How do I write extension methods that can be used globally in my Node.JS application?

Take for example this "StartsWith" extension:

if (typeof String.prototype.startsWith != 'function') {
    String.prototype.startsWith = function (str) {
        return this.slice(0, str.length) == str;
    };
}

If I was writing a web app, I would stick that code in an ExtensionMethods.js page that I imported on a web page within my site.

But what about the case of using this on the server with Node.js?

Thanks!