Building a custom Contentful Rich Text editor: the round-trip problem

Contentful lets you replace the UI of a Rich Text field with your own App. That sounds like a front-end job — build a nicer editor, mount it in the field location, done. It isn't. The editor UI is the easy 20%. The hard 80% is a small file most people never think about: the layer that converts between your editor's internal model and Contentful's stored document — losslessly, in both directions. I built one on top of PlateJS (which is a Slate editor underneath), and this post is about that conve