Design Patterns

CHAPTER 3 ■ ENCAPSULATION AND INFORMATION HIDING 31
},
getIsbn: function() {
return this._isbn;
},
setIsbn: function(isbn) {
if(!this.checkIsbn(isbn)) throw new Error('Book: Invalid ISBN.');
this._isbn = isbn;
},
getTitle: function() {
return this._title;
},
setTitle: function(title) {
this._title = title || 'No title specified';
},
getAuthor: function() {
return this._author;
},
setAuthor: function(author) {
this._author = author || 'No author specified';
},
display: function() {
...
}
};
In this example, all of the attributes have been renamed. An underscore is added to the
beginning of each, signifying that it is intended to be private. This is still a valid variable name
in JavaScript, since the underscore is a legal first character in an identifier.
This naming convention can be applied to methods as well. Let’s say that a programmer
using your class is having a hard time creating an instance because he keeps getting “Invalid
ISBN” errors. He could use the public method checkIsbn to run through each possible character
for the checksum digit (there are only ten) until he finds one that passes, and use that to
create an instance of Book. You should prevent that sort of behavior because it is likely that the
ISBN created will still be invalid. To do this, you can change the method declaration from this
checkIsbn: function(isbn) {
...
},
to this
_checkIsbn: function(isbn) {
...
},