NodeJS require singletons

a.js

const common = require('./common')
module.exports.add = (e) => common.add(e)
module.exports.get = () => common.get()

b.js

const common = require('./common')
module.exports.add = (e) => common.add(e)
module.exports.get = () => common.get()

common.js

const c = [];
module.exports.add = (n) => c.push(n)
module.exports.get = () => c

index.js

// run this file with node

// require both a and b,
const a = require('./a');
const b = require('./b');

// both a and b require common,
a.add('a')
b.add('b')

console.log('a', a.get())
console.log('b', b.get())