A simple JSON stringify function which requires Array.prototype.map method.
The following users provided lots of ideas and improvements of the code.
@Kambfhase, @hartrock, @atk, @haochi, @jed
Actually, I just provided a deficient version at first. The other versions were created by these users(especially @atk). You may see the discussion below.
OK, got it down to 140bytes anyway... by reorganizing the test structure, I golfed away the final crucial byte:
I'll leave it to you to a) create the annotated version b) applaud us (I'd never made this alone) on this achievement.