why both tsconfig-esm.json and tsconfig.json?

[xmlking]

Frist of all, thanks for a gready blog post Distributing an Angular Library - The Brief Guide blog
just wonder why you have two config files i.e., tsconfig-esm.json and tsconfig.json in this repo.
Is there any specific need for it?
your package.json only refers to tsconfig-esm.json

[mgechev]

It's used by ngc, which was incorrectly referencing to tsconfig-esm.json.