javascript

Chapter 6: Object-Oriented Programming
determine if the prototype needs to be initialized by checking for the existence of a method that should
be available. Consider this example:
function Person(name, age, job){
//properties
this.name = name;
this.age = age;
this.job = job;
//methods
if (typeof this.sayName != “function”){
Person.prototype.sayName = function(){
alert(this.name);
};
}
}
var person = new Person(“Nicholas”, 29, “Software Engineer”);
person.sayName();
The highlighted section of code inside the constructor adds the sayName() method if it doesn ’t already
exist. This block of code is executed only the first time the constructor is called. After that, the prototype has
been initialized and doesn ’ t need any further modification. Remember that changes to the prototype are
reflected immediately in all instances, so this approach works perfectly. The if statement may check for
any property or method that will be present once initialized — there ’ s no need for multiple if statements
to check each property or method; any one will do. This pattern preserves the use of instanceof in
determining what type of object was created.
You cannot overwrite the prototype using an object literal when using the dynamic
prototype pattern. As described previously, overwriting a prototype when an
instance already exists effectively cuts off that instance from the new prototype.
Parasitic Constructor Pattern
The parasitic constructor pattern is typically a fallback when the other patterns fail. The basic idea of this
pattern is to create a constructor that simply wraps the creation and return of another object while
looking like a typical constructor. Here ’ s an example:
function Person(name, age, job){
var o = new Object();
o.name = name;
o.age = age;
o.job = job;
o.sayName = function(){
alert(this.name);
(continued)
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