: When the side effect directly affects the layout, such as when you're manipulating DOM elements or synchronizing visual state.

• Blocking rendering for layout adjustments: If you need to ensure the DOM is in a certain state before it's visible to the user, such as resizing elements or adjusting layout for animations.

Key Differences Between useEffect and useLayoutEffect

When to Use useEffect vs. useLayoutEffect

1. Use useEffect for:
• Non-blocking operations: You want your UI to update quickly and run side effects afterward.
• Data fetching: Fetch data from an API after the component renders.
• Logging: Send data or log information without affecting the visual flow.
• Event listeners: Set up and clean up listeners (like window resizing, or API polling) that don't affect the layout.

2. Use useLayoutEffect for:
• DOM measurements or manipulations: When you need to calculate the size of an element before it’s painted on the screen.
• Synchronous layout changes: If you need to apply a change before the component becomes visible, like adjusting scroll positions or element sizes for animations.
• Complex UI interactions: Such as animations that require exact measurements before a reflow occurs or ensuring that layout adjustments happen without visual flickers.

Best Practices for Using useLayoutEffect

1. Use it sparingly: Since useLayoutEffect runs synchronously after DOM mutations but before the browser paints, it can delay visual updates and impact performance. Overuse of useLayoutEffect can lead to jank (stuttering, flickering) if it blocks the browser from rendering content in time.

2. Manipulate the DOM synchronously: Use useLayoutEffect only when you need to read layout or style values from the DOM (e.g., getBoundingClientRect(), measuring an element’s size or position) and then immediately modify it. This ensures that your changes happen before the browser repaints.

3. Avoid unnecessary blocking: Only resort to useLayoutEffect when the DOM information or mutation must be performed before the next paint. Stick with useEffect for most tasks, like fetching data, side effects not tied to layout, or any non-blocking operations.

4. Common Use Cases:
• Measuring elements’ dimensions or positions before displaying the updated result.
• Synchronizing animations that require DOM measurements or styles that need to be applied before the content becomes visible.