Professional JavaScript For Web Developers

Chapter 3
Scope
Programmers in any language understand the concept of scope, meaning the area in which certain variables
are accessible.
Public, protected, and private
In traditional object-oriented programming, a lot of focus is placed on the public and private scopes. An
object’s properties in the public scope can be accessed from outside the object, meaning that after a developer
creates an instance of the object, that property can be used. Properties in the private scope, however,
can only be accessed from within the object itself, meaning that these properties don’t exist to the outside
world. This also means that subclasses of the class defining the private properties and methods
can’t access them either.
More recently, another scope has become popular: protected. Although different languages have different
rules for the protected scope, it generally is used to define properties and methods that act private except
that they are accessible by subclasses.
The discussion of these scopes in reference to ECMAScript is almost a moot point because only one
scope of these three exists: the public scope. All properties and methods of all objects in ECMAScript are
public. You must take great care, therefore, when defining your own classes and objects. Keep in mind
that all properties and methods are public by default.
This problem has been tackled by many developers online trying to come up with effective property
scoping schemes. Due to the lack of a private scope, a convention was developed to indicate which properties
and methods should be considered private. This convention involves adding two underscores
before and after the actual property name. For example:
obj.__color__ = “red”;
In this code, the color property is intended to be private. Remember, adding these underscores doesn’t
change the fact that the property is public; it just indicates to other developers that it should be considered
private.
Some developers also prefer to use a single underscore to indicate private members, such as
obj._color.
Static is not static
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The static scope defines properties and methods accessible all the time from one location. In Java, classes
can have static properties and methods that are accessible without instantiating an object of that class,
such as java.net.URLEncoder, whose function encode() is a static method.
Strictly speaking, ECMAScript doesn’t have a static scope. It can, however, provide properties and methods
on constructors. Remember, constructors are just functions. Functions are objects, and objects can
have properties and methods. For instance:
function sayHi() {
alert(“hi”);
}