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CHAPTER 9 WRITING BETTER JAVASCRIPTI have used Modernizr to test some indicator that jQuery has already been loaded and use the nopeproperty to load the JavaScript file if it hasn’t. Applying this technique to my tiny example web app willmake everything work. But it isn’t a real solution, and while the new problem I created occurs lessfrequently, it is much harder to track down.The underlying problem is that I am still just trying to make my code work. If utils.js is the only filethat uses this technique, then everything will be fine, with the exception that the mapProducts functionmay not be defined in a timely enough manner if the jQuery library does need to be loaded and there is adelay in the request. However, if this technique is used in more than one file, then there is a very subtlerace condition. Imagine that there are two files that use Modernizr to test for jQuery: fileA.js andfileB.js. Most of the time, the sequence of events will be this:1. The browser executes the code in fileA.js, which tests for jQuery. jQueryhasn’t been loaded, so Modernizr requests the file and then executes thecomplete function.2. The browser executes the code in fileB.js, which tests for jQuery. jQuery hasbeen loaded via fileA.js, and Modernizr executes the complete functionwithout needing to load any files.However, Modernizr requests are asynchronous, which means that the browser will continue toexecute JavaScript code while Modernizr waits for the response from the server. So, if the timing is justright, the sequence will really be as follows:1. The browser executes the code in fileA.js, which tests for jQuery. jQueryhasn’t been loaded, so Modernizr requests the file.2. The browser continues to execute code while Modernizr is waiting and beginsprocessing fileB.js. The Modernizr request from fileA.js hasn’t completedyet, so fileB.js causes Modernizr to make a second request for the jQuery file.3. The fileA.js request completes, jQuery is loaded, and the fileA.js completefunction is executed.4. The fileB.js request completes, jQuery is loaded for a second time, and thefileB.js complete function is executed.Any properties that the complete function in fileA.js adds to the jQuery $ shorthand will be lostwhen Modernizr loads jQuery again. This sequence occurs infrequently, but when it does, it can kill theweb app by deleting essential functionality required in at least one of the JavaScript files. You mightthink that infrequent problems are acceptable, but infrequent can still be a serious issue when your webapp has millions of users.Using the Asynchronous Module DefinitionThe only real way to eliminate race conditions and duplicated library loading is to deal withdependencies in a coordinated way, and this means taking responsibility for loading dependencies outof individual JavaScript files and consolidating them. The best model for doing this is the AsynchronousModule Definition (AMD), which I’ll explain and demonstrate in the sections that follow.244www.it-ebooks.info
CHAPTER 9 WRITING BETTER JAVASCRIPTDefining an AMD ModuleDefining a module is pretty simple and hinges on the use of the define function. Listing 9-11 shows howI have created a module in a new file called utils-amd.js. You don’t have to include amd in the file name;that’s just my preference because I like to make it as obvious as possible to the consumers of my codethat they are dealing with AMD. Providing the define function is the responsibility of the AMD loader. Asan author of AMD modules, you can rely on the define function being present without having to worryabout which loader is being used or how the function is implemented.Listing 9-11. The utils-amd.js Filedefine(['jquery-1.7.1.js'], function() {return {mapProducts: function(func, data, indexer) {$.each(data, function(outerIndex, outerItem) {$.each(outerItem[indexer], function(itemIndex, innerItem) {func(innerItem, outerItem);});});},composeString: function(bindingConfig) {var result = bindingConfig.value;if (bindingConfig.prefix) { result = bindingConfig.prefix + result; }if (bindingConfig.suffix) { result += bindingConfig.suffix;}return result;}};});The define function creates an AMD module. The first argument is an array of the libraries that thecode in the module depends on. The second argument is a function, known as the factory function, thatcontains the module code. Only one AMD module can be defined in a file, and since I like to keep thefunctionality defined in a module narrowly focused, my utils-amd.js file contains just the mapProductsand composeString functions. (I’ll return to some of the other code from utils.js in a while.)An AMD module can rely on all of the declared dependencies being loaded before the factoryfunction is executed. In this case, I have declared a dependency on jquery-1.7.1.js, and I can assumethat this JavaScript file will be loaded and jQuery will be available for use when I set up my mapProductsand composeString functions. The result from the factory function is an object whose properties are thefunctions I want to export for use elsewhere in the web app. Any variables or functions that I define andthat are not part of the result object will be tidied up when the factory function has executed withoutpolluting the global namespace.• Tip Notice that there is no namespace in my module. One of the nice features of AMD is that it is up to theconsumer of my module to decide how to refer to the functionality that I define, as I’ll demonstrate in the nextsection.245www.it-ebooks.info
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Contents at a Glance• About the A
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CHAPTER 1 GETTING READYWhat Do You
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CHAPTER 1 GETTING READY{{each prop
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C H A P T E R 2Getting StartedIn th
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CHAPTER 3 ADDING A VIEW MODELCreat
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CHAPTER 3 ADDING A VIEW MODEL$('#b
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C H A P T E R 4Using URL RoutingIn
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CHAPTER 4 USING URL ROUTINGwe requ
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CHAPTER 4 USING URL ROUTINGThe app
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C H A P T E R 5Creating Offline Web
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CHAPTER 5 CREATING OFFLINE WEB APP
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C H A P T E R 6Storing Data in the
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C H A P T E R 7Creating Responsive
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CHAPTER 7 CREATING RESPONSIVE WEB
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Page 193 and 194: CHAPTER 7 CREATING RESPONSIVE WEB
Page 195 and 196: C H A P T E R 8Creating Mobile Web
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Page 271 and 272: • CONTENTSUsing Persistence in th
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