g.js

function def() {
  var myx;
  Object.defineProperty(global, 'x', {
    get: function() {
      return myx;
    },
    set: function(val) {
      console.log('setter!');
      myx = val;
    }
  });
}

def();

var vm = require('vm');
vm.runInThisContext('var x = function x() {}', 'var.js');
vm.runInThisContext('function x() {}', 'decl.js');