On iOS the default output (NSLog) is by default disabled on the project. You can re-enable it by editing the scheme you are using, going to the
Arguments tab, there you will see a
OS_ACTIVITY_MODE flag, you can clear the value and you will start seeing all the logs.
If you don’t want to use NSLog (or print on Swift I suppose) you can use the RCTLog functions:
#import <React/RCTLog.h> // Your code RCTLogWarn(@"Your message")
On Android you can resort to the RNLog functions:
import com.facebook.react.util.RNLog; // Your code RNLog.a("Your message");