JavaScript构造函数的工作原理、与普通函数的区别及原型优化

function Person(){
    this.name = "zqq";
    this.age = 28;
}
var p = new Person();

var p = {};

Person.call(p)

p.__proto__ = Person.prototype

var p = new Person();
var p = Person();

return的是五种简单数据类型：String,Number,Boolean,Null,Undefined的话，构造函数会忽略return的值，依然返回this对象；而普通函数会返回return后面的值
 
function Person(){
    var a;
    this.name = "zqq";
    this.age = 28;
    return a;
}
var p = new Person();//返回this对象
var p = Person();//因为a没初始化，所以a是undefined，而undefined属于简单数据类型，所以返回undefined，String,Number,Boolean,Null同理
 
 
如果return的是引用类型：Array,Date,Object,Function,RegExp,Error的话，构造函数和普通函数都会返回return后面的值
 
function Person(){
    var arr = [];
    this.name = "zqq";
    this.age = 28;
    return arr;
}
var p = new Person();//返回arr空数组，Date,Object,Function,RegExp,Error同理
var p1 = new Person();//返回arr空数组，Date,Object,Function,RegExp,Error同理

function CreateObj(uName) {
              this.userName = uName;
              this.showUserName = function () {
              return this.userName;
            }
        }
 
var obj1 = new CreateObj('ghostwu');
var obj2 = new CreateObj('卫庄');
 
console.log( obj1.showUserName === obj2.showUserName ); //false

function CreateObj(uName) {
  this.userName = uName;
  this.showUserName = showUserName;
}
function showUserName() {
  return this.userName;
}
var obj1 = new CreateObj('ghostwu');
var obj2 = new CreateObj('卫庄');
console.log(obj1.showUserName === obj2.showUserName); //true

function CreateObj(uName) {
  this.userName = uName;
}
CreateObj.prototype.showUserName = function () {
  return this.userName;
}
var obj1 = new CreateObj('ghostwu');
var obj2 = new CreateObj('卫庄');
console.log(obj1.showUserName === obj2.showUserName); //true