2

This is sort of a follow up of from this question: Python like inheritance for JavaScript

But I phrased it wrong and made it seem like I wanted classical inheritance in JavaScript when I just wanted to figure out how to do it similarly to Python.

This is what I have so far but it isn't working as I would like it to.

var util = require("util")

function FirstClass() {
    this.name = "First Class";
}

FirstClass.prototype.sayName = function(){
    this.callFunction("some_function", "hello")
}

FirstClass.prototype.callFunction = function(){
    var func = arguments[0]; 
    var args = Array.prototype.slice.call(arguments).slice(1, arguments.length)  
    if (this[func] !== undefined){ 
        this[func].apply(null, args); 
    } 
}

function SubClass(){}
util.inherits(SubClass, FirstClass)
SubClass.prototype.some_function = function(msg){
    console.log(msg, this.name) // "hello undefined"
}

var sub_class = new SubClass();
sub_class.sayName();

When I try sub_class.sayName() It calls correctly and inherited sayName right, but when I try using the this keyword to show the name it doesn't work and prints undefined next to 'hello', so how would I fix it so it will show 'hello FirstClass'?

I was thinking this have all the variables and objects that FirstClass has since it inherits from it but it doesn't. Is that how prototypal inheritance is suppose to work? If so is there any way I can view/change parent variables from the child class?

I've tried looking up prototypal inheritance for JavaScript to figure it out but I'm not quite grasping the concept since I'm use to classical inheritance. Does anyone know how to inherit from the parent class and change it's variables from a child class?

Also if anyone has a good link to look at so I can understand prototypal inheritance better for future references that would be great.

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4
  • 1
    I'd be interested in knowing the answer to this question as well. I've worked with prototypes, just not with inheritances yet. Commented Jun 12, 2014 at 19:29
  • @Grimbode: When you work with prototypes, you're doing inheritance. Commented Jun 12, 2014 at 19:30
  • @cookie monster, then what is the use of inherits()? Commented Jun 12, 2014 at 19:31
  • 1
    It's just a helper function that abstracts away a little boilerplate code when making the prototype object of one function inherit from another. github.com/joyent/node/blob/master/lib/util.js#L628-L638 But irrespective of how you set up a prototype chain, when you add a member to an object used as the prototype of another, the other object inherits that property. Commented Jun 12, 2014 at 19:37

3 Answers 3

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4

You are missing this part:

function SubClass(){
   FirstClass.apply(this);
}

This would be the equivalent to a super invocation in traditional class inheritance.

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2 Comments

Thank you, this worked. I'm surprised I didn't find this sooner it seems simple enough.
@user3234209: This is only part of the problem. The null issue in the other answer needs to be addressed as well.
2

Your problem lies here: this[func].apply(null, args);

It should read: this[func].apply(this, args);

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3 Comments

I just changed it and it is still printing undefined.
I realized Edwin's answer worked because I changed null to this, so this helped solve my problem thank you.
Yay, I am glad I could help you! Thanks for mentioning this.
1

Here is a simplified example of how to inherit and extend a class. I very highly recommend checking Javascript Design Patterns for much better examples of how to create Classes with inheritance.

Now, as I was not familiar with what util was supposed to be doing, this example demonstrates how to achieve the result with native JS and no helpers:

JSFiddle here: http://jsfiddle.net/YAj5R/

function FirstClass() {
    this.name = "First Class";
}

FirstClass.prototype.sayName = function () {
    this.callFunction("some_function", "hello")
}

FirstClass.prototype.callFunction = function () {
    var func = arguments[0];
    var args = Array.prototype.slice.call(arguments).slice(1, arguments.length)
    if (this[func] !== undefined) {
        this[func].apply(this, args);
    }
}

function SubClass() {
    var parent = new FirstClass();

    // this is lightweight example of an "extend" method
    for(var attr in parent) {
        // if this subclass has not over-written an attr,
        // copy the attributes of FirstClass to SubClass
        if(!this[attr]) {
            this[attr] = parent[attr];
        }
    }

    return this;
}

SubClass.prototype.some_function = function (msg) {
    console.log(msg, this.name);
}

var sub_class = new SubClass();
sub_class.sayName();
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2 Comments

This approach, though, might fail to define any non-enumerable attributes in FirstClass.
@user3234209: You won't want to do it this way in practice. It defeats the point of using prototypal inheritance in the first place where you don't need the for(var attr in parent) { at all, and changes made to FirstClass.prototype won't be seen on the SubClass objects. The util.inherits can mostly be replaced with two short lines of code.

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