CODEWITHSUNDEEP
javascriptinheritancemethodscallprototype-programming
sergionni
sergionni

Reputation: 13510

Child class can't call its parent's prototype method

I'm learning JS prototypes.

From Java language point I expect,that SpecificRectangle object will have access to area() method,due to area() is the method of its parent(Rectangle class) prototype.

function Rectangle(w,h){
 this.width = w;
 this.height=h;
}
Rectangle.prototype.area = function(){return this.width*this.height}

function SpecificRectangle(w,h,angle){
  Rectangle.call(this,w,h);
  SpecificRectangle.prototype=new Rectangle();
}

var specrec = new SpecificRectangle(7,8,45);

All at all I can't call area() method on SpecificRectangle instance.
Standard JS error got:

TypeError: specrec.area is not a function
[Break On This Error] specrec.area()

What is the explanation and reason of such encapsulation?

Upvotes: 3

Views: 3431

Answers (2)

Krzysztof
Krzysztof

Reputation: 16140

You should copy base class prototype. Eg:

function Rectangle(w,h){
    this.width = w;
    this.height=h;
}
Rectangle.prototype.area = function(){return this.width*this.height}

function SpecificRectangle(w,h,angle){
    Rectangle.call(this,w,h);
}
function SpecificRectangleProto(){}
SpecificRectangleProto.prototype = Rectangle.prototype;
SpecificRectangle.prototype = new SpecificRectangleProto();

var specrec = new SpecificRectangle(7,8,45);
alert(specrec.area);

I suggest to extract extend method from some framework. For example ExtJS. With such method you can extend class like this:

SpecificRectangle = extend(Rectangle, {
    constructor: function(w,h,angle){
        SpecificRectangle.superclass.constructor.call(this,w,h);
    }
});

Upvotes: 1

Didier Ghys
Didier Ghys

Reputation: 30666

Honestly i don't know the exact reason but you need to set the prototype outside the constructor function:

function SpecificRectangle(w, h, angle) {
    Rectangle.call(this,w,h);
}

SpecificRectangle.prototype = new Rectangle();
SpecificRectangle.prototype.constructor = SpecificRectangle; // Otherwise instances of SpecificRectangle would have a constructor of Rectangle

Working example here.

Edit following the comment by @herby:

It seems indeed that the upper method could break the prototypal inheritance depending on how the super-class constructor is built (see this article).

A more robust solution is to use Object.create (source - thanks herby)

// in case Object.create does not exist
if (typeof Object.create !== 'function') {
    Object.create = function(o) {
        var F = function() {};
        F.prototype = o;
        return new F();
    };
}

function Rectangle(w, h) {
    this.width = w;
    this.height = h;
}
Rectangle.prototype.area = function() {
    return this.width * this.height
}

function SpecificRectangle(w, h, angle) {
    Rectangle.call(this, w, h);
}

SpecificRectangle.prototype = Object.create(Rectangle.prototype);
SpecificRectangle.prototype.constructor = SpecificRectangle;

var r = new SpecificRectangle(100, 50, 30);
alert(r.area());

Updated example on jsfiddle

Upvotes: 3

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