function people(word){
    this.word = word;
    this.say = function(){
      console.log(this.word);
    }
    this.capacity = function(){
      return this.say;
    }
  }
  var p = new people('Hello World');
  var res = p.capacity();
  console.log(res)//ƒ (){console.log(this.word);}
  console.log(res())//undefined

如上带吗,我new了一个people,返回的res 是一个function
但是为什么 我执行这个res为undefined,求解,我想的应该打印出来 hello world

如果改成这样呢
function people(word){

this.word = word;
this.say = function(){
  console.log(this.word);
}()
this.capacity = function(){
  return this.say;
}

}
var p = new people('Hello World');
var res = p.capacity(); //undefined
为什么res是undefined