Running React Native on Multiple Devices or Emulators: Troubleshooting the "Error in react native start"
Scenario: You're developing a React Native app and are ready to test it on various devices, but when you try to run react-native start you encounter an error. This error message usually stems from the fact that you're trying to run your app on more than one device or emulator simultaneously, which can lead to conflicts.
Example Code (Error Message):
$ react-native start
info Starting server...
info Starting Metro Bundler...
Starting the development server on http://localhost:8081
info Waiting for debugger to connect...
ERROR: Error: listen EADDRINUSE: address already in use :::8081
Understanding the Problem:
The EADDRINUSE error means that the port 8081 (the default port for the React Native development server) is already being used by another process. This can happen if you have another React Native project running, or if you have a web server or other application running on the same port.
Solutions and Insights:
-
Check Running Processes:
- On Windows: Use Task Manager (Ctrl+Shift+Esc) and look for processes using port 8081.
- On macOS: Use Activity Monitor (found in /Applications/Utilities) and filter by "Port" to find the process using 8081.
- On Linux: Use the command
netstat -a | grep :8081to see what process is using the port.
Once identified, terminate the process to free up the port.
-
Change the Port:
- In
package.json: Modify thestartscript to use a different port:"start": "react-native start --port 8082" - In
android/app/build.gradle: Set thedevServerPortto a different port.... applicationVariants.all { variant -> variant.outputs.all { outputFileName = "app-${variant.name}-${variant.versionName}-${variant.versionCode}.apk" // ... def variantName = variant.name if (variantName.toLowerCase().contains("debug")) { signingConfig signingConfigs.debug // ... manifestPlaceholders = [ devServerPort: "8083" // Change the port number ] } } } ...
- In
-
Simultaneous Development:
- For multiple devices/emulators: To run your app on multiple devices simultaneously, use the "Expo Go" app for each device, which connects to the same development server.
- For different projects: It's generally recommended to run each React Native project on a separate port. Use the method described in the previous solution to achieve this.
-
Firewall:
- In rare cases, your firewall might be blocking the development server. Temporarily disable your firewall or configure it to allow access to port 8081 (or the port you're using).
Additional Considerations:
- Use a different port for each project: This avoids conflicts and provides a more organized development environment.
- Keep your environment clean: Regularly close applications and processes you're not actively using to avoid port conflicts.
- Consider a dedicated development machine: If you find yourself frequently running into port conflicts, a dedicated machine specifically for React Native development might be helpful.
References:
Conclusion:
Running React Native on multiple devices or emulators requires careful consideration of port usage and potential conflicts. By understanding the cause of the error and following these solutions, you can seamlessly develop and test your app on multiple platforms.
