[Quoth]

Most people (about 99.99% of readers) using ebooks just want to read actual books on a portable device for convenience (phones) or on eink for readability and convenience. They expect the content to be the same as paper. They don't mind if the formatting is a bit simpler. Few obsess about it being identical format to a particular paper edition, that is usually publisher or author egotism. The important thing is easy to consume (read) content.

The only "more" they expect is easy TOC access, bookmarks, search, library management and way down the list is highlights and annotation.
They want to be able to change the font size.

Formatting most ebooks needs no javascript or human editing, just conversion of a properly styled docx. Then the epub, awz3 and kfx are similar to original. Even the mobi can be usably similar.

I've edited / written javascript since late 1990s, also later Adobe EMCAscript (Flash based applications that were not video). I've produced multimedia interactive content many years earlier. Books, even if poems, plays or textbooks don't need javascript at all ever. I've coded for servers where indeed the content might be in a database. Horrid files with a mix of coldfusion, SQL stored procedures, java, php, html and javascript. Fortunately ebooks are simpler.

An interactive application with a lot of reading content can use javascript as way to implement it. However that is then really needing a web page, so an iOS/Android framework you just add your text & images content to and edit the controls/logic is easier to distribute to Google, Apple & Amazon stores, though you can wrap your JS based web thing as an app (but on iOS to date that is a problem).