Question

I am experimenting with JavaScript Inheritance. Basically, I am following this tutorial.

I see that, with the code there, the Person class is instantiated twice. Please have a look at this fiddle.

What I did is comment out:

Person.call(this)

And the inheritance is working just fine.

In the original code, the line

Person.call(this)

is used. Is there a need of calling parent constructor with child scope?

Could you please also give some explanation, I am new to OO JavaScript.

Thanks a lot.

EDIT:

My code in the fiddle is as follows:

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

Running the example you provided only calls the Person() constructor once during initial execution of the script when the line “Student.prototype = new Person();” is executed.

If we modify your script to create a second student and separate the setup from the instantiation: 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));

​

This code gives:

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

Which shows you that the Person constructor is only being called once, and is never called by instantiating a Student. This might be desirable in your case (I don’t know enough javascript to tell you whether it’s ‘proper’ form or not).