如何在JavaScript中使用父对象调用子方法？

Question

I want to call child method using parent object in javascript how can I do it?

var Shape = function () { }
    Shape.prototype.draw = function () {
        return "Generic shape called ";
    }

    var Circle = function () { }
    Circle.prototype = Object.create(Shape.prototype);
    Circle.prototype.draw = function () {
        return "I am from Circle";
    }

    var Rectangle = function () { }
    Rectangle.prototype = Object.create(Shape.prototype);
    Rectangle.prototype.draw = function () {
        return "I am from Rectangle";
    }

    var Triangle = function () { }
    Triangle.prototype = Object.create(Shape.prototype);

I want to access any overridden method like Rectangle, Circle any one but using Shape object like in c# what we can do: Shape shape=new Circle() shape.draw(); it will call overridden method of the child class.I want to do the same thing in javascript how can I achieve it

Answer 1

 var Shape = function () { } Shape.prototype.draw = function () { return "Generic shape called "; } Shape.prototype.val = 10; var Circle = function () { } Circle.prototype = Object.create(Shape.prototype); Circle.prototype.draw = function () { return "I am from Cirlce"; } var Rectangle = function () { } Rectangle.prototype = Object.create(Shape.prototype); Rectangle.prototype.draw = function () { return "I am from Rectangle"; } var Triangle = function () { } Triangle.prototype = Object.create(Shape.prototype); Triangle.prototype.draw = function () { return "I am from Triangle"; } //Using Up casting (access value of parent object) var newCircle = new Circle(); alert("newCircle: "+newCircle.draw()+" : "+newCircle.val); 

Answer 2

If you are using any subclass of Shape in your code ( you can check at runtime with instanceof operator) you can simple invoke the method. Js runtime would automatically call the right method from the prototype chain. So, if you have an object in code called myshape , you can:

if(myshape instanceof Shape){

    // invoke draw method
   myshape.draw();
}