One of the things I've been noticing as I vaguely and casually keep up with technology news is that JavaScript seems to be showing up more and more as an extension language, especially in heavy-usage environments. The most recent example is Cloudflare Workers, but there are plenty of other places that support it, such as AWS Lambda. One of the reasons for picking JavaScript here is adequately summarized by Cloudflare:

After looking at many possibilities, we settled on the most ubiquitous language on the web today: JavaScript.

JavaScript is ubiquitous not just in the browser but beyond it. Node.js is popular on the server, Google's V8 JavaScript engine is apparently reasonably easy to embed into other programs, and then there's at least Electron as an environment to build client-side applications on (and if you build your application in JavaScript, you might as well allow people to write plugins in JavaScript). But ubiquity isn't JavaScript's only virtue here; another is that it's generally pretty fast and a lot of people are putting a lot of money into keeping it that way and speeding it up.

(LuaJIT might be pretty fast as well, but Lua(JIT) lacks the ecology of JavaScript, such as NPM, and apparently there are community concerns.)

This momentum in JavaScript's favour seems pretty strong to me as an outside observer, especially since its use in browsers insures an ongoing supply of people who know how to write JavaScript (and who probably would prefer not to have to learn another language). JavaScript likely isn't the simplest option as an extension language (either to embed or to use), but if you want a powerful, fast language and can deal with embedding V8, you get a lot from using it. There are also alternatives to V8, although I don't know if any of them are particularly small or simple.

(The Gnome people have Gjs, for example, which is now being used as an implementation language for various important Gnome components. As part of that, you write Gnome Shell plugins using JavaScript.)

Will JavaScript start becoming common in smaller scale situations, where today you'd use Lua or Python or the like? Certainly the people who have to write things in the extension language would probably prefer it; for many of them, it's one fewer language to learn. The people maintaining the programs might not want to embed V8 or a similar full-powered engine, but there are probably lighter weight alternatives (there's at least one for Go, for example). These may not support full modern JavaScript, though, which may irritate the users of them (who now have to keep track of who supports what theoretically modern feature).

PS: Another use of JavaScript as an 'extension' language is various NoSQL databases that are sometimes queried by sending them JavaScript code to run instead of SQL statements. That databases are picking JavaScript for this suggests that more and more it's being seen as a kind of universal default language. If you don't have a strong reason to pick another language, go with JavaScript as the default. This is at least convenient for users, and so we may yet get a standard by something between default and acclamation.