Question

Could I run rake assets:precompile for specific JavaScript file?

Otherwise the full precompile lasts for 5 minutes and makes quick changes in JavaScript files very annoying.

Answer 1

Short: You can’t.

During precompilation Rails goes through the Application.js file and merges all imports into one so just changing one file is simply not possible due to the compression that goes on in there. (It doesn’t do anything to files not referenced from application.js)

Next up: You should not have to run rake assets:precompile during development when doing quick fixes. Only on deployment where (depending on your patience) it should be no problem having the task run 5 minutes.

You should be using the development environment during development where asset precompilation is not necessary because Rails will serve the assets unmerged and un-minified.
If you are running the Rails build in web-server through rails s this should be by default, but you can explicitly start the rails server using:

rails s RAILS_ENV=development

If the assets are still not correctly displayed or you see errors make sure you have config.assets.debug = true