a.js

var b = require('./b')

console.log('B', b);

module.exports = {
  doThingA: function () {
    console.log('thing A');
    b.doThingB();
  }
}

b.js

var a = require('./a')

module.exports = {
  doThingB: function () {
    console.log('thing B');
  }
}

main.js

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

a.doThingA();