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;

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