I heard it on a podcast and I am going to butcher the explanation but they explained it as knowing the “abstract machine”.

When I started learning AppleSoft Basic back in the 80s, I knew how to optimize my BASIC programs because I knew how the interpreter worked and so knew assembly.

When I started learning C#, I knew both C, Win APIs and how the CLR worked.

But for a web developer who communicates with backend servers, I think it’s important to understand HTTP.

I've heard a similar idea on Hanselminutes. The way he states it is that everything is layers of abstractions, and in order to be great at whatever your chosen layer is, you should be pretty good at the layer below it and of passable quality (think junior dev level) at the layer below that.

Actually it might have been Hanselminutes. Do you know which episode?

I've heard it a handful of times on various episodes but I know he's brought it up when speaking to folks who work primarily one layer below him.

What podcast was that? Sounds legit! I'm always on the hunt for new dev focused podcasts.

I don’t remember the episode unfortunately. It was an episode of Software Engineering Radio (http://www.se-radio.net/)

I would think one level down from React is the DOM API, and one level down from that is the rendering engine (i.e. knowing about how DOM operations affect repaints, whether repaints block the main thread, etc.

Language-wise, I'd say babel "javascript" is the highest level of abstraction. One level below is understanding what things like JSX and class properties transpile to. One level below is understanding JIT strategies (e.g. when hidden classes deopt), memory usage of various patterns, etc. One level below is understanding specific semantics of high level APIs (e.g. exact accuracy of a high res timer implementation), or regexp memory footprint by looking at browser source code.

For a react developer I'd say one level is understanding reactive design and maybe how you would implement it in pure JavaScript without react. One level below that would be how you would implement JavaScript and maybe a bit of knowledge how the engine works and shadow Dom.