for-in循环、Object.keys()、Object.getOwnPropertyNames()三者枚举属性时的区别

1. for-in

对象属性的 Enumerable 表示属性是否可以通过 for-in 循环返回，默认为true
对象自身上的及其原型链上的可枚举属性都可以遍历到

function Person() {}
    Person.prototype.name = 'person'
    Person.prototype.age = 20
    Person.prototype.sayName = function(){
      console.log(this.name);
    }
    
    Object.prototype.say = function() {
      console.log('Object.prototype');
    }
    
    const p1 = new Person()
    p1.work = 'worker'
    p1.name = 'hhh'
    console.log(p1);
    
    for (let i in p1) {
      console.log(i);    //work name age sayName say 
    }
    
    for (let i in Person.prototype) {
      console.log(i);   //age name sayName say
    }

2. Object.keys()

该方法接受一个对象，返回该对象所有可枚举属性的字符串数组

console.log(Object.keys(p1));  // ['work', 'name']
console.log(Object.keys(Person.prototype));  // ['age', 'name', 'sayName']
console.log(Object.keys(Object.prototype));  // ['say']

3. Object.getOwnPropertyNames()

可列出所有实例属性，无论是否可以枚举

console.log( Object.getOwnPropertyNames(p1));  // ['work', 'name']
console.log( Object.getOwnPropertyNames(Person.prototype));  // ['constructor', 'age', 'name', 'sayName']