爲什麼我們需要在JavaScript中調用父構造函數繼承

Question

我正在嘗試JavaScript的繼承。基本上，我正在遵循this教程。

我看到，在那裏的代碼中，Person類被實例化了兩次。請看this fiddle。

我所做的是註釋掉：

Person.call(this) 

而且繼承工作得很好。

在原始代碼中，行

Person.call(this) 

被使用。是否需要使用子範圍調用父構造函數？

請你也給一些解釋，我是OO JavaScript的新手。

非常感謝。

編輯：

我在小提琴代碼如下：

function Person(gender) { 
    this.gender = gender; 
    document.write('Person instantiated</br>'); 
} 

Person.prototype.walk = function(){ 
    document.write("is walking</br>"); 
}; 

Person.prototype.sayHello = function(){ 
    document.write("Hello</br>"); 
}; 

Person.prototype.sayGender = function(){ 
    document.write(this.gender + "</br>"); 
}; 



function Student() { 
    //Person.call(this); 
    document.write('Student instantiated</br>');   
} 
Student.prototype = new Person(); 

Student.prototype.constructor = Student; 

Student.prototype.sayHello = function(){ 
    document.write("Student says Hello</br>"); 
} 
Student.prototype.sayGoodBye = function(){ 
    document.write("Student says goodbye</br>"); 
} 


var student1 = new Student(); 
student1.sayHello(); 
student1.walk(); 
student1.sayGoodBye(); 

document.write(student1 instanceof Person); 
document.write("</br>"); 
document.write(student1 instanceof Student); 

Answer 1

運行，你只提供的示例調用人（）構造函數一次在腳本初始執行期間，當行「Student.prototype = new Person（）;」被執行。

如果我們修改腳本創建第二個學生和設置從實例中分離：bit: http://jsfiddle.net/anacW/

function Person(gender) { 
    this.gender = gender; 
    document.write('Person instantiated</br>'); 
} 

Person.prototype.walk = function(){ 
    document.write("is walking</br>"); 
}; 

Person.prototype.sayHello = function(){ 
    document.write("Hello</br>"); 
}; 

Person.prototype.sayGender = function(){ 
    document.write(this.gender + "</br>"); 
}; 



function Student() { 
    //Person.call(this); 
    document.write('Student instantiated</br>');   
} 
Student.prototype = new Person(); 

Student.prototype.constructor = Student; 

Student.prototype.sayHello = function(){ 
    document.write("Student says Hello</br>"); 
} 
Student.prototype.sayGoodBye = function(){ 
    document.write("Student says goodbye</br>"); 
} 

document.write("*** Building student1 *** </br>"); 
var student1 = new Student(); 
student1.sayHello(); 
student1.walk(); 
student1.sayGoodBye(); 

document.write("*** Building student2 ***</br>"); 
var student2 = new Student(); 
student2.sayHello(); 
student2.walk(); 
student2.sayGoodBye(); 

document.write("*** InstanceOf Tests ***</br>"); 
document.write("student1 is Person?: " + (student1 instanceof Person)); 
document.write("</br>"); 
document.write("student1 is Student?: " + (student1 instanceof Student)); 
document.write("</br>"); 
document.write("student2 is Person?: " + (student2 instanceof Person)); 
document.write("</br>"); 
document.write("student2 is Student?: " + (student2 instanceof Student)); 

該代碼給出：

Person instantiated 
*** Building student1 *** 
Student instantiated 
Student says Hello 
is walking 
Student says goodbye 
*** Building student2 *** 
Student instantiated 
Student says Hello 
is walking 
Student says goodbye 
*** InstanceOf Tests *** 
student1 is Person?: true 
student1 is Student?: true 
student2 is Person?: true 
student2 is Student?: true 

，向您顯示Person構造函數只被調用一次，並且永遠不會通過實例化Student來調用。這可能是你的理想情況（我不知道足夠的javascript來告訴你它是否是'正確'的形式）。

Answer 2

在這裏，我如何做到這一點：

Foo = function() {} 
Foo.prototype.sayHello = function() { console.log("hello"); } 

Bar = function() {} 
Bar.prototype = new Foo(); 
Bar.prototype.sayBye = function() { console.log("bye"); } 

var oBar = new Bar(); 
oBar.sayHello(); // "hello" 
oBar.sayBye(); // "bye" 

Answer 3

是的，你會需要它。在你的例子中，所有學生都具有相同的性別，只有一個人實例化，不管實例化學生的數量如何。

更好的方式是：

function Student() { 
    // gives this Student properties of one (new) Person: 
    Person.call(this); 
    document.write('Student instantiated</br>');   
} 
// does not create a Person, just makes Students have Person prototype features 
Student.prototype = Object.create(Person.prototype);