[AgentME]

This completely matches my experience. I've worked on a 200k+ line javascript codebase that originally used just vanilla HTML DOM management and jQuery for everything, and I've helped introduce React into it over the years.

Every time the old code needed to place a view inside of another view, it was a fresh adventure in the exact way that sort of extremely common routine operation shouldn't be. Understanding a module always meant understanding its approach to managing its DOM elements before you even got to its business logic. Modules often chose between having simple-but-janky code that re-created DOM elements from scratch every time anything in the view changed, or having more-efficient-but-complex-and-buggy code that intelligently updated only the changed elements but often had inconsistencies with the first-render code causing some values to not work if they got set while the view was already mounted. People tried to establish some conventions across a few modules, but the chosen conventions often had significant drawbacks or only worked in specific cases because the people making the conventions didn't have the experience of library/framework authors.

I remember working with a coworker sketching out an idea of a convention we could develop and use for updating text and substituting in translations and live values. One of my main goals was to allow our modules to have the same code for the first-render and subsequent renders without having to replace all the module's elements with new ones. It was originally intended for just updating text, but I started to realize it would have to handle arbitrary HTML and embedding views from other modules to really be useful. The idea felt like it would need special cases to handle that, and I couldn't get the idea further. Then I later found React, and I realized it was a generalized approach to accomplish exactly what we were trying to design. I got the green light to use it in a few new modules, and even coworkers who weren't yet familiar with React could see how the modules were much more focused on the actual business logic rather than being absolutely filled with so much unique DOM-manipulation code. Everyone was sold on it within a few months, and people that needed to make changes to old modules would often choose to move the code over to React first to make it simpler to work on.