Software that parses source code and outputs only the signatures the function names and what types they receive and output

Question

I would like a program that reads a program, "folds" the functions, and just gives me the signatures/top level comments (optionally) of the functions, and just the function signatures, this can be done for a variety of programming languages, but I'd like it for JavaScript, C++, and Rust.

Think about it as a .h file generator, except it works in languages that don't need one, (JavaScript)