I think it's because we specify 'class': JSXRefRole(fix_parens=True) instead of fix_parens=False (which is what the Python domain does). Since the types are rendered as if they were class references, they get an additional () suffix). I need to check this but I guess that's one reason.

EDIT: ok it's not that. But looking at the HTML we have:

<code class="xref js js-func docutils literal notranslate"><span class="pre">string()</span></code>

so string is treated as a js-func. I'll investigate more.

EDIT 2: Found a fix.

The issue is with JSCallable:

    doc_field_types = [
        TypedField(
            'arguments',
            label=_('Arguments'),
            names=('argument', 'arg', 'parameter', 'param'),
            typerolename='func',
            typenames=('paramtype', 'type'),
        ),
        GroupedField(
            'errors',
            label=_('Throws'),
            rolename='func',
            names=('throws',),
            can_collapse=True,
        ),
   ...

The role is func instead of class. But changing it into class leads to lots of not-found references. Could be fine to change it but this could annoy existing builds, so we could first suppress the reference errors arising from those places (it surprises me though that the func role didn't actually lead to ref errors as well).

@picnixz The missing reference warning about js:func is silenced by the custom nitpick_ignore in doc/conf.py.

So I think changing the rolename from func to class will not annoy existing builds, how about just changing it and fixing the fix_parens? I am happy to file a PR.