Javascript对象添加赋值操作符

Question

我试图在javascript对象中使用加法赋值操作符，以便将学生选举中的票数相加。鉴于以下情况：

var votes = {
  "Alex": { president: "Bob", vicePresident: "Devin", secretary: "Gail", treasurer: "Kerry" },
  "Bob": { president: "Mary", vicePresident: "Hermann", secretary: "Fred", treasurer: "Ivy" },
  "Cindy": { president: "Cindy", vicePresident: "Hermann", secretary: "Bob", treasurer: "Bob" },
  "Devin": { president: "Louise", vicePresident: "John", secretary: "Bob", treasurer: "Fred" },
  "Ernest": { president: "Fred", vicePresident: "Hermann", secretary: "Fred", treasurer: "Ivy" },
  "Fred": { president: "Louise", vicePresident: "Alex", secretary: "Ivy", treasurer: "Ivy" },
  "Gail": { president: "Fred", vicePresident: "Alex", secretary: "Ivy", treasurer: "Bob" },
  "Hermann": { president: "Ivy", vicePresident: "Kerry", secretary: "Fred", treasurer: "Ivy" },
  "Ivy": { president: "Louise", vicePresident: "Hermann", secretary: "Fred", treasurer: "Gail" },
  "John": { president: "Louise", vicePresident: "Hermann", secretary: "Fred", treasurer: "Kerry" },
  "Kerry": { president: "Fred", vicePresident: "Mary", secretary: "Fred", treasurer: "Ivy" }
};

我正在尝试预先形成一个for/in循环，以便计数和创建一个新对象，其中包含候选人的姓名和他们拥有的选票数：

var President = {};
for (var student in votes) {
  President[votes[student].president] += 1;
};

您可能已经知道，它为我提供了所有的NaN值。

如何在每次迭代中获得属性的密钥增加？

Answer 1

问题是您没有初始化该值。所以当你第一次遇到一个名字的时候，你有一个未定义的President[votes[student].president]，所以undefined + 1会给你NaN，然后每个添加都会给你NaN，因为NaN + 1是NaN

var President = {}, president;
for (var student in votes) {
    president = votes[student].president;
    if (!President[president]) {
        President[president] = 0;
    }
    President[president] += 1;
};

Answer 2

您只需要在增加值之前用值初始化President[votes[student].president]。也许是这样的：

if(President[votes[student].president]) {
  President[votes[student].president] += 1
} else {
  President[votes[student].president] = 1;
}

Answer 3

更改此代码，如您所见，需要初始化变量;)

    var President = {};
    for (var student in votes) {
      if(!President[votes[student].president]){
         President[votes[student].president] = 0; 
      }
      President[votes[student].president] += 1;
    };