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

Answer: 
You have two options: 

1. Debug remotely

iOS: Cmd + ctrl + z to open menu and select "Debug remotely"

Android: Cmd + M to open menu and select "Debug remotely"

2. Or run one of these commands:

```
react-native log-ios
react-native log-android

```
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).