How to Optimize useInsertionEffect in React

This article provides a comprehensive guide on how to optimize the useInsertionEffect hook in React. Introduced in React 18, this highly specialized hook allows CSS-in-JS library creators to inject styles into the document before any layout effects run. Read on to understand its precise timing, why optimization is critical to prevent rendering bottlenecks, and the best practices for using it efficiently.

Understand the Timing of useInsertionEffect

To optimize useInsertionEffect, you must first understand when it executes. It runs synchronously before useLayoutEffect and before the browser paints the screen. At this stage, the React fiber tree has been mutated, but the DOM has not yet been read by layout effects.

useInsertionEffect(() => {
  // Runs first: Inject styles here
}, []);

useLayoutEffect(() => {
  // Runs second: Read DOM layout here
}, []);

useEffect(() => {
  // Runs third: Asynchronous effects
}, []);

By injecting <style> tags during useInsertionEffect, you ensure that when useLayoutEffect fires, the browser has already processed the new style rules. This prevents style recalculations from triggering multiple times during a single render pass.

Only Use It for CSS-in-JS Libraries

The most important optimization for useInsertionEffect is to avoid using it for general application logic. It does not have access to refs and cannot schedule state updates safely.

If you are fetching data, manipulating the DOM directly, or managing component state, use useEffect or useLayoutEffect instead. Restricting useInsertionEffect solely to dynamic style injection keeps your application’s rendering pipeline lean.

Avoid State Updates Inside the Hook

Do not update component state inside useInsertionEffect. Because this hook runs synchronously before the layout phase, triggering a state update here forces React to recalculate the component tree immediately, causing severe performance degradation. Keep the hook pure and focused strictly on inserting static or pre-computed style rules into the DOM.

Memoize Generated Styles

If your CSS-in-JS library generates styles dynamically based on props, recalculating and reinjecting these styles on every render is highly inefficient.

Optimize this process by caching generated CSS rules. Before inserting a new <style> tag, check if a style tag with the same hash or rule set already exists in the document head.

const styleCache = new Set();

function useDynamicStyle(cssRule) {
  useInsertionEffect(() => {
    if (styleCache.has(cssRule)) {
      return;
    }

    const styleTag = document.createElement('style');
    styleTag.textContent = cssRule;
    document.head.appendChild(styleTag);
    styleCache.add(cssRule);

    return () => {
      document.head.removeChild(styleTag);
      styleCache.delete(cssRule);
    };
  }, [cssRule]);
}

Implement Proper Cleanup

To prevent memory leaks and DOM bloating, always return a cleanup function to remove injected styles when the component unmounts. This is especially important in single-page applications (SPAs) where users navigate through many different views that may use unique, temporary styles.