# Custom Web Index

You might've noticed that there's no call to ReactDOM.render anywhere in your Redwood App (v0.26 and greater). That's because Redwood automatically mounts your <App /> in web/src/App.js to the DOM. But if you need to customize how this happens, you can provide a file called index.js in web/src and Redwood will use that instead.

To make this easy to do, there's a setup command that'll give you the file you need where you need it:

yarn rw setup custom-web-index

This generates a file named index.js in web/src that looks like this:

// web/src/index.js

import ReactDOM from 'react-dom'

import App from './App'
/**
 * When `#redwood-app` isn't empty then it's very likely that you're using
 * prerendering. So React attaches event listeners to the existing markup
 * rather than replacing it.
 * https://reactjs.org/docs/react-dom.html#hydrate
 */
const rootElement = document.getElementById('redwood-app')

if (rootElement.hasChildNodes()) {
  ReactDOM.hydrate(<App />, rootElement)
} else {
  ReactDOM.render(<App />, rootElement)

This is actually the same file Redwood uses internally. So even if you don't customize anything any further than this, things will still work the way the should!