app.js

var s = require('./index').service;
var submod = require('./index').submod;

s.hoge();
submod.hoge();

index.js

module.exports.service = require('./mod.js');
module.exports.submod = require('./submod');

mod.js

var hoge = null;
module.exports = {
  'hoge':function(){
    if (!hoge) {
      console.log('construct!');
      hoge = "hoge"
    }
    console.log(hoge);
  }
};

run.sh

node app.js

construct!
hoge
hoge

submod.js

var m = require('./mod');
module.exports = {
  hoge: function(){m.hoge();}
};