A quick example of circular dependency with node.js. This code will crash unexpectedly because module A is only partially loaded by C.

gistfile1.js

// a.js
var moduleB = require('./b');

function ModuleA() {
}

ModuleA.hello = function () {
  console.log('hello!');
};

module.exports = ModuleA;

// b.js
var moduleC = require('./c');

function ModuleB() {
}

ModuleB.hello = function () {
  moduleC.hello();
};

module.exports = ModuleB;

// c.js
var moduleA = require('./a');

function ModuleC() {
}

ModuleC.hello = function () {
  moduleA.hello();
};

module.exports = ModuleC;

// main.js
var moduleA = require('./a'),
    moduleB = require('./b');

moduleB.hello();