lib: refactor wrap_js_stream for ES6/readability

PR-URL: #16158
Backport-PR-URL: #16626
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>

Author: addaleax