How is that the context? In the sense of the intersection between Javascript and JSON, trailing commas have pretty much always worked in C, which is something all of us who write code that generates C rely on, like a lot.

I googled and trailing comma was allowed since C89, I was wrong. For code that generates code I always did something like this cause of that mistake:

  { a, b
  , c
  ...
  , z
  };