I have an API project where I've been successfully using Jest (v26.x because code coverage doesn't work with v27.x on Windows 10 for me) for unit testing. I've run into a situation where I'm trying to achieve sufficient code coverage on a module I'm testing, but not all functions in that module have been exported, so this is impeding my ability to achieve this.

I came across the NPM package rewire, which worked beautifully, as it allowed me to mock any functions I wanted. The only problem is that code coverage doesn't acknowledge the lines tested via rewire and the devs in the Jest GitHub said they don't plan to support this.

So then I saw a couple of posts that people were successfully using babel-plugin-rewire to achieve the same thing as rewire, but that code coverage lines were successfully being acknowledged with this package. However, I cannot get this setup to work.

I think I understand the basics of Babel, as it pertains to browsers. It will take newer JS code and transpile/polyfill so that older JS code is generated that will work with older browsers of your choice.

But then I started seeing stuff about babel-jest and Babel being integrated into Jest and so on. Aside from the fact that I can't get babel-plugin-rewire to work, I feel like I don't even understand the basics of how Babel and Jest work together, or why Jest needs Babel.

What is the relationship between Babel and Jest?