Is the search_param directive here for whitelisting the parameters that we want to transfer? Would there be an issue transferring all of them blindly?

Not sure to understand.
It's here to build a python list from the string, but yes we could probably transfer the whole string and directly build a javascript array from it.
Do you think of an other way ?

What I mean is the following (removing the search_param directive and transferring all query parameteres to the iframe)

window.jupyterliteConcatSearchParams = (iframeSrc) => {
  const baseURL = window.location.origin;
  const iframeUrl = new URL(iframeSrc, baseURL);

  let pageParams = new URLSearchParams(window.location.search);

  pageParams.keys().forEach(param => {
    value = pageParams.get(param);
    if (value !== null) {
      iframeUrl.searchParams.append(param, value);
    }
  });

  return iframeUrl.toString().replace(baseURL, '');
}

I am not sure it makes sense to forward all query parameters. But it would simplify the code and user API.

OK thanks, I understand your first comment. Yes, the idea is to whitelist the parameters.

I am not sure it makes sense to forward all query parameters. But it would simplify the code and user API.

It's better to have at least the option (like a flag) to let user allow the transfer or not.
I don't know which is the best.

Maybe we could add an option to transfer all the parameters(=all for example, since = can probably not be used as parameter name), or use only a boolean value to transfer all or none.

Do you have an opinion on it @martinRenou ?

Indeed, we could have the search_params be either a list of strings (whitelist) or a boolean (True=transfer all, False=transfer none).

Also, I wonder if this could be implemented as an option for other directives, instead of a global search_params directive, like:

.. jupyterlite::
   :search_params: True

Or

.. replite::
   :search_params: ["theme", "code"]

This way it will be more granular

Would it make sense to provide a practical example that really applies to JupyterLite? Like "theme" for replite?

Why not, but I don't really know how it can be displayed to user.

The simplest solution I can find is to use something like

%%javascript
alert(window.location.search);

in a notebook, formatting a bit better the attributes.

Otherwise I didn't find a way to share value between javascript and python in a notebook, using xeus kernel.
It would be better to display the values in an output rather than using an alert.

Ok, let's merge it as is for now!