My first suggestion is that you do not define your endpoint with /:variable.  This will then match any route you create that follows that pattern.  You should instead use something like /pages/:page to get a specific page
Then, you should use the URL parameter functionality in Express to include a URL parameter.  
Define your route like this:  
app.get("/pages/:page", function (req, res) {
    var pg = undefined;
    if (req.query.page) {
        pg = req.query.page;
    }
}
You can then access the page in req.query.page if it exists and do whatever you want with that value.
So for example, you could submit a request with localhost/pages/123?page=3.  
req.query.page would equal 3 and req.params.page would equal 123.