a.js

const b = require('./b')
class A {
  constructor () {

  }

  echo () {
    console.log(b.hello)
  }

  lang () {
    return 'ja'
  }
}
module.exports = A;

b.js

const A = require('./a')
class B {
  constructor () {
    throw new Error('aaa')
  }

  static get hello () {
    const a = new A()
    if (a.lang() === 'ja') {
      return 'こんにちは〜'
    }
    return 'hello'
  }
}
module.exports = B;

> const A = require('./a')
> const a = new A()
> a.echo
TypeError: A.lang is not a function
    at Function.get hello [as hello] (/private/tmp/scratch/b.js:8:11)
    at A.echo (/private/tmp/scratch/a.js:8:18)
    at repl:1:3
    at sigintHandlersWrap (vm.js:22:35)
    at sigintHandlersWrap (vm.js:96:12)
    at ContextifyScript.Script.runInThisContext (vm.js:21:12)
    at REPLServer.defaultEval (repl.js:313:29)
    at bound (domain.js:280:14)
    at REPLServer.runBound [as eval] (domain.js:293:12)
    at REPLServer.<anonymous> (repl.js:513:10)

つ https://google.com/?q=circular+dependency+node.js