The usage of ./cmd/frontend says:

  -dev
    	enable developer mode (reload templates on each page load, serve non-minified JS/CSS, etc.)

However, from some experimentation this doesn't appear to be happening. If I modify a .js file and re-run the frontend, the modification is not applied until I compile the .js file into a minified version with ./devtools/compile_js.sh.

Perhaps I'm holding it wrong?

A quick look at the code makes me believe this indeed isn't implemented. While in devMode the server calls parsePageTemplates anew each time a template is requested, the template files directly refer to minified JS, e.g.:

  <script>
    loadScript("/static/js/playground.min.js");
  </script>