Client-side development - jQuery

A framework is something that usually forces a certain way of implementing a solution, whereas jQuery is just a tool to make implementing what you want to do easier.

a library - a collection of functions which are useful when writing web apps. Your code is in charge and it calls into the library when it sees fit. 

E.g., jQuery.

frameworks - a particular implementation of a web application, where your code fills in the details. 

The framework is in charge and it calls into your code when it needs something app specific. 

E.g., durandal, ember, etc.

The purpose of jQuery is to make it much easier to use JavaScript on your website.

jQuery takes a lot of common tasks that require many lines of JavaScript code to accomplish, and wraps them into methods that you can call with a single line of code.

jQuery also simplifies a lot of the complicated things from JavaScript, like AJAX calls and DOM manipulation.

The jQuery library contains the following features:

HTML/DOM manipulation

CSS manipulation

HTML event methods

Effects and animations

AJAX

Utilities

The advantages of jQuery

The main advantage of jQuery is that it is much easier than its competitors. You can add plugins easily, translating this into a substantial saving of time and effort. In fact, one of the main reasons why Resig and his team created jQuery was to buy time (in the web development world, time matters a lot).

The open source license of jQuery allows the library to always have constant and fast support, constantly publishing updates. The jQuery community is active and extremely hardworking.

Another advantage of jQuery over its competitors such as Flash and pure CSS is its excellent integration with AJAX.

In summary:

jQuery is flexible and fast for web development

It comes with an MIT license and is Open Source

It has an excellent support community

It has Plugins

Bugs are resolved quickly

Excellent integration with AJAX

The disadvantages of jQuery

One of the main disadvantages of jQuery is the large number of published versions in the short time. It does not matter if you are running the latest version of jQuery, you will have to host the library yourself (and update it constantly), or download the library from Google (attractive, but can bring incompatibility problems with the code).

In addition to the problem of the versions, other disadvantages that we can mention:

jQuery is easy to install and learn, initially. But it’s not that easy if we compare it with CSS

If jQuery is improperly implemented as a Framework, the development environment can get out of control.

jQuery Selectors

jQuery selectors allow you to select and manipulate HTML element(s).

jQuery selectors are used to "find" (or select) HTML elements based on their name, id, classes, types, attributes, values of attributes and much more. It's based on the existing CSS Selectors, and in addition, it has some own custom selectors.

All selectors in jQuery start with the dollar sign and parentheses: $().

The element Selector

The jQuery element selector selects elements based on the element name.

You can select all <p> elements on a page like this:

$("p")

Example

When a user clicks on a button, all <p> elements will be hidden:

Example

$(document).ready(function(){

  $("button").click(function(){

    $("p").hide();

  });

});

The #id Selector

The jQuery #id selector uses the id attribute of an HTML tag to find the specific element.

An id should be unique within a page, so you should use the #id selector when you want to find a single, unique element.

To find an element with a specific id, write a hash character, followed by the id of the HTML element:

$("#test")

Example

When a user clicks on a button, the element with id="test" will be hidden:

Example

$(document).ready(function(){

  $("button").click(function(){

    $("#test").hide();

  });

});

The .class Selector

The jQuery .class selector finds elements with a specific class.

To find elements with a specific class, write a period character, followed by the name of the class:

$(".test")

Example

When a user clicks on a button, the elements with class="test" will be hidden:

Example

$(document).ready(function(){

  $("button").click(function(){

    $(".test").hide();

  });

});

Common Selectors Overview

Example	Classification	Explanation
h1	Type Selector	Selects an element by its type
.tagline	Class Selector	Selects an element by the class attribute value, which may be reused multiple times per page
#intro	ID Selector	Selects an element by the ID attribute value, which is unique and to only be used once per page

Child Selectors Overview

Example	Classification	Explanation
article h2	Descendant Selector	Selects an element that resides anywhere within an identified ancestor element
article > p	Direct Child Selector	Selects an element that resides immediately inside an identified parent element

Sibling Selectors Overview

Example	Classification	Explanation
h2 ~ p	General Sibling Selector	Selects an element that follows anywhere after the prior element, in which both elements share the same parent
h2 + p	Adjacent Sibling Selector	Selects an element that follows directly after the prior element, in which both elements share the same parent

