@neurobashing eslint with plugins + crank up the strictness (very easy to run with no rules enabled by accident)
you can run TS on unannotated js code it will find a few problems.
I'm looking for a "property testing" tool in js, havent' found one I like (it's unit tests with smarter random data generation)
sonar never seems to find much to complain about
Without type annotations, I'd put my effort into more & more unit tests/higher coverage.