Handling HTTP requests in isomorphic javascript

I have a purely external javascript project that contains several models that encapsulate various interactions with various RESTful web services. My goal is to pull these models into my own npm module so I can use them server side in a new node application I am writing.

The models use an XMLHttpRequest object which will obviously be undefined on the server. I cannot require ("http") in my models, because then the browser will throw an error when trying to create for the client.

How can I handle HTTP requests that will work on both the server and client? I would like something like:

var ajax = {
    get: function (url, opts) {
        if (typeof XMLHttpRequest === 'undefined') {
            // is node app
            var http = require('http');
            ...
        } else {
            // is browser app
            var xhr = new XMLHttpRequest();
            ...
        }
    },
    post: ...
};