Attribute Selectors Overview

Example	Classification	Explanation
a[target]	Attribute Present Selector	Selects an element if the given attribute is present
a[href="http://google.com/"]	Attribute Equals Selector	Selects an element if the given attribute value exactly matches the value stated
a[href*="login"]	Attribute Contains Selector	Selects an element if the given attribute value contains at least once instance of the value stated
a[href^="https://"]	Attribute Begins With Selector	Selects an element if the given attribute value begins with the value stated
a[href$=".pdf"]	Attribute Ends With Selector	Selects an element if the given attribute value ends with the value stated
a[rel~="tag"]	Attribute Spaced Selector	Selects an element if the given attribute value is whitespace-separated with one word being exactly as stated
a[lang|="en"]	Attribute Hyphenated Selector	Selects an element if the given attribute value is hyphen-separated and begins with the word stated

The Document Object Model (DOM) is a programming API for HTML and XML documents. It defines the logical structure of documents and the way a document is accessed and manipulated. In the DOM specification, the term "document" is used in the broad sense - increasingly, XML is being used as a way of representing many different kinds of information that may be stored in diverse systems, and much of this would traditionally be seen as data rather than as documents. Nevertheless, XML presents this data as documents, and the DOM may be used to manage this data.

The Document Object Model originated as a specification to allow JavaScript scripts and Java programs to be portable among web browsers. Dynamic HTML was the immediate ancestor of the Document Object Model, and it was originally thought of largely in terms of browsers. However, when the Document Object Model Working Group was formed, it was also joined by vendors in other domains, including HTML or XML editors and document repositories. Several of these vendors had worked with SGML before XML was developed; as a result, the Document Object Model has been influenced by SGML Groves and the HyTime standard. Some of these vendors had also developed their own object models for documents in order to provide programming APIs for SGML/XML editors or document repositories, and these object models have also influenced the Document Object Model. 

Benefits of using jQuery

* Search Engine Optimized – While search engines are getting better at being able to read content within some Flash, everything within jQuery is setup as text. This means it is completely readable to all the search engines, exposing all your keyword rich content.

* Save Time – Five lines of jQuery are equivalent to 25 lines of conventional JavaScript code. This means smaller files and faster loading web pages.

* Plug-ins – There are an abundance of plug-ins on the web that make creating special effects simple and fast for web developers.

* Help? – With an abundance of plug-ins comes with an abundance of help. There is a large helpful support community on the web to help you quickly remedy any bug issues.

* That was easy! – jQuery has easy implementation for web developers in comparison to other applications.

* Cross Browser Friendly – jQuery is currently the most popular JavaScript library and works in all browsers.

* FREE! – free, open source software.

* Mobile Devices – jQuery is supported by any mobile device whose web browser supports JavaScript. A lot of mobile devices like iPads and iPhones don’t run Flash at all.

Simplifies AJAX

* Wow Factor – Web developers use jQuery to make web pages more exciting, interactive, cleaner, and more user friendly. Make your users go WOW!

JavaScript libraries

Many JavaScript libraries use $ as a function or variable name, just as jQuery does. In jQuery's case, $ is just an alias for jQuery, so all functionality is available without using $. If you need to use another JavaScript library alongside jQuery, return control of $ back to the other library with a call to $.noConflict(). Old references of $ are saved during jQuery initialization; noConflict() simply restores them.

If for some reason two versions of jQuery are loaded (which is not recommended), calling $.noConflict( true ) from the second version will return the globally scoped jQuery variables to those of the first version.

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<script src="other_lib.js"></script> <script src="jquery.js"></script> <script> $.noConflict(); // Code that uses other library's $ can follow here. </script>

This technique is especially effective in conjunction with the .ready() method's ability to alias the jQuery object, as within callback passed to .ready() you can use $ if you wish without fear of conflicts later:

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<script src="other_lib.js"></script> <script src="jquery.js"></script> <script> $.noConflict(); jQuery( document ).ready(function( $ ) { // Code that uses jQuery's $ can follow here. }); // Code that uses other library's $ can follow here. </script>

If necessary, you can free up the jQuery name as well by passing true as an argument to the method. This is rarely necessary, and if you must do this (for example, if you need to use multiple versions of the jQuery library on the same page), you need to consider that most plug-ins rely on the presence of the jQuery variable and may not operate correctly in this situation.

