function addPropertyToNewObject(obj, key, value) {
  const newObject = Object.assign({}, obj);
  newObject[key] = value;
  return newObject;
}

const myObj = {
  one: 1,
  two: 2,
  three: 3
};

const newObject = addPropertyToNewObject(myObj, 'four', 4);

console.log(newObject);
console.log(myObj);