To achieve that kind of behavior — where a user submits a form once and later returns to update it — you’ll need to create a custom Odoo website module.

Here’s how it works:

• Backend Logic: You need a custom controller in Python that checks whether the logged-in user already has a record in your model. If they do, it fetches and displays that record in the form. If not, it shows an empty form for first-time submission.
• Form Submission: The same controller can handle both creation and update, depending on whether a record for that user already exists.
• Frontend Form: The form itself can be rendered using a QWeb template. You can prefill fields with existing values if the user has already submitted it.
• JavaScript? Not always required. If you're okay with a full-page reload on form submission and rendering, you can handle everything using just Python and QWeb. But if you want a smoother experience — like updating the form without page reload (AJAX), live validations, or auto-saving — then yes, you’ll need to write custom JavaScript for that.

So, you don’t need JavaScript just to show or update a form, but you do need it for dynamic or real-time interactions.