在構造函數中動態賦值原型不起作用

Question

當我們使用「new」運算符爲javascript中的對象創建實例時會發生什麼？並且，在創建過程中，何時分配了構造函數的原型？ 我嘗試動態地分配在構造一個新的原型，但結果頗爲怪異：

function Person(name, age){//the super class constructor 
    this.name = name; 
    this.age = age; 
    if (typeof this.sayName != "function") {//set the function in prototype so Person's instance or subType can share just one instance of function(since function in javascript is Object indeed) 
     Person.prototype.sayName = function(){ 
      console.log("my name is ", this.name); 
     } 
    } 
} 
//if I assign Student's prototype here, not in its constructor, it will be OK, but why not encapsulate it within the constructor if I can? 
//Student.prototype = new Person(); 
function Student(name, age, school, grade){//the subType constructor 
    Person.call(this, name, age);//get the super class property 
    this.school = school;//init the subType property 
    this.grade = grade; 
    if (!(Student.prototype instanceof Person)) {//just assign the prototype to Student for one time 
     Student.prototype = new Person(); 
    } 
} 

let person1 = new Student("Kate", 23, "Middle school", "3.8"); 
let person2 = new Student("Gavin", 23, "Middle school", "3.8"); 
let person3 = new Student("Lavin", 23, "Middle school", "3.8"); 
person1.sayName();//Uncaught TypeError: person1.sayName is not a function 
person2.sayName();//my name is Gavin 
person3.sayName();//my name is Lavin 

1. 自「sayName（）」可以分配到人的原型，我們可以得出結論，原型已經準備好當構造函數代碼執行
2. 基於第1點，在構造函數Student（）中，爲什麼我無法替換原始原型？（person1將找不到sayName函數）
3. 當構造函數的原型被分配了嗎？我可以在哪裏更換默認原型？

Answer 1

當我們用 「新」 運營商在JavaScript的對象創建一個實例，會發生什麼？

繼MDN文檔，你的情況：

當代碼新的學生（...）執行第一次，下面的事情發生：

1. '的新對象創建，繼承自Student.prototype
2. 構造函數Student用指定的參數調用，並且綁定到新創建的對象上...

基本上，你不能在構造函數中改變第一個實例的原型。它已被設置爲Student.prototype。您可以從任何地方調用設置原型代碼，但在開始創建新實例之前。

下一個實例將使用Person原型創建。

Answer 2

function Person(name, age, parents){//the super class constructor 
 
\t this.name = name; 
 
\t this.age = age; 
 
} 
 

 
// This is must be outside the constructor/function Person 
 
Person.prototype.sayName = function(){ 
 
\t console.log("my name is ", this.name); 
 
} 
 

 
// Is Student created inside Student constructor? 
 
//Student.prototype = new Person(); 
 
function Student(name, age, school, grade){//the subType constructor 
 
\t Person.call(this, name, age);//get the super class property 
 
\t this.school = school;//init the subType property 
 
\t this.grade = grade; 
 
} 
 

 
// Check this to understand why this is a good way to inherit in js http://stackoverflow.com/a/17393153/1507546 
 
Student.prototype = Object.create(Person.prototype); 
 

 
let person1 = new Student("Kate", 23, "Middle school", "3.8"); 
 
let person2 = new Student("Gavin", 23, "Middle school", "3.8"); 
 
let person3 = new Student("Lavin", 23, "Middle school", "3.8"); 
 
person1.sayName();//Uncaught TypeError: person1.sayName is not a function 
 
person2.sayName();//my name is Gavin 
 
person3.sayName();//my name is Lavin