.onion/ is the term you are looking for. There must be prefix/suffix match text comparison functions, if it breaks the URL down. Scheme, address, path and query. I find it hard to believe that JavaScript lacks an equivalent of Go's url.URL type. Then it is literally just a suffix match on the address element.
Discussion
ws://ws/oxtrdevav64z64yb7x6rjg4ntzqjhedm5b5zjqulugknhzr46ny2qbad.onion
The issue is that there's an extra ws/ in the URL, which actually makes xxx.onion part of the path, not the host.
JavaScript does have a URL class too. To be more rigorous, I should extract the host and match the suffix there.