No biggie, I just named an example. Even "real" programmers, whatever definition you want to use for that, don't know all languages.

But yes, many languages do use // as comments and particularly the Javascript environment it originally stems from does. Python (#) and SQL (--) are the only examples I interact with frequently enough to know off the top of my head (but SQL still recognises /* ... */ for delimited comments). XML/HTML also has <!-- --> for comments, but that's not so much a programming and more of a description language.