Indeed, I've worked on billing system that relied heavily on pure JavaScript. Not even modern flavors with map/reduce, etc. - ECMAScript 5. It worked surprisingly well and our bottleneck wasn't the runtime but rather the databases we were constantly upserting to.

It sounds kinda crazy but with good change control, documentation, good relationship with the ETL team - it was pretty maintainable.

Heh I worked one of these. We handled arithmetic in the DB tho. Lot of PL/pgSQL running under the typescript, TS was more of like a middleware or API layer for things that could change more frequently. Finance code is to some extent transcribing of regulation & law into code and we kept all that in postgres.

Crossed fingers, in my experience.