Comment by jchw

If you were going to use a C library, the most logical is QuickJS since it has Python bindings, is small, perfectly fast enough for the kind of needs yt-dl has, and it has excellent coverage of the standard and passes conformance tests.

That said I think a decent Python-native JS interpreter isn't that bad of an idea, it definitely needs a separate project and a more sophisticated architecture but it's an attainable goal.