First, D3 is not primarily a DOM library or UI framework; it’s a loose collection of tools for data visualization, and is designed to work with other UI libraries.
Second, the main difference with d3-selection (the part of D3 that does focus on the DOM) is that it focuses on transformation rather than representation. You describe changes that you want to apply to the DOM, rather than the desired current state of the DOM. This makes it harder to use (more complexity) than more declarative frameworks (UI as pure functions of state) such as React. But the advantage is that you can control exactly what happens as you change the DOM, which can make it faster, and makes it easier to animate transitions.
Can you comment on why you think d3 is harder to use than a paradigm like react? Isn’t UI also a function of data in d3? When you make changes to the data the tree (DOM) is also updated. That’s why the differences are blurry to me.
I would be curious to see how d3 scales in complexity for reactive UI apps... such as todo lists, forms, pages with buttons, etc.
I’m saying selections are harder than React, not that D3 is harder than React (because you can use React with D3 without selections). And selections are harder because, although they are functions of data, they are used to apply transformations (deltas) rather than declare representation (state). And as I say in the tutorial, thinking in terms of transformations is most useful when you want animated transitions and to optimize incremental updates. So for example I try to let Observable’s dataflow handle as much state as possible, rather than doing everything in event listeners.
Second, the main difference with d3-selection (the part of D3 that does focus on the DOM) is that it focuses on transformation rather than representation. You describe changes that you want to apply to the DOM, rather than the desired current state of the DOM. This makes it harder to use (more complexity) than more declarative frameworks (UI as pure functions of state) such as React. But the advantage is that you can control exactly what happens as you change the DOM, which can make it faster, and makes it easier to animate transitions.