Design Patterns

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CHAPTER 17 ■ THE CHAIN OF RESPONSIBILITY PATTERN
var responseObject = this.firstGenreCatalog.findBooks(requestObject);
return responseObject.results;
}
// Otherwise, search through all books.
else {
var results = [];
for(var isbn in this.catalog) {
if(!this.catalog.hasOwnProperty(isbn)) continue;
if(this.catalog[isbn].getTitle().match(searchString) ||
this.catalog[isbn].getAuthor().match(searchString)) {
results.push(this.catalog[isbn]);
}
}
return results;
}
},
checkoutBook: function(book) { ... },
returnBook: function(book) { ... },
addBook: function(newBook) { ... }
};
The findBooks method creates an object that encapsulates all of the information regarding
the request, including a list of the genres to search, the search terms, and an empty array
to hold any results that are found. The obvious question is, why bother to create this object
when you could just as easily pass these pieces of information as separate arguments? You create
the object mostly because it is much easier to keep track of all the data if it is in a single place.
You need to keep this information intact through all of the links in the chain, and encapsulating
it in a single object makes it easier to do that. In this example, you will pass this same object
back to the client as the response. This helps in keeping the results together with the terms
and genres you are searching through, in the event that you fire off more than one search
through the chain at a time.
You will now implement the findBooks method in the GenreCatalog superclass. This method
is used by all subclasses and shouldn’t need to be overridden. The code is a bit complex, so we
will go through it line by line:
/* GenreCatalog class, used as a superclass for specific catalog classes. */
var GenreCatalog = function() { // implements Catalog
this.successor = null;
this.catalog = [];
this.genreNames = [];
};
GenreCatalog.prototype = {
_bookMatchesCriteria: function(book) { ... }
handleFilingRequest: function(book) { ... },
findBooks: function(request) {
var found = false;
for(var i = 0, len = request.genres.length; i < len; i++) {