It has so-called JS client. But all the docs or demos are written from the point of Python developer.
Does bokeh has standalone, non-python JavaScript API, and is it used by anyone in non-python environments?
Does bokeh has standalone, non-python JavaScript API
As of late 2019: Somewhat! (See below for more context)
and is it used by anyone in non-python environments?
Yes, definitely, though pure-BokehJS usage levels are still low compared to Python APIs. Improving the JS story is a 2020 goal.
The Bokeh project was started in 2012 with the explicit goal of providing Python developers a way to publish interactive visualizations in the web, without themselves having to get into "web tech", i.e. JavaScript. As such, the BokehJS library (which has always existed) was originally mostly a largely undocumented implementation detail. It didn't really help that the Bokeh developers themselves were not JS experts at the time. (Some of us still are not!)
As things progressed, and features like CustomJS callbacks and the ability to make custom extensions were added, the BokehJS side of things became more and more publicly exposed. That said, until fairly recently, BokehJS development has been very fast and furious and we were not in any position to provide guarantees around core API stability or expend resources on documentation that would likely be out of date very quickly. As two examples, in the last year BokehJS was completely re-written in TypeScript, which rendered any old CoffeeScript extensions or callbacks deprecated. Additionally the entire layout system was re-vamped to afford much higher performance.
For some time, there has been a fairly stable "high level" API for BokehJS, and you can find details of that in the Developing with JavaScript chapter of the users guide. Additionally, all the low level "models" and their properties are 100% aligned up between Python and JS, so the Python Reference Guide actually has all the information you might need to use models on the JS side as well.
We are very interested in improving BokehJS for pure JS usage in the coming year. We have been getting some very helpful issues from folks actually using BokehJS directly. Some major hurdles will be overcome with the upcoming 2.0 release, but there will still be work to do to really provide a great user-experience for JS devs. This is actually a fantastic opportunity for any interested JS devs to have a big impact by offering their input, advice, and collaboration. Anyone so interested should head over to the Bokeh project Discourse.