The muscle memory is so strong, when writing Javascript/Typescript I type # by accident sometimes.
But then, // is one of the most commonly used operators in another language I use, so it's weirdly symmetrical :)
Like ||, but // tests definedness instead of truthiness.
The equivalent in several other languages is ??, which is uglier imho, but it's just syntax. https://en.wikipedia.org/wiki/Null_coalescing_operator
I use // and ?? often as it's conceptually clean, fast and concise. When available, it arises naturally in logic like caches, memoizations and defaults. //= is pretty useful too, and I'm surprised it's not more widely available.