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How to make your web apps perform ameliorate

As each year passes, developers tread further into the brave new globe of JavaScript-heavy web sites and applications. Using lawmaking libraries like jQuery and frameworks such equally Backbone.js or Ember.js, together with a Swiss Army knife of reusable plug-ins, it’s possible to simplify the core aspects of JS development in gild to gratuitous u.s. up to build richer user experiences that are both functional and a joy to employ.

Each JavaScript file nosotros add together to a project introduces an actress level of complication, notably in terms of how we manage that file’s human relationship with the residuum of the JS files in the codebase. We may write some lawmaking in one file to interact with a page using a jQuery plug-in from a split file, which in turn relies on jQuery being present and loaded from another file.

As the size of the project grows, so do the number of possible connections between files. We say that any JavaScript file that requires another file to be nowadays for it to part correctly has a dependency on that file.

The RequireJS homepage contains the library files plus plenty of documentation and examples to help y’all learn more later on finishing this tutorial

Management styles

Most often we manage our dependencies in a linear and transmission way. At the end of our HTML file, before the closing </body> tag, we commonly list our JavaScript files in order, starting with the near generic library and framework files and working through to the most application-specific files. We need to ensure that each file is listed after its dependencies to prevent errors when trying to access variables defined in other script files that aren’t loaded yet.

As the number of files grows, this method of dependency management becomes increasingly difficult to maintain, especially if you wish to remove a file without affecting any other code relying on information technology.

A more robust method is needed for managing dependencies in larger websites and applications. This tutorial volition explicate how to exercise and so using RequireJS, a JavaScript module loader created to solve this exact problem, which has an added advantage of on-demand script file loading.

The BBC is a proponent of RequireJS and has its own documentation site for their developers to refer to when building JavaScript modules

The BBC is a proponent of RequireJS and has its ain documentation site for their developers to refer to when building JavaScript modules

The RequireJS library is based upon the Asynchronous Module Definition (AMD) API, a cantankerous-language unified manner of defining code blocks together with their dependencies, which is gaining a lot of traction in the industry (it’s used on sites such every bit BBC, Hallmark, Etsy and Instagram, among others).

Let’s start by creating a simple HTML page, naming information technology index.html, and coding in it a very uncomplicated form, which posts an email accost to a separate page called give thanks-you.html. I’ll let you create this page yourself – or you can download an example, together with some example CSS styles, from the tutorial aught file package.

            <!doctype html> <html> <caput>   <meta charset="utf-8">   <championship>Mailing list</title>   <link rel="stylesheet" href="styles/chief.css"> </caput> <body>   <course action="thank-you lot.html" id="grade" method="post">     <h1>Join our mailing list</h1>     <label for="e-mail">Enter your electronic mail address</label>     <input type="text" proper name="email" id="email" placeholder="east.g. [email protected]">     <input type="submit" value="Sign up">   </course> </body> </html>

Before we go any farther, let’s download a re-create of RequireJS. (At the time of writing, the electric current version of this library is two.1.4, and it’s supported in all the major web browsers dorsum to Internet Explorer six, Firefox ii, Safari 3.2, Chrome 3 and Opera 10.)

Now, before we add RequireJS to our page, allow’due south review which JavaScript files nosotros’re going to need for our awarding and organise them into an appropriate folder structure.

The final page we're building represents a newsletter sign-up form, which will only submit if the email address provided is in a valid format

The last folio nosotros’re building represents a newsletter sign-up grade, which will only submit if the email accost provided is in a valid format

Nosotros’ll start with our main application script file, which, using jQuery, volition listen for the submit event on the HTML form and perform grade validation when this occurs, simply allowing the form to continue submitting if there are no errors. Therefore, we volition have iii JavaScript files in improver to RequireJS:

  • Latest version of jQuery (jquery-ane.ix.0.js)
  • Course validation script jQuery plugin (validation-plugin.js)
  • Our main application script file (chief.js)
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Let’s arrange these files together with RequireJS and the balance of the files for the tutorial into a sensible folder structure:

–thank-you lot.html
–chief.css (Add your ain CSS files)

Annotation that we’ve placed all our JavaScript files together into a scripts folder and placed RequireJS at the root level of this folder together with the master awarding script file. All other reusable third party scripts are stored together in the lib sub-folder.

Up and running

Get RequireJS loaded and prepare on the HTML folio by adding a <script> tag to the end of our HTML folio, just before the closing </body> tag, pointing to the location of the library. Although we could then add multiple <script> tags after this point containing the remainder of our code, we can instead rely on the asynchronous file loading feature of RequireJS.

Past adding a data-chief attribute to our <script> tag, we tin can specify the location of the main application script for our page. When RequireJS initialises, it will load any file referenced within that attribute value automatically and asynchronously. We just then demand have one <script> tag on the page:

            <script src="scripts/require.js" data-primary="scripts/chief"></script>

Notation that, every bit in our data-main attribute here, it’s fine to exclude the .js file extension when referencing any files with RequireJS, because it assumes this extension by default.

