I would say C first. You need to learn the fundamentals:
- pointers
- allocation
- reference vs value
- recursion
- stack
- panics, errors, error propagation
- data structures
Many devs don't know it and they are honestly just clueless about anything they are doing. They just want to make it work.
JavaScript is just too high level, and makes you think you are immune to these low level concepts, but you are not.
And not only that, but also good practices, like:
- git
- linting
- types of tests and how to implement them
- working with third party dependencies