model.js

module.exports = {
   find: function() {
       return undefinedVar;
   }
};

rewire.test.js

var rewire = require('rewire');
var model = rewire('./model');
var expect = require('chai').expect;

describe('rewire', function(){
    before(function(){
        model.__set__({
            undefinedVar: 'hello'
        });
    });
    it('should be able to set an undefined variable', function(){
        var out = model.find();
        expect(out).to.eql('hello');
    });
});