The developers of jQuery added support for RequireJS. No special code is required to use it as a dependency in your modules

The developers of jQuery added support for RequireJS. No special lawmaking is required to use information technology as a dependency in your modules

Reusable blocks of code or modules are divers with RequireJS using its define method, which follows this pattern:

            define(   moduleName, // optional, defaults to name of file   dependencies, // optional array listing this file'due south dependencies   function(params) {     // Function to execute once dependencies accept been loaded     // params contains return values from the dependencies   } );

All that’south required to be present within a call to the define method is a part containing the module code. Typically, we would create a separate file for each module in our code base of operations, and by default the module name would exist identified in RequireJS by its file proper name.

If your code relies on other code to part correctly (for example, a jQuery plug-in requires jQuery), you should list these and so-chosen dependencies in an assortment within the define method before the function parameter. The names to utilise in this array commonly correspond to the file names of the dependencies relative to the location of the RequireJS library file itself.

In our case, if we wanted to list jQuery as a dependency of another module, we would include it in the dependencies assortment like and so:

            define(["lib/jquery-ane.9.0"], function($) {});

We don’t need to specify the file extension, and so we get out this off when list our dependencies. Incidentally, if this is present on the page, recent versions of jQuery incorporate lawmaking to register themselves equally modules using the define method so we do not demand to write whatsoever special lawmaking to plow the jQuery library into the format nosotros need for utilize with RequireJS.

Whatsoever render values provided by dependent scripts are passed through to the module function lawmaking itself through input parameters. But these parameters passed through to this part should be used within this module, so as to properly encapsulate the code with its dependencies.

We now accept a style to codify the relationship between our module lawmaking and the lawmaking to which it depends. It’s this relationship that tells RequireJS to load in all the code essential to our module office before executing that function.

Getting the names right

We know that jQuery has a convention of naming its files with the version number included, equally you’ve seen in a higher place. If we had numerous references (throughout multiple files) to jQuery every bit a dependency, we would exist creating a maintenance problem for ourselves should we wish to update the site’southward version of jQuery at a later date. Nosotros would have to make changes to all of these files to friction match the new file proper noun.

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Fortunately, RequireJS enables us to get around this issue by defining alternative names for certain modules. We’re able to create a single module proper name mapping to our versioned file name, so we tin can employ that name throughout our files in place of the straight file name. This is fix within the RequireJS configuration object.

Allow’southward start our principal application script file (main.js), creating this module proper name to file name mapping for jQuery:

            requirejs.config({   paths: {     "jquery": "lib/jquery-1.ix.0"   } });

Nosotros tin at present apply the module name jquery in our dependency arrays in other modules, and this volition map to the specific jQuery file.

RequireJS is used across a number of popular sites, including Instagram, pictured here

RequireJS is used across a number of popular sites, including Instagram, pictured here

Content delivery networks

Many developers prefer to reference a copy of jQuery from one of the many global content commitment networks (CDNs) around the web. With the right conditions, this decreases the time taken to download the file and increases the likelihood that the file may be buried on the user’southward machine already if they have previously visited another website that loads the same version of jQuery from the aforementioned CDN.

RequireJS allows yous to link to modules hosted on other domains merely by using its URL in the dependencies assortment. Simply nosotros can simplify the direction of our external file dependencies using the same configuration setting we used to configure jQuery earlier.

We will replace the last code snippet with a new one to reference jQuery from Google’southward CDN – while withal allowing it to fall back to a local version of the file should the external file neglect to load. We chain the list of fallback scripts using an assortment:

            requirejs.config({   paths: {     "jquery": [       "",       // If the CDN fails, load from this local module instead       "lib/jquery-1.ix.0"     ]   } });

Now nosotros have our file construction in place and RequireJS loaded and configured on our page, it’s time to think nearly how the files inside our application depend on each other. We’ve established that our main awarding script is dependent on jQuery in gild to set upwardly the form submit handler and the validation script to validate that form. Since the validation script volition be built every bit a jQuery plugin, it too is dependent on jQuery. We can draw this with a table:

We tin now write the lawmaking for our validation jQuery plug-in module within the validation-plugin.js file, specifying jQuery as its merely dependency. The module checks whether the value of a given field is in the typical format of an electronic mail address, and that it returns true if it’s a valid and false if not:

            define(["jquery"], function($) {   $.fn.isValidEmail = function() {     var isValid = true,       regEx = /\[email protected]\South+\.\Southward+/;     this.each(part() {       if (!regEx.examination(this.value)) {         isValid = false;       }     });     return isValid;   }; });

We omitted the optional module proper noun parameter from our telephone call to the ascertain method, so the module will be registered with the proper name of the file relative to the location of RequireJS. Information technology can be referenced in other dependencies past the module proper name lib/validation-plugin.

