CODEWITHSUNDEEP
javascript
M.Svrcek
M.Svrcek

Reputation: 5635

Calling supertype methods in javascript

I'm reading book The Principles of Object-Oriented Javascript and I'm stuck at part, where accessing supertype methods is explained. I have defined two functions on super prototype(redeclared toString and new function getArea) and extended them whit child class, in which i want to change super methods while using their return value. Code look like this:

function Rectangle(length, width) {
    this.length = length;
    this.width = width;
}
Rectangle.prototype.getArea = function () {
    return this.length * this.width;
};
Rectangle.prototype.toString = function () {
    return "[Rectangle " + this.length + "x" + this.width + "]";
};

// inherits from Rectangle
function Square(size) {
    Rectangle.call(this, size, size);
}
Square.prototype = Object.create(Rectangle.prototype, {
    constructor: {
        configurable: true,
        enumerable: true,
        value: Square,
        writable: true
    }
});

// call the supertype method
Square.prototype.toString = function () {
    var text = Rectangle.prototype.toString.call(this);
    return text.replace("Rectangle", "Square");
};
Square.prototype.getArea = function () {
    var area = Rectangle.prototype.getArea().call(this);
    return area * 2;
};

var rectangle = new Rectangle(2, 2);
console.log(rectangle); //instance of Rectangle
console.log(rectangle.toString()); // [Rectangle 2x2]
console.log(rectangle.getArea()); //4

var square = new Square(2);
console.log(square); //instance of Square
console.log(square.toString()); // [Square 2x2]
console.log(square.getArea()); //expected 8, got ERROR

Problem is, square.getArea() throws the error

undefined is not a function

Can someone explain me why? Why toString works and getArea not?

Upvotes: 2

Views: 959

Answers (1)

thefourtheye
thefourtheye

Reputation: 239453

In this line

var area = Rectangle.prototype.getArea().call(this);

you are trying to invoke the result of getArea() as a function with call (which is not possible, as the result of getArea will be a number and numbers don't have call method, only Function objects have that). But, you should be invoking the getArea function itself, like this

var area = Rectangle.prototype.getArea.call(this);

Upvotes: 3

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