如何将内部对象中的键与其父对象合并(取消嵌套)

function mergeKeys(obj) {
  for (let key in obj) {
    if (typeof obj[key] === 'object') {
      obj = {
        ...obj,
        ...obj[key]
      };
      delete obj[key];
    }
  }
  return obj;
}

const user = {
  name: 'John Doe',
  age: 30,
  address: {
    city: 'New York',
    country: 'USA'
  }
};

const mergedUser = mergeKeys(user);
console.log(mergedUser);

{
  name: 'John Doe',
  age: 30,
  city: 'New York',
  country: 'USA'
}