1. Introduction
When React introduced concurrent rendering in React 18, new hooks emerged to handle different stages of a component’s lifecycle in the rendering pipeline. One such hook is useInsertionEffect, which lets you perform effects synchronously before DOM mutations occur.
Why does this matter? In many cases, you want to inject or modify styles (or perform other side effects) before the browser paints. This ensures that the user sees the correct styling without flickers or layout shifts. In React 19, useInsertionEffect continues to serve a niche—but essential—role, especially for CSS-in-JS libraries and similar use cases.
2. What Is useInsertionEffect?
useInsertionEffect is a special React hook that runs its effect synchronously before any DOM updates are committed to the screen. It has a very specific use case, primarily intended for styling libraries that inject CSS dynamically so that the user never experiences a flash of unstyled content.
Key Points
- Runs Before: The DOM is updated and the browser paints changes to the screen.
- Synchronously: React does not batch or defer this effect; it blocks rendering until it completes.
- For CSS or Critical Side Effects: Typically, you’ll use this to insert styles.
- Warning: Because it happens so early, you shouldn’t perform heavy computations or schedule major side effects here, as it can block rendering.
3. Basic Signature
useInsertionEffect(setup, dependencies?)- setup: A function that contains your side effect code.
- dependencies?: An optional array of values that determine when this effect re-runs. If omitted, useInsertionEffect runs after every render.
Return Value: Like other effect hooks, setup may return a cleanup function that runs before the next insertion effect call or when the component unmounts.
4. Example Use Case: CSS-in-JS
Imagine you have a CSS-in-JS library that needs to dynamically insert a <style> tag into the document. You want to do this before the user sees any layout shift. Here’s an example of how useInsertionEffect might be used in a simplified scenario:
import React, { useInsertionEffect } from "react";
function useDynamicStyles(cssString) {
useInsertionEffect(() => {
// Create a style element and insert it before DOM updates
const styleEl = document.createElement("style");
styleEl.textContent = cssString;
document.head.appendChild(styleEl);
// Cleanup by removing the style element
return () => {
document.head.removeChild(styleEl);
};
}, [cssString]);
}
export default function DynamicStyledComponent({ color }) {
// Generate some CSS on the fly
const css = `
.dynamic-box {
width: 100px;
height: 100px;
background-color: ${color};
}
`;
// Use our custom hook to insert styles before paint
useDynamicStyles(css);
return <div className="dynamic-box"></div>;
}
How It Works:
- The useInsertionEffect hook creates and appends a <style> element to the <head> before the DOM updates are committed.
- This ensures that when the user sees the <div> with className "dynamic-box", it’s already styled with the correct color.
5. Differences from Other Effect Hooks
5.1 useEffect
- Timing: Runs after the browser paints.
- Common Usage: Data fetching, subscriptions, logging, side effects that don’t affect layout.
- Async: In concurrent rendering, React may delay useEffect calls if necessary.
5.2 useLayoutEffect
- Timing: Runs synchronously after the DOM is updated but before the browser repaints.
- Common Usage: Measuring layout, scroll positions, or performing DOM manipulations that must happen before the user sees the updated screen.
5.3 useInsertionEffect
- Timing: Runs before the DOM is mutated.
- Primary Usage: Inserting or updating things (often styles) so that the final DOM we commit to the screen is already in the correct state.
- Caution: Because it blocks rendering, it should be used sparingly and primarily for styling injections (e.g., CSS-in-JS).
6. Best Practices
- Limit Scope
- Only use useInsertionEffect for critical style operations or similar tasks that genuinely require insertion before the DOM is updated.
- Avoid Heavy Logic
- Because it blocks rendering, performing big computations can degrade performance and cause jank.
- Depend on Minimal Values
- If your effect depends on multiple states or props, ensure you only include necessary dependencies. Re-running too often might cause repeated style insertions and performance issues.
- Don’t Replace useLayoutEffect or useEffect
- Most side effects—like data fetching or event listeners—should remain in useEffect.
- Layout measurements or DOM read/write tasks are still best in useLayoutEffect.
- useInsertionEffect should be treated as a specialized tool for injecting styles or “pre-insertion” tasks.
- Check for Hydration Mismatches
- If you’re using SSR or Next.js, ensure that any style insertion done on the server matches what you do in useInsertionEffect to avoid hydration warnings.
7. Potential Pitfalls
- Overusing the Hook
- If you handle too many operations here, you’ll block rendering unnecessarily. Keep it minimal.
- Missing Cleanup
- Always return a cleanup function if you’re inserting elements (e.g., <style> tags). Otherwise, you’ll accumulate multiple style elements on re-renders or risk memory leaks.
- CSS Conflicts
- When dynamically inserting CSS, be mindful of scoping, naming collisions, or using consistent rules across the app.
- Unsupported in Older React Versions
- useInsertionEffect was introduced in React 18, so it won’t work in older versions. Make sure you’re using React 18 or 19+.
8. Example: Scoped Styling in a Library
Below is a more robust, hypothetical example of a scoped styling approach for a library that re-renders styles whenever props change:
import React, { useInsertionEffect, useState } from "react";
function useScopedStyles({ bgColor, textColor }) {
useInsertionEffect(() => {
const scopeClass = "scoped-" + bgColor.slice(1) + textColor.slice(1);
const css = `
.${scopeClass} {
background-color: ${bgColor};
color: ${textColor};
padding: 10px;
border-radius: 4px;
}
`;
// Insert the style
const styleEl = document.createElement("style");
styleEl.textContent = css;
document.head.appendChild(styleEl);
// Cleanup
return () => {
document.head.removeChild(styleEl);
};
}, [bgColor, textColor]);
}
export default function StyledButton() {
const [bg, setBg] = useState("#228B22");
const [fg, setFg] = useState("#FFF");
useScopedStyles({ bgColor: bg, textColor: fg });
function handleBgChange(e) {
setBg(e.target.value);
}
function handleFgChange(e) {
setFg(e.target.value);
}
return (
<div>
<input type="color" value={bg} onChange={handleBgChange} />
<input type="color" value={fg} onChange={handleFgChange} />
<button className={`scoped-${bg.slice(1)}${fg.slice(1)}`}>Click Me!</button>
</div>
);
}
- We define a hook that uses useInsertionEffect to dynamically create a unique class based on the given colors.
- Whenever the user changes bg or fg, we re-insert a new style block with updated rules, removing the old one.
- This can be beneficial if you need ephemeral or dynamic styling without re-render flickers.
9. Conclusion
useInsertionEffect is a powerful yet highly specialized hook in React 19. It ensures certain side effects (like style insertion) can happen before the browser sees any layout changes, preventing unwanted flickers or reflows. If you’re building a CSS-in-JS library, or need to dynamically manage critical style injections, useInsertionEffect is your go-to.
Key Takeaways
- Synchronous, Pre-Commit Hook: Runs before the DOM updates, giving you a chance to inject styles.
- Primarily for CSS: The main use case is dealing with dynamic or critical CSS.
- Use Sparingly: Inserting too much logic here can block rendering and hurt performance.
- React 18+: Ensure your project is on React 18 or React 19 to use this hook.
By using useInsertionEffect judiciously, you can create snappy user experiences with minimal flashing or layout shifts, maintaining a smooth and polished UI.
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