# Capture App Start Errors | Sentry for React Native

By default, the React Native SDK initializes the native SDK underneath the `init` method called on the JS layer. As a result, the SDK has a current limitation of not capturing native crashes that occur prior to the `init` method being called on the JS layer.

Starting with SDK version 8.0.0, you can initialize Sentry natively before JavaScript loads, enabling capture of app start errors and crashes that occur during:

* Native module initialization
* JavaScript bundle loading
* Early React Native bridge setup

This feature uses a `sentry.options.json` configuration file and native initialization APIs that read from this file.

##### SDK Version Requirement

This feature requires Sentry React Native SDK version 8.0.0 or higher.

## [Configuration File](https://docs.sentry.io/platforms/react-native/manual-setup/app-start-error-capture.md#configuration-file)

Create a `sentry.options.json` file in your React Native project root with the same options you currently have in `Sentry.init`:

`sentry.options.json`

```json
{
  "dsn": "https://key@example.io/value",
  "debug": true,
  "environment": "production",
  "tracesSampleRate": 1.0,
  "enableTracing": true
}
```

##### Options Merging

Options from `sentry.options.json` are merged with options from `Sentry.init()` in JavaScript. Options specified in JavaScript take precedence over the configuration file, allowing you to override settings at runtime.

## [Android Setup](https://docs.sentry.io/platforms/react-native/manual-setup/app-start-error-capture.md#android-setup)

Initialize Sentry in your `MainApplication` class:

```kotlin
import io.sentry.react.RNSentrySDK

class MainApplication : Application(), ReactApplication {
    override fun onCreate() {
        super.onCreate()
        RNSentrySDK.init(this)
        // ... rest of your initialization code
    }
}
```

## [iOS Setup](https://docs.sentry.io/platforms/react-native/manual-setup/app-start-error-capture.md#ios-setup)

Initialize Sentry in your `AppDelegate` before starting React Native so app start crashes are captured.

```objective-c
#import <RNSentry/RNSentry.h>

@implementation AppDelegate

- (BOOL)application:(UIApplication *)application
    didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    [RNSentrySDK start];
    return [super application:application didFinishLaunchingWithOptions:launchOptions];
}

@end
```

## [Expo Setup](https://docs.sentry.io/platforms/react-native/manual-setup/app-start-error-capture.md#expo-setup)

If you're using Expo, you can enable native initialization using the Expo plugin. First, create a `sentry.options.json` file in your project root (same location as your `app.json`) [as above](https://docs.sentry.io/platforms/react-native/manual-setup/app-start-error-capture.md#configuration-file).

Add the `useNativeInit` option to your Expo plugin configuration:

`app.json`

```json
{
  "expo": {
    "plugins": [
      [
        "@sentry/react-native/expo",
        {
          "useNativeInit": true
        }
      ]
    ]
  }
}
```

If you need to use environment variables or dynamic configuration, you can use `app.config.js` with the `withSentry()` wrapper instead of the array form in `app.json`:

`app.config.js`

```javascript
import { withSentry } from "@sentry/react-native/expo";

export default withSentry(config, {
  useNativeInit: true,
});
```

When `useNativeInit` is set to `true`, the Expo plugin automatically:

* Adds `RNSentrySDK.init()` to your Android `MainApplication`
* Adds `RNSentrySDK.start()` to your iOS `AppDelegate`