Spot the bug! Why doesn't doSomething print "new function"

gistfile1.js

function MyObject(fun){
  this.fun = fun;
}

MyObject.prototype.doSomething = function() {
  this.fun();
};

var f = function (){console.log('Original function')};

var myObj = new MyObject(f)

f = function (){console.log('new function')}

myObj.doSomething();

anyway: it's not a bug :)
it's about closuring and javascript execution.