JSX unnamed export default is subtly broken #10077
Description
🐛 bug report
If I write component with default export and without name like: export default function(){...} the component is getting unmounted/mounted instead of just rerendered. It is different to the default export with a name like export default function Foo(){...}. The name is not important. It happens only in watch and serve mode.
🎛 Configuration (.babelrc, package.json, cli command)
{
"parcel": "^2.13.3",
"react": "^19.0.0",
"react-dom": "^19.0.0"
}🤔 Expected Behavior
It should be the same like with name.
In the following code sample I expect to see only Test mounting once and then repeating of Rendering...
😯 Current Behavior
In watch and serve it differs from the named function.
In the sample code I am getting repeatedly:
Rendering...
Test unmounting
Test mounting
💁 Possible Solution
🔦 Context
💻 Code Sample
index.html
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
</head>
<body>
<div id="root"></div>
<script type="module" src="index.jsx"></script>
</body>
</html>index.jsx
import React from 'react';
import ReactDOM from 'react-dom/client';
import { App } from './app.jsx'
ReactDOM.createRoot(
document.querySelector('#root')
).render(<App />)app.jsx
import { useState, useEffect } from 'react';
import Test from "./test.jsx"
export function App() {
const [foo, setFoo] = useState();
console.debug("Rendering...")
// every second change state randomly
useEffect(function () {
const interval = setInterval(() => Promise.resolve().then(_=>setFoo(Math.random())), 1000)
return () => clearInterval(interval)
}, [])
return (<Test />)
}test.jsx
import { useEffect } from 'react';
export default function () {
// This just prints when it is mounting/unmounting
useEffect(() => {
console.log("Test mounting");
return () => console.log("Test unmounting")
}, []);
return <span style={{ color: "red" }}>Lorem ipsum</span>
}🌍 Your Environment
| Software | Version(s) |
|---|---|
| Node | v23.6.0 |
| npm/Yarn | npm - 11.0.0 |
| Operating System | win11 |