func.func1 is not a function how to use funcition from function?

function foo(a) {
  console.log(a);
  return a;
}

const fooSpy = spy(foo);

fooSpy('test'); // return: test
fooSpy.callCount(); // out: 1
fooSpy.calledWith('test'); // out: true
fooSpy.calledWith('test123'); // out: false
fooSpy.returned('test'); // out: true


function spy(func) {
  let fooSpy = {
    callCount: function() {
      console.log('testss')
    }
  }
  console.log(func);
  if (func === undefined) {
    console.log(1);
  }
  spy.prototype.callCount = function() {
    console.log('win')
  };
  return func;
}
fooSpy.prototype.callCount = function() {
  console.log('win')
};