Lately, I have been playing with few JavaScript frameworks and in today’s modern web applications it is very common that we are including tons of JavaScript files in our application. One of the thing that plays important role in application performance is how fast these script files are delivered into the browsers. I have extensively blogged on combining, compression and caching of JavaScript files in my old blog, in this post I will show you, how you can achieve parallelism in delivering the scripts in the browser with the Head JS library. If you do not know what parallel script downloading is and how does it impact on page speed, then I would suggest to read this article of the YSlow creator. In short, when a browser encounters a script tag in a page it halts its rending until it downloads the script file, the parallelism is actually archived by adding the script dynamically or by XHR depending upon the browser it is running.
Though there are several libraries like HeadJS which support parallelism in script downloading like LABjs, ControlJS but I am bit partial to it due to jQuery like syntax. Other than script parallelism HeadJS also has few more features as like Modernizr but I will only show the script loading part in this post.
Now, in a typical ASP.NET MVC Application we usually load our shared scripts in the master page and specific scripts in content page, beside that we also want our content page script runs after the master page scripts. So we do have two tasks in hand first serialize the script files so that independent script files download first then dependent files and then serialize the script statements in document.ready.
So, the first thing we will do is add the HeadJS script loader in the head of the master layout file, like the following:
<head>
<title>@ViewBag.Title</title>
<script src="@Url.Script("head.load")"></script>
</head>
The Script is just an extension method of UrlHelper which just returns the regular or minified version of the script depending the debug compilation. But in your real application you should group, compress, cache and if possible create a different domain(e.g. static.yoursite.com or assets.yoursite.com) to host the scripts. Here is the code of the Script method:
public static string Script(this UrlHelper instance, string scriptFileName)
{ const string Path = "~/Scripts/";
return instance.Content(Path + scriptFileName +
(instance.RequestContext.HttpContext.IsDebuggingEnabled ? ".js" :
".min.js"));
}
Now, at the bottom of the master layout, I have added the following snippet:
<script>
head.js("@Url.Script("jquery-1.4.4")", function() {
head.js("@Url.Script("jquery-ui")")
.js("@Url.Script("jquery.unobtrusive-ajax")")
.js("@Url.Script("jquery.validate")",
"@Url.Script("jquery.validate.unobtrusive")")
.js("@Url.Script("jquery.customSharedPlugin")");
@RenderSection("scriptFiles", false)
}); head.ready(function() {
$('#main').customSharedPlugin();
@RenderSection("scriptOnReady", false)
}); </script>
</body> </html>
For the shake of simplicity I am mostly using the scripts which comes with the default ASP.NET MVC project, in your real application it may vary. Now, in the above example I am first loading the jQuery as my other scripts depends upon it and in the callback which fires when the script is loaded I am loading the other depend scripts and these are the script files that are loaded simultaneously without blocking each other. Now when I am done with layout file script loading, I am using the new Razor RenderSction to include the content page scripts. But as I am marking the section as optional by passing false in the second argument, it is perfectly okay if the content page does not has its own script files. But if the content page has scripts then it is included in the following way:
@section scriptFiles {
head.js("@Url.Script("jquery.customPlugin1")"); }
Next, the head.ready, it is executed when all of the scripts are loaded and like the script loading we are first calling the layout page specific code then using the RenderSection(also making it as optional) so that the content page can inject its script. Now, in the Content page:
@section scriptOnReady {
$('#main > h2').customPlugin1(); }
So when the page renders it will render the following:
<script>
head.js("/Scripts/jquery-1.4.4.js", function() {
head.js("/Scripts/jquery-ui.js")
.js("/Scripts/jquery.unobtrusive-ajax.js")
.js("/Scripts/jquery.validate.js", "/Scripts/jquery.validate.unobtrusive.js")
.js("/Scripts/jquery.customSharedPlugin.js");
head.js("/Scripts/jquery.customPlugin1.js");
}); head.ready(function() {
$('#main').customSharedPlugin();
$('#main > h2').customPlugin1(); });
</script>
Happy reading.
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