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)//? (){console.log(this.word);}  console.log(res())//undefined如上帶嗎,我new了一個(gè)people,返回的res 是一個(gè)function但是為什么 我執(zhí)行這個(gè)res為undefined,求解,我想的應(yīng)該打印出來 hello world如果改成這樣呢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為什么res是undefined