I would like to take this opportunity to point out that the author of "Eloquent JavaScript", Marijn Haverbeke, also created both CodeMirror and ProseMirror [1], which are both extremely well-regarded open source code/text editing tools. I can especially recommend ProseMirror after spending a lot of time with it recently. It's an actually sane wrapper around contenteditable.

[1] https://prosemirror.net/