Pro JavaScript for Web Apps pdf - EBook Free Download

CHAPTER 9 WRITING BETTER JAVASCRIPTUnderstanding Directly Resolved DependenciesThe tiny web app works because jQuery has been loaded long before I call the mapProducts function. Thesituation would be different if I rewrote the web app to use a different toolkit. Most programmers do thesame thing when they first understand that assumed dependencies are a problem: they assume controlof the situation and take direct action to fix it. Listing 9-9 shows a typical solution.Listing 9-9. Taking Direct Action to Resolve Assumed Dependencies(function() {function createNamespace(namespace) {...code removed for brevity...};var utilsNS = createNamespace("cheeselux.utils");Modernizr.load({load: 'jquery-1.7.1.js',complete: function() {utilsNS.mapProducts = function(func, data, indexer) {$.each(data, function(outerIndex, outerItem) {$.each(outerItem[indexer], function(itemIndex, innerItem) {func(innerItem, outerItem);});});}}})...code removed for brevity...})();In this listing, I have taken responsibility for resolving my dependency on jQuery by usingModernizr to load it before creating my mapProducts function. (The load property in a Modernizr.loadobject specifies that the JavaScript file should always be loaded.)In doing this, I have transformed an assumed dependency into a directly resolved dependency. Adirectly resolved dependency is when I rely on another JavaScript library and I take direct action to makemy code work, usually by loading the library myself.Understanding the Problems Caused by Resolving a DependencyDirectly resolving a dependency causes a lot of problems. First, I created an assumed dependency onModernizr to ensure that jQuery is loaded, which isn’t a huge step forward. But the real damage is that Ihave made sure that the mapProducts function works; however, in doing so, I have undermined thestability of the web app itself.To see the problem, load the web app, and reload the page a few times. There are two issues. If theweb app works, you have encountered just the least serious one, which is that the jQuery library hasbeen loaded twice. You can see this in the browser developer tools or in the console output from theNode.js server that prints out each URL that is requested. Here is the list of files loaded by the web app asreported by the server, with annotations to highlight the two loads for jQuery:240www.it-ebooks.info