Prototypal Inheritence in Node JS

Question

Being from the classical Inheritance Background(C#,Java etc.) , I am struggling with the Prototypal Way of doing it. I am not understanding the basics also . Please explain and correct me on the following code block.

    var util = require ('util'); 

function Student(choiceOfStream) { 
    this.choiceOfStream = choiceOfStream;
}

Student.prototype.showDetails = function() {
    console.log("A student of "+this.choiceOfStream+" has a major in "+this.MajorSubject);
}

function ScienceStudent() {
    Student.call(this,"Science");
}

function ArtsStudent() {
    Student.call(this,"Arts");
}

util.inherits(ScienceStudent,Student);
util.inherits(ArtsStudent,Student);

var ArtsStudent = new ArtsStudent(); 
var ScienceStudent = new ScienceStudent(); 

ScienceStudent.prototype.MajorSubject = "Math";
ArtsStudent.prototype.MajorSubject = "Literature";

console.log(ArtsStudent.showDetails());
console.log(ScienceStudent.showDetails());

But the error I am getting is

What was I missing ?

jfriend00 · Accepted Answer · 2016-01-09 20:56:44Z

1

There is no standard this.super_ property so I'm not sure where you got that from. If you're using util.inherits(), you can see a nice simple example of how to use it in the nodejs doc for util.inherits().

And, here's how your code could work:

var util = require ('util');

function Student(choiceOfStream) { 
    this.choiceOfStream = choiceOfStream;
}

Student.prototype.showDetails = function() {
    console.log("A student of "+this.choiceOfStream+" has a major in "+this.MajorSubject);
}

function ScienceStudent() {
    Student.call(this, "Science");
    this.majorSubject = "Math";
}

function ArtsStudent() {
    Student(this,"Arts");
    this.majorSubject = "Literature";
}

util.inherits(ScienceStudent,Student);
util.inherits(ArtsStudent,Student);

FYI, in ES6 syntax, there is a super keyword which is part of a new way of declaring Javascript inheritance (still prototypal) you can read about here.

edited Jan 9, 2016 at 20:56

answered Jan 9, 2016 at 20:50

jfriend00

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6 Comments

StrugglingCoder Over a year ago

But still gives me an error ... saying cannot set property "MajorSubject" for undefined . Please see the edited question.

jfriend00 Over a year ago

@StrugglingCoder - Because you are still apparently trying to use the .super_ property which does not exist. See the initializer for .majorSubject I added to each constructor. In general, function properties (e.g. methods) should be on the prototype and data properties should be on the object itself initialized in the constructor. This is because things on the prototype are shared between ALL instances of that object type which is perfect for methods, but is usually not what you want for data.

StrugglingCoder Over a year ago

Why can't I just add it to the respective prototypes ? Got it . Thanks

jfriend00 Over a year ago

@StrugglingCoder - See what I had already added to my previous comment. You can put data properties on the prototype, but it is not the usual practice because it does not usually behave the way people want data properties to behave.

jfriend00 Over a year ago

@StrugglingCoder - your next problem is that when you do this: var ArtsStudent = new ArtsStudent(); , you are reassigning the constructor function. Use a different name for your instance variable. A good convention to follow is that Constructor functions start with uppercase and all other properties or variables start with lowercase.

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