Essentials of Javascript - Cultural View

JavaScript syntax 40
Inheritance
JavaScript supports inheritance hierarchies through prototyping in the manner of Self.
In the following example, the Derive class inherits from the Base class. When d is created as a Derive, the reference
to the base instance of Base is copied to d. base.
Derive does not contain a value for aBaseFunction, so it is retrieved from Base when aBaseFunction is accessed.
This is made clear by changing the value of base.aBaseFunction, which is reflected in the value of d.aBaseFunction.
Some implementations allow the prototype to be accessed or set explicitly using the proto slot as shown below.
function Base() {
}
this.anOverride = function() {alert("Base::anOverride()");};
this.aBaseFunction = function() {alert("Base::aBaseFunction()");};
function Derive() {
}
this.anOverride = function() {alert("Derive::anOverride()");};
base = new Base();
Derive.prototype = base; // Must be before new Derive()
d = new Derive(); // Copies Derive.prototype to d instance's hidden
prototype slot.
base.aBaseFunction = function() {alert("Base::aNEWBaseFunction()")}
d.anOverride(); // Derive::anOverride() d.aBaseFunction(); //
Base::aNEWBaseFunction() alert(d.aBaseFunction ==
Derive.prototype.aBaseFunction); // true
alert(d. proto == base); // true in Mozilla-based implementations
but false in many other implementations
The following shows clearly how references to prototypes are copied on instance creation, but that changes to a
prototype can affect all instances that refer to it.
function m1() {return "One ";}
function m2() {return "Two ";}
function m3() {return "Three ";}
function Base() {}
Base.prototype.yyy = m2;
bar = new Base();
alert("bar.yyy " + bar.yyy()); // bar.yyy Two
function Top(){this.yyy = m3}
ttt = new Top();