To me, this is the only thing that seems like change in behavior.

So i understand, before if there was no fetch passed into the config and no global fetch available, we'd have not failed in the constructor, but failed later inside of postJSON when we tried to execute the undefined thing. Correct?

But our change here catches this in the constructor and yells before trying to use it.

Correct. These days it's unlikely there's no fetch available but rather than have an internal Node exception bubble up. I wanted to fail fast with a clear exception message

Changed this a little in ecae723.

this.fetch = config.fetch || fetch!

This will raise a Uncaught ReferenceError: fetch is not define - which is easily googlable