Expo Docs

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registerRootComponent

Sets the main component for Expo to use for your app.
Note: Prior to SDK 18, it was necessary to use registerRootComponent directly, but for projects created as of SDK 18 or later, this is handled automatically in the Expo SDK.

  • component (ReactComponent) -- The React component class that renders the rest of your app.

No return value.
Note: Expo.registerRootComponent is roughly equivalent to React Native's AppRegistry.registerComponent, with some additional hooks to provide Expo specific functionality.

  • Before continuing, make sure your project is running on SDK 18 or later.
  • Open up main.js (or if you changed it, whatever your "main" is in package.json).
  • Set "main" to "node_modules/expo/AppEntry.js".
  • Delete the Expo.registerRootComponent call from main.js and put export default before your root component's class declaration.
  • Rename main.js to App.js.

You can set the "main" in package.json to any file within your project. If you do this, then you need to use registerRootComponent;export default will not make this component the root for the Expo app if you are using a custom entry file.
For example, let's say you want to make "src/main.js" the entry file for your app -- maybe you don't like having JavaScript files in the project root, for example. First, set this in package.json:
{
  "main": "src/main.js"
}
Then in "src/main.js", make sure you call registerRootComponent and pass in the component you want to render at the root of the app.
import Expo from 'expo';
import React from 'react';
import { View } from 'react-native';

class App extends React.Component {
  render() {
    return <View />;
  }
}

Expo.registerRootComponent(App);