iOS

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")

Android

On Android you can resort to the RNLog functions:

import com.facebook.react.util.RNLog;

// Your code
RNLog.a("Your message");