If you start with a simple file design.js, save it in a folder and use editor of your choice.
open https://openjscad.xyz/ and make sure you check the option "Enable Auto-reload"
drag and drop the file into the window and you will see the model
change your js file and save, changes will be picked-up automatically.
openjscad checks for changes every second, but if you want to see changes even faster you can use livereload.
one way to use livereload is via nodejs
if you have nodejs just run
npm -g install livereload
go to the folder with your design and run:
livereload
you must do the livereload step before you drag and drop design into openjscad window.