Bug: In the unmount function returned by createRoot, an imprecise comparison statement causes the first unmount failed, affecting subsequent program operations.

[swqddx]

React version: 19.1.0

Steps To Reproduce

1. Start a react app, and unmount， it failed.
   This can cause some problems when applied as a multi-instance child in a micro frontend

Link to code example:

The current behavior

The expected behavior

[pcentric]

🧨 The Root Cause

You're seeing:

TypeError: Cannot read properties of undefined (reading '_internalRoot')

This is happening because:

args = this._internalRoot;
if (null !== args) {

fails when this is not the actual root instance (or has been destroyed already), and this._internalRoot is undefined.

This can occur:

• If you call root.unmount() twice

• Or call unmount() on a stale/non-ReactDOMRoot object

• Or in a micro-frontend setup where React apps are mounted/unmounted multiple times without cleanup

---

✅ Fix / Workaround

Here’s how to safely unmount a React 18+ root without triggering this bug:

✅ 1. Track the root object explicitly

// index.tsx or micro-frontend mount script
let root: ReactDOM.Root | null = null;
export function mount(container: HTMLElement) {

if (!root) {

root = ReactDOM.createRoot(container);

root.render(<App />);

}

}
export function unmount() {

if (root) {

try {

root.unmount();

} catch (err) {

console.error("Failed to unmount:", err);

} finally {

root = null;

}

}

}

✅ 2. Ensure unmount() is never called twice

If you're in a micro frontend (like single-spa or Qiankun), check whether the container has already been removed before calling unmount().

if (container && document.body.contains(container)) {
  unmount();
}

---

🧪 Bonus Tip for Tests

If you're using createRoot in unit or integration tests, you must ensure you call unmount() exactly once per root, or use cleanup() from @testing-library/react to avoid this issue.

---

📌 Summary

Issue	Fix
root.unmount() fails	Store root in a variable and check it exists before unmounting
Called twice / stale root	Guard with if (root) and clear reference after unmount
Microfrontend mount/unmount loop	Verify DOM container before unmount

Let me know your micro frontend framework or app setup, and I can tailor a safer wrapper for it.