共同点：可以用来遍历对象

输入
for…in 输出自身以及原型链上可枚举的属性 key2、key1
Object.keys 获取对象自身可枚举的属性键
等价于for in+hasOwnProperty的效果		 b
Object.getOwnPropertyNames 获取对象自身的全部属性名 b c

目标：遍历child中的属性

  1. var parent = Object.create(Object.prototype, {
  2.     key1: {
  3.         value: 1,
  4.         writable: true,
  5.         enumerable: true,
  6.         configurable: true            
  7.     }
  8. });
  10. var child = Object.create(parent, {
  11.     key2: {
  12.         value: 2,
  13.         writable: true,
  14.         enumerable: true,
  15.         configurable: true
  16.     },
  17.     key3: {
  18.         value: 3,
  19.         writable: true,
  20.         enumerable: false,
  21.         configurable: true
  22.     }
  23. });

for…in

  1. for (let k in child) {
  2.     console.log(k);
  3. }
  4. // key2
  5. // key1

for…in + hasOwnProperty

  1. for (let k in child) {
  2.   if (child.hasOwnProperty(k)) {
  3.     console.log(k);
  4.   }
  5. }
  6. // key2

Object.keys

  1. Object.keys(child);
  2. // ['key2']

Object.getOwnPropertyNames

  1. Object.getOwnPropertyNames(child);
  2. // (2) ['key2', 'key3']