gistfile1.js

function setup(method)
{
  method = fake;
};
function fake()
{
  return "faked";
}
function real()
{
  return "real";
}

setup(real);
console.log(real() == "faked");

/* this works but isn't as pretty */
function setup(method)
{
  return fake;
};
...
real = setup(real);
console.log(real() == "faked");

var test = {
   real: function() { return "real"; }
};

function mock(object, name, mocked) {
    object[name] = mocked;
}

console.log(test.real() == "faked");
mock(test, "real", function() { return "faked"; });
console.log(test.real() == "faked");