https://www.cnblogs.com/laiqun/p/5645801.html
注意：其文章中说 instanceof 是 constructor 的语法糖，是有待斟酌的：
function Person () {}
Person.prototype = {
name: 'Jim',
age: '21'
}
var person = new Person()
console.log(person instanceof Person) //true
console.log(person.constructor === Person) //false