If you are not bundling your code to begin with which you wouldn't be without a build step; I think there are ways to reload when a file is changed. Like an extension that speaks to a file system watcher.
Hot [module] reloading refers to dynamically swapping in an updated module or asset without reloading the page. When building nontrivial stuff HMR is pretty huge for productivity.
E.g. I do a lot of procedural music via WebAudio, and being able to modify the code and hear the results as the song continues to play is obviously pretty useful.