I understand why and how this works, but it always catches me out. I wonder how obvious it is to others? Feel free to comment below.

readme.md

This code outputs the following:

---------- local
one

---------- ref
one

---------- before re-require

---------- local
one

---------- ref
two

---------- after required

---------- local
one
two

---------- ref
two
two

index.js

'use strict';

var mod = require('./module');
var fn = mod.fn;

log('local');
// calling the local variable - this is a copy of the reference to `one`
fn();

log('ref');
// calling the *reference* to `one`
mod.fn();

setTimeout(function () {
  log('before re-require');

  log('local');
  fn();

  log('ref');
  mod.fn();

  var mod2 = require('./module');
  var fn2 = mod.fn;

  log('after required');

  log('local');
  fn();
  fn2();

  log('ref');
  mod.fn();
  mod2.fn();

}, 2000);

function log(s) {
  console.log('\n---------- ' + s);
}

module.js

'use strict';

function one() {
  console.log('one');
}

function two() {
  console.log('two');
}

module.exports = {
  fn: one
};

setTimeout(function () {
  module.exports.fn = two;
}, 1000);

Got it ! Tricky though, and must say its not that obvious,
If I understand it correctly, if the first call to mod.fn(); [line-12] takes more than 1 seconds, it will output two instead of one ! ?