Test

HTML

<script src="https://rawgit.com/eu81273/jsfiddle-console/master/console.js"></script>

JavaScript

const foo = () => {
  let OnDone = null;

  const Do = function() {
    console.log('Done');
    console.log(OnDone);
    if (typeof OnDone === 'function')
      OnDone();
  }

  return {Do, OnDone};
}

let f = foo();
f.OnDone = () => {
  console.log('OnDone');
};

f.Do();