用带有对象的常规 for 循环替换 for in

Question

出于好奇，我目前正在尝试用常规 for 循环替换我拥有的 for in 循环，但没有成功。我总是得到undefined。这甚至可以用 javascript 对象实现吗？

我循环的示例对象：

var el = {
    1: {type: "fish", commonName: "clownfish", scientificName: "sdasd", gender: "m", price: 1.99},
    2: {type: "fish", commonName: "dragonfish", scientificName: "dada", gender: "f", price: 2.99}
};

我的工作方法：

for (var element in el) {
   if (el[element].type === type && el.hasOwnProperty(element)) {
       elementNum++;
   }
}

总是让我着迷的简单 for 循环方法Cannot read property 'type' of undefined：

for(var i = 0, x = Object.keys(el).length; i < x; i++) {
   if (el[i].type === type && el.hasOwnProperty(i)) {
      elementNum++;
   }
}

Answer 1

您从0 开始循环，而整个对象中的第一个键是1。

var el = {
  1: {
    type: "fish",
    commonName: "clownfish",
    scientificName: "sdasd",
    gender: "m",
    price: 1.99
  },
  2: {
    type: "fish",
    commonName: "dragonfish",
    scientificName: "dada",
    gender: "f",
    price: 2.99
  }
};

var keyLength = Object.keys(el).length;

// I've simplified the for loop
for (var i = 1; i <= keyLength; i++) {
  console.log(el[i].commonName);
}

Answer 2

我建议直接遍历对象的键。

function getCount(type) {
    return Object.keys(el).reduce(function (r, k) {
        return r + +(el[k].type === type);
    }, 0);
}

var el = { 1: { type: "fish", commonName: "clownfish", scientificName: "sdasd", gender: "m", price: 1.99 }, 2: { type: "fish", commonName: "dragonfish", scientificName: "dada", gender: "f", price: 2.99 } };

console.log(getCount('fish'));