JavaScript 2.0-The Complete Reference, Second ... - freecodingtutorial

function Robot(flying, action)
{
}
if (flying == true)
this.hasJetpack = true;
if (action)
this.actionValue = action;
We have added a new property to the prototype, actionValue. This property has a default value
that can be overridden by passing a second argument to the constructor. If a value for action is
passed to the constructor, invoking doAction() will show its value rather than the default
("Intruders beware!"). For example,
var guard = new Robot(true, "ZAP!");
guard.doAction();
results in ―ZAP!‖ being alerted rather than ―Intruders beware.‖
Dynamic Types
A very important aspect of the prototype is that it is shared. That is, there is only one copy of
the prototype that all objects created with the same constructor use. An implication of this is
that a change in the prototype will be visible to all objects that share it! This is why default
values in the prototype are overridden by instance variables, and not changed directly.
Changing them in the prototype would change the value for all objects sharing that prototype.
Modifying the prototypes of built-in objects can be very useful. Suppose you need to repeatedly
extract the third character of strings. You can modify the prototype of the String object so that
all strings have a method of your definition:
String.prototype.getThirdChar = function()
{
}
return this.charAt(2);
You can invoke this method as you would any other built-in String method:
var c = "Example".getThirdChar(); // c set to 'a'
Class Properties