Here is an image of the top lines of a simple(ish) app.js (not using the bluetooth stuff yet - all commented out)
So no biggie as app seems to work regardless, but I'm interested in why I get it and what can be done to remove it.
Some previous questions seem related to the use of something called Jest (no idea what that is - I suppose I should look it up?) Jest Testing React Native cannot use import statement outside of a module
Putting it here for benefit of others who may come across it.