Adding to @carljm's comment about running the tests locally in a browser, we could create a test HTML file for the admin package and a test HTML file for the gis package.  This way changing large portions of JavaScript code would still only require opening one or two HTML files in a web browser to confirm that everything is still green.  Also remember that this only needs to be done for the few developers that touch the JavaScript.

One thing local testing in this way would not provide however is code coverage reports.  JavaScript code coverage tools need to modify the JS before execution to inject coverage-measuring code.  Therefore a command-line step would be needed to prepare the JavaScript for code coverage.

Modifying the JavaScript in minor ways may not warrant checking code coverage.  The Python code is not locally executed under coverage by default and the JavaScript code doesn't necessarily need to be either.