How to deal with cyclic dependencies in Node.js

I've been working with nodejs lately and still getting to grips with the module system, so apologies if this is an obvious question. I want code roughly like the below:

a.js (the main file run with node)

var ClassB = require("./b");

var ClassA = function() {
    this.thing = new ClassB();
    this.property = 5;
}

var a = new ClassA();

module.exports = a;

b.js

var a = require("./a");

var ClassB = function() {
}

ClassB.prototype.doSomethingLater() {
    util.log(a.property);
}

module.exports = ClassB;

My problem seems to be that I can't access the instance of ClassA from within an instance of ClassB.

Is there any correct / better way to structure modules to achieve what I want? Is there a better way to share variables across modules?