`.enter()` and `.exit()` is not covered at all, simply outsourced to the guide/demo by Mike Bostock, which is actually a really nice demo however it also omits explaining the mechanism.
I also specifically didn't want to go into the data join patterns for two main reasons:
1. there are lots of other articles that talk about it in depth, and 2. these days, I largely don't use it at all.
This is partially why I wanted to write this up, because the DOM manipulation parts can seem like an integral part of D3, but there are really only 2 modules devoted to it. I mostly use D3 for its utility methods, and a JS framework like React to handle all of the rendering, and I really want other people to be able to discover those functions without first "learning d3".
These days I also tend not to use d3 for DOM manipulation, but only for its powerful helper functions (d3-scale, d3-array, d3-shape, d3-geo etc.)
I even try not to import d3-selection except for very specific cases.
I’m using Vue for DOM manipulation, and computed properties play very well with d3 helper functions (i.e. a scale which auto-updates itself reactively). In a way it’s very similar to ObservableHQ’s reactivity. Most examples from Observable can be relatively easily converted to Vue with computed properties.