From a practical standpoint, WASM is "pure JS" insofar as if I am browsing libraries and see one advertising itself as "pure JS", by convention that to me means

From a practical standpoint, WASM is "pure JS" insofar as if I am browsing libraries and see one advertising itself as "pure JS", by convention that to me means "no FFI" or "potentially works in browser". The only place I've seen "pure JS" used in the JS world is differentiating e.g. a Postgres client implementation that does or doesn't depend on the specific version & configuration of libpq you have on your current system. That's about runtime deps, not about source language.