regular javascript is actually okay in my book. It's just node.js that I despise because of all it's weirdness. Javascript should just be able to be opened in a browser and run with no fuss.
Yes, but then the browser will fetch them with separate HTTP requests. You might not want to wait for 10 requests to finish before your web app can load.
Also if you want to use the features of TypeScript or SASS (many people do), you will need to turn that code into something the browser will understand.