Question: How do you debug react-native when it is running on device ?

Answer: 
Figured our React Native debugging for Android. Steps to for someone struggling with this -

1. Launch your RN App
2. Shake device
3. Select "Dev Settings"
4. Select "Debug server host & port for device" Put in your system's IP address.
5. Shake device
6. Select "Debug JS remotely"
7. Chrome will open a new tab with the address "<http://localhost:8081/debugger-ui>"
8. If this doesn't happen, check your port and open a new tab and enter the above address with your port number.
9. Open #Chrome DevTools (Cmd+Alt+I on #MacOSX)
10. Select the Console Tab
11. Shake Device
12. Select "Reload"

View all your console logs or use "debugger;" in your JS for breakpoint debugging.
If you are working on **Visual Studio Code** then you can install `React Native Tools` extension (provided by `Microsoft`) and debug your app like any other IDE.

Read all steps in detail in my [this answer](https://stackoverflow.com/a/56233781/1055241).