Convention works when the culture is there, but I think you're right a dash of typescript and a class or interface definition could go a long ways.

I think the maintainability comes from easy debugging. Stack traces are sensible and the code is straightforward. Look at a React stack trace and nothing in the trace will tell you much about _your_ code.

I'd also point out that this looks like it's about seven years old. We've shifted a lot of norms in that time.