How to Debug useInsertionEffect in React

Debugging useInsertionEffect in React can be challenging due to its unique execution timing before DOM mutations. This article provides a straightforward guide on how to effectively debug this hook, understand its execution order, and resolve common issues related to dynamic CSS-in-JS injection.

Understand the Hook’s Timing

To debug useInsertionEffect successfully, you must understand exactly when it runs. It fires synchronously before any DOM mutations and before useLayoutEffect or useEffect. Because it runs before the DOM is updated, you cannot access DOM nodes via refs inside this hook; doing so will yield null or outdated elements.

Track Execution Order with Console Logs

The most direct way to debug timing issues is by tracking the sequence of execution. Place console logs in your component’s hooks to verify that they are firing in the correct order:

useInsertionEffect(() => {
  console.log('1. useInsertionEffect runs');
}, []);

useLayoutEffect(() => {
  console.log('2. useLayoutEffect runs');
}, []);

useEffect(() => {
  console.log('3. useEffect runs');
}, []);

If the logs do not appear in this exact sequence, or if useInsertionEffect fires more times than expected, inspect your dependency array for values that change too frequently.

Inspect the Document Head

Because useInsertionEffect is primarily designed for injecting <style> tags into the DOM before layout calculations occur, you should verify the DOM output. Open your browser’s Developer Tools, navigate to the Elements tab, and expand the <head> section. Trigger the component render and watch if the <style> tags are being injected, updated, or removed correctly during mount and unmount cycles.

Use the Browser Debugger

You can pause execution at the exact moment of style insertion by placing a debugger; statement inside the hook:

useInsertionEffect(() => {
  debugger; // Execution pauses here
  insertStyles(rules);
}, [rules]);

When the browser pauses execution, inspect the call stack to see what triggered the render and check the local variables to ensure the CSS rules being injected are correct.

Avoid Common Pitfalls

If you encounter bugs, verify that you are not committing these common mistakes: