useLayoutEffect | Advanced Hooks Tutorial

React JS: Advanced Hooks - `useLayoutEffect`

useLayoutEffect is a React Hook that's very similar to useEffect, but it fires synchronously after all DOM mutations. This makes it useful for reading layout from the DOM and synchronously re-rendering. It's less commonly used than useEffect because it can block visual updates if used improperly.

Understanding the Difference: `useEffect` vs. `useLayoutEffect`

useEffect: Runs asynchronously after the browser has painted the screen. This means the user might briefly see an intermediate state before the effect runs and updates the UI. Ideal for most side effects like data fetching, subscriptions, and manual DOM manipulations that don't require precise timing.
useLayoutEffect: Runs synchronously after all DOM mutations but before the browser paints the screen. This means the user won't see any intermediate states. Ideal for reading layout information from the DOM (e.g., element dimensions, scroll position) and making changes that need to be applied immediately to avoid flickering or visual inconsistencies.

Key takeaway: If your effect needs to read or modify the DOM and you want to avoid visual flickering, useLayoutEffect is the way to go. Otherwise, stick with useEffect.

Syntax

The syntax for useLayoutEffect is identical to useEffect:

import { useLayoutEffect, useRef } from 'react';

function MyComponent() {
  const elementRef = useRef(null);

  useLayoutEffect(() => {
    // Code to run after DOM mutations, before paint
    if (elementRef.current) {
      const width = elementRef.current.offsetWidth;
      // Do something with the width, potentially updating state
      console.log("Element width:", width);
    }

    // Optional cleanup function
    return () => {
      // Code to run when the component unmounts or before the effect re-runs
    };
  }, [/* dependencies */]); // Dependency array
  
  return <div ref={elementRef}>My Component</div>;
}

Callback Function: The first argument is a function that contains the side effect logic.
Dependency Array: The second argument is an optional array of dependencies. The effect will only re-run if any of the values in the dependency array have changed since the last render. If the array is empty ([]), the effect runs only once after the initial render. If no array is provided, the effect runs after every render.
Cleanup Function (Optional): The callback function can return another function, which is the cleanup function. This function is called when the component unmounts or before the effect re-runs due to a change in dependencies. It's used to clean up any resources created by the effect (e.g., removing event listeners, canceling subscriptions).

Use Cases

Here are some common scenarios where useLayoutEffect is beneficial:

Reading DOM Dimensions: Calculating the width or height of an element and using that information to adjust the layout.

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

function DynamicHeightComponent() {
  const elementRef = useRef(null);
  const [height, setHeight] = useState(0);

  useLayoutEffect(() => {
    if (elementRef.current) {
      setHeight(elementRef.current.offsetHeight);
    }
  }, []);

  return (
    <div ref={elementRef}>
      Content that determines the height.
      <div style={{ height: `${height}px`, backgroundColor: 'lightgray' }}>
        Height is dynamically set based on content.
      </div>
    </div>
  );
}

Synchronous DOM Manipulation to Avoid Flickering: Making changes to the DOM that need to be applied immediately to prevent visual glitches. For example, adjusting scroll position or focusing an element.

import { useLayoutEffect, useRef } from 'react';

function AutoFocusComponent() {
  const inputRef = useRef(null);

  useLayoutEffect(() => {
    if (inputRef.current) {
      inputRef.current.focus();
    }
  }, []);

  return <input ref={inputRef} type="text" />;
}

Synchronizing with Third-Party Libraries: Interacting with libraries that require synchronous DOM access.
Measuring and Adjusting Layout Before Paint: If you need to measure an element's size and then immediately adjust another element's position based on that measurement, useLayoutEffect is crucial.

Performance Considerations & When Not to Use `useLayoutEffect`

Blocking Updates: Because useLayoutEffect runs synchronously, it can block the browser from painting the screen. This can lead to performance issues if the effect's logic is complex or time-consuming. Avoid heavy computations within useLayoutEffect.

Server-Side Rendering (SSR): useLayoutEffect will trigger a warning during server-side rendering because the DOM is not available on the server. You can conditionally run it only on the client-side:

import { useLayoutEffect, useRef } from 'react';
import { isClient } from 'next/headers'; // Or similar for your SSR framework

function MyComponent() {
  const elementRef = useRef(null);

  useLayoutEffect(() => {
    if (isClient) { // Only run on the client
      // ... your useLayoutEffect logic ...
    }
  }, []);

  return <div ref={elementRef}>My Component</div>;
}

Prefer useEffect when possible: If you don't need synchronous DOM access, always prefer useEffect. It's more performant and less likely to cause issues.

Summary

useLayoutEffect is a powerful hook for specific scenarios where synchronous DOM access is required. However, it's important to understand its performance implications and use it judiciously. Always consider whether useEffect can achieve the same result before resorting to useLayoutEffect. Prioritize performance and avoid blocking the browser's paint cycle whenever possible.

React JS: Advanced Hooks - useLayoutEffect

Understanding the Difference: useEffect vs. useLayoutEffect