I'm looking forward to the split of google-gax into two modules :) For now, I'm hesitant of adding yargs as a runtime dependency. Even though it's not used by the code in runtime, it still adds a whole megabyte (!!!) to node_modules:

fenster-macbookpro:y fenster$ cat package.json 
{
  "dependencies": {
    "yargs": "^17.7.1"
  }
}
fenster-macbookpro:y fenster$ npm install 

added 16 packages, and audited 17 packages in 226ms

2 packages are looking for funding
  run `npm fund` for details

found 0 vulnerabilities
fenster-macbookpro:y fenster$ du -sh node_modules/
1.0M	node_modules/

which will increase not only the download size, but also a chance of us getting security vulnerability bugs. Since it's just about one extra command line option, would it be possible to parse argv manually? A simple for loop over argv should be enough here.

style nit: it will be a little bit more readable if we do

let options;
if (isJs) {
  options = { ... };
} else {
  options = { ... };
}
const output = uglify.minify(content, options);

so we call minify just once in the code.

Here I'm wondering if the second require of the same filename will just take it from cache. Maybe either invalidate the cache (https://stackoverflow.com/questions/9210542/node-js-require-cache-possible-to-invalidate) or just copy it into another file just to be sure.

Would you also want to change this in the test above? I just copied your pattern for the previous test: