Made an account to point out you can use a third-party package to handle possessive quantifiers for you. As of 2024, it appears to be regularly updated.
It would apply to the question like this:

    const regex = require("regex");

    const defaultRouteRegex = regex`/\/{1}+/`;
    app.get(defaultRouteRegex, function (req, res) {
        res.render("index"); 
    });