Property Read/Write Asymmetry

When you put a property in a constructor's prototype object, the property can be read just as if it were part of each object instance created with that constructor. That is, you can access the area method in the example above using the expression obj1.area() no matter where area was defined (constructor prototype or object instance). However, when you write to a property the prototype properties are not accessible. Instead, writing to a property that has the same name as a prototype property will create a local property in that object instance instead of modifying the constructor's prototype property.

Assume you start with the following object constructor:

Circle(radius):={
.radius=radius;
Circle.prototype.area=Circle_area;
}
Circle_area(none):=PI*.radius^2

The following statements illustrate how various versions of the area method are accessed:

var obj=new Circle(5); // create a new object
obj.area(); // call Circle_area
obj.area=My_area; // create new method in obj
obj.area(); // call My_area
delete obj.area; // delete local area method
obj.area(); // call Circle_area
Circle.prototype.area=My_area;// replace prototype method
obj.area(); // call My_area

In the example above, you can read the value of the constructor's prototype method named area using the expression

obj.area

However, to write to this method, you must use the expression

Circle.prototype.area

In contrast, the local method named area that is stored in the object instance can be accessed for both reading and writing using the expression

obj.area