RequireJS supports the loading of script dependencies from external URLs, including accessing jQuery directly from Google's hosted libraries site on its CDN

RequireJS supports the loading of script dependencies from external URLs, including accessing jQuery directly from Google’s hosted libraries site on its CDN

Completing the code

Now we’ve established our dependencies, it’s fourth dimension to finish off the lawmaking in our main awarding script file. We’re non going to utilize the ascertain method here. Instead we’re going to employ the require method of RequireJS. The two methods have identical patterns, but they differ in the mode they’re intended to be used.

The former is used to declare modules for later employ, while the latter is used to load dependencies for immediate execution without needing to create a reusable module from it. This latter case suits our master awarding script, which will be executed once just.

Here, beneath the configuration code in our principal.js file, nosotros declare our code for attaching to the submit event of our HTML form, performing validation using our new plug-in script and allowing the form to submit if the provided email address is valid:

            require(["jquery", "lib/validation-plugin"], function($) {   var $form = $("#class"),     $email = $("#e-mail");   $form.on("submit", function(east) {     e.preventDefault();     if ($email.isValidEmail()) {       $course.get(0).submit();     } else {       $email.addClass("fault").focus();     }   });   $e-mail.on("keyup", function() {   $email.removeClass("mistake");   }); });

When this code is executed in the browser, the jQuery library will be loaded first, followed past our validation plug-in module. Yous will recall that when we defined our validation module, we specified that it, too, had a dependency on jQuery.

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One of the really potent features of RequireJS is that, if it comes across a dependency that has already been referenced, it will utilize the stored value from memory rather than downloading it over again. This enables usa to properly define our dependencies without affecting the amount of data downloaded.

In one case the dependencies have loaded, the part is executed – with any return values from the dependencies passed through as parameters. Because we defined the validator module to be a jQuery plug-in, nosotros specified no return value; it will be added to the jQuery $ variable (as is common with other plug-ins).

RequireJS is based on the Asynchronous Module Definition (AMD) API, described in more detail on its own GitHub project page

RequireJS is based on the Asynchronous Module Definition (AMD) API, described in more particular on its own GitHub project folio

(opens in new tab)

Going further

Nosotros can extend our code to take advantage of the fact that RequireJS tin load JavaScript dependencies on need at the exact indicate in time they are needed. Rather than load in our validation plug-in on folio load (equally nosotros’re doing at present), we merely really need that plug-in loaded and available when the user is submitting the form. Doing this would boost page load performance by reducing the corporeality of data downloaded and code executed when the folio is requested.

RequireJS enables us to attain this simply by calling the require method, at the bespeak at which nosotros wish to download extra dependencies. We can rewrite our main awarding script, removing the dependency on the validation plug-in on page load, and adding it into the betoken at which the user attempts to submit the form.

If the user never submits the grade, this file is never loaded.

            require(["jquery"], function($) {  var $form = $("#course"),    $electronic mail = $("#e-mail");  $form.on("submit", office(due east) {    e.preventDefault();    require(["lib/validation-plugin"], role() {       if ($e-mail.isValidEmail()) {         $form.get(0).submit();       } else {         $email.addClass("error").focus();       }     });   });   $electronic mail.on("keyup", function() {     $email.removeClass("error");   }); });

When the user attempts to submit the form, the validator plug-in is loaded and validation attempted. If the user attempts validation a 2d time, the validator plug-in is already loaded so it will not be downloaded again.

Of course, form submission couldn’t occur until the file has downloaded, and then, if the plug-in script were a large file, this may impact on the perceived responsiveness of the page.

Should you wish, you can counteract this by downloading the validator plug-in at the point at which the user first focuses on the text field in the form. This should requite the browser enough time to download the plug-in file, so information technology’south ready for the bespeak at which the user attempts to submit the form.

RequireJS supports loading of JavaScript files dynamically on demand as your application needs them, reducing the number of HTTP requests on page load

RequireJS supports loading of JavaScript files dynamically on demand as your application needs them, reducing the number of HTTP requests on folio load


In this tutorial, I’ve taken you through how to build a simple page that uses RequireJS to simplify the management of code file dependencies and enable the filibuster of script file loading until it’due south needed. Non only does this arroyo brand information technology easier to manage your code as it grows, it too allows you to meliorate the performance of your web applications past merely loading the lawmaking needed at the exact moment it’south required.

RequireJS is capable of more than I’ve covered here. I encourage you to read the documentation on the library homepage to learn how to apply the many configuration options, how to give modules alternative names, how to load and shop data from JSONP spider web services directly, and how to load and manage multiple versions of the same module at the aforementioned fourth dimension (among many other features).

This approach to lawmaking dependency direction and script file loading is an emerging best practice in today’s world of ever-growing JavaScript-heavy codebases for websites and applications. Learn more and adopt information technology in your own sites to reap the benefits. Happy coding!

Dennis Odell

This article originally appeared in net magazine issue 240.

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