CODEWITHSUNDEEP
javascript
Reema Joshi
Reema Joshi

Reputation: 13

access to parent from a child object

i am trying to figure out how the code below works. Can somebody explain what does it mean when it says " create an uber property that points to the parent's prototype object." What exactly does it mean when it says "this.constructor.uber" points to parent's prototype? i've been scratching my head for a while now. i would be really thankful if somebody could explain what's happening in function Shape(). 

function Shape() {}
// augment prototype
Shape.prototype.name = 'shape';
Shape.prototype.toString = function () {
    var result = [];
    if (this.constructor.uber) {
        result[result.length] = this.constructor.uber.toString();
    }
    result[result.length] = this.name;
    return result.join(', ');
};


function TwoDShape() {}
// take care of inheritance
var F = function () {};
F.prototype = Shape.prototype;
TwoDShape.prototype = new F();
TwoDShape.prototype.constructor = TwoDShape;
TwoDShape.uber = Shape.prototype;
// augment prototype
TwoDShape.prototype.name = '2D shape';

function Triangle(side, height) {
    this.side = side;
    this.height = height;
}
// take care of inheritance
var F = function () {};
F.prototype = TwoDShape.prototype;
Triangle.prototype = new F();
Triangle.prototype.constructor = Triangle;
Triangle.uber = TwoDShape.prototype;
// augment prototype
Triangle.prototype.name = 'Triangle';
Triangle.prototype.getArea = function () {
    return this.side * this.height / 2;
}

var my = new Triangle(5, 10);
my.toString()

Output: "shape,2Dshape,Triangle"

Upvotes: 1

Views: 457

Answers (1)

Pablo
Pablo

Reputation: 380

In Javascript inheritance, there is no support for accessing a super method, so what I can see here with uber, is having access to its parent methods.
Name it parent as following:
TwoDShape.parent = Shape.prototype

That way you can have direct access to a parent property: TwoDShape.parent.name should return 'shape'.

What toString() is actually doing here, is:

var result = []; //declare an array
if (this.constructor.parent) { //if there is a parent prototype
    result[0] = this.constructor.parent.toString(); //set the 0 position of result with the return value of parent's toString() method.
}
result[result.length] = this.name; //set the position (0 or 1, if we had a parent it will be 1, otherwise 0) with the current name.
return result.join(', '); //return the names joined with a comma

Note that I changed the uber to parent so its more readable. the first assignment of result will always be 0, so it was not necessary the call to result.length.

What the calls would return for each object?:
Shape.toString();: 'shape' (there's no parent)
TwoDShape.toString();: 'shape, 2D shape' ('shape' for the call to Shape.toString();, and '2D shape' for its own name, joined).
Triangle.toString();: 'shape, 2D shape, Triangle' ('shape, 2D shape' for the call to TwoDShape.toString();, and 'Triangle' for its own name, joined).

Upvotes: 1

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