---
title: index.[tj]s
sidebar_position: 4
---
# index.[tj]s

The identifier for the entry point when the application uses custom `bootstrap`.

In general, the [`App.[tj]sx`](/apis/app/hooks/src/app) hook file can already meet our needs. When we need to add custom behavior before `bootstrap` or completely take over the webpack packaging entry, we can place `index.[tj]s` in the `src` or entry directory. There are two cases below:

## Add custom behavior before bootstrap

Just export a function in `src/index.[tj]s`:

```js title=src/index.js
import { bootstrap } from '@modern-js/runtime';

export default App => {
  // do something before bootstrap...
  bootstrap(App, 'root');
};
```

## Completely take over the webpack entry

When there is no default export function in `src/index.[tj]sx?`, the file is the real webpack packaging entry file and the code can be organized directly as using create-react-app and other scaffolding tools:

```js title=src/index.jsx
import React from 'react';
import ReactDOM from 'react-dom';
import App from './App';

ReactDOM.render(<App />, document.getElementById('root'));
```
