Object.keys方法

在实际开发中，我们有时需要知道对象的所有属性;
ES5 引入了Object.keys方法，成员是参数对象自身的（不含继承的）所有可遍历（ enumerable ）属性的键名。

• 传入对象，返回属性名

var data={a:1,b:2,c:9,d:4,e:5};
  console.log(Object.keys(data));//["a", "b", "c", "d", "e"]
  Object.keys(data).map((key,item)=>{
    console.log(key,data[key]);//key=>属性名  data[key]=>属性值
});

• 传入字符串，返回索引

var str = 'ab1234';
console.log(Object.keys(obj)); //[0,1,2,3,4,5]

• 传入数组 返回索引

var arr = ["a", "b", "c"];
  console.log(Object.keys(arr)); // console: ["0", "1", "2"]

• 构造函数 返回空数组或者属性名

function Pasta(name, age, gender) {
   this.name = name;
   this.age = age;
   this.gender = gender;
   this.toString = function () {
      return (this.name + ", " + this.age + ", " + this.gender);
  }
}

console.log(Object.keys(Pasta)); //console: []

var spaghetti = new Pasta("Tom", 20, "male");
console.log(Object.keys(spaghetti)); //console: ["name", "age", "gender", "toString"]