如何删除对象中未定义的键和值？

Question

如何从对象中删除undefined的键和值？

例如我有

var x = {
 firstname: undefined,
 lastname: 'blabla'
}

如何获取没有未定义的对象？以最有效，最快捷的方式？

结果应为：

x = {
 lastname: 'blabla'
}

Answer 1

var x = {
 firstname: undefined,
 lastname: 'blabla'
}

for (let key in x) {
  if (x[key] === undefined) {
   delete x[key];
  }
}