Examples:

Map the original object that was referenced by $ back to $.

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jQuery.noConflict(); // Do something with jQuery jQuery( "div p" ).hide(); // Do something with another library's $() $( "content" ).style.display = "none";

Revert the $ alias and then create and execute a function to provide the $ as a jQuery alias inside the function's scope. Inside the function the original $ object is not available. This works well for most plugins that don't rely on any other library.

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jQuery.noConflict(); (function( $ ) { $(function() { // More code using $ as alias to jQuery }); })(jQuery); // Other code using $ as an alias to the other library

Create a different alias instead of jQuery to use in the rest of the script.

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var j = jQuery.noConflict(); // Do something with jQuery j( "div p" ).hide(); // Do something with another library's $() $( "content" ).style.display = "none";

Completely move jQuery to a new namespace in another object.

1 2	var dom = {}; dom.query = jQuery.noConflict( true );

Result:

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// Do something with the new jQuery dom.query( "div p" ).hide(); // Do something with another library's $() $( "content" ).style.display = "none"; // Do something with another version of jQuery jQuery( "div > p" ).hide();

Load two versions of jQuery (not recommended). Then, restore jQuery's globally scoped variables to the first loaded jQuery.

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<!doctype html> <html lang="en"> <head> <meta charset="utf-8"> <title>jQuery.noConflict demo</title> <script src="https://code.jquery.com/jquery-1.10.2.js"></script> </head> <body> <div id="log"> <h3>Before $.noConflict(true)</h3> </div> <script src="https://code.jquery.com/jquery-1.6.2.js"></script> <script> var $log = $( "#log" ); $log.append( "2nd loaded jQuery version ($): " + $.fn.jquery + "<br>" ); // Restore globally scoped jQuery variables to the first version loaded // (the newer version) jq162 = jQuery.noConflict( true ); $log.append( "<h3>After $.noConflict(true)</h3>" ); $log.append( "1st loaded jQuery version ($): " + $.fn.jquery + "<br>" ); $log.append( "2nd loaded jQuery version (jq162): " + jq162.fn.jquery + "<br>" ); </script> </body> </html>

Advanced Plugin Concepts

linkProvide Public Access to Default Plugin Settings

An improvement we can, and should, make to the code above is to expose the default plugin settings. This is important because it makes it very easy for plugin users to override/customize the plugin with minimal code. And this is where we begin to take advantage of the function object.

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// Plugin definition. $.fn.hilight = function( options ) { // Extend our default options with those provided. // Note that the first argument to extend is an empty // object – this is to keep from overriding our "defaults" object. var opts = $.extend( {}, $.fn.hilight.defaults, options ); // Our plugin implementation code goes here. }; // Plugin defaults – added as a property on our plugin function. $.fn.hilight.defaults = { foreground: "red", background: "yellow" };

Now users can include a line like this in their scripts:

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// This needs only be called once and does not // have to be called from within a "ready" block $.fn.hilight.defaults.foreground = "blue";

And now we can call the plugin method like this and it will use a blue foreground color:

1	$( "#myDiv" ).hilight();

As you can see, we've allowed the user to write a single line of code to alter the default foreground color of the plugin. And users can still selectively override this new default value when they want:

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// Override plugin default foreground color. $.fn.hilight.defaults.foreground = "blue"; // ... // Invoke plugin using new defaults. $( ".hilightDiv" ).hilight(); // ... // Override default by passing options to plugin method. $( "#green" ).hilight({ foreground: "green" });

linkProvide Public Access to Secondary Functions as Applicable

This item goes hand-in-hand with the previous item and is an interesting way to extend your plugin (and to let others extend your plugin). For example, the implementation of our plugin may define a function called "format" which formats the hilight text. Our plugin may now look like this, with the default implementation of the format method defined below the hilight function:

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// Plugin definition. $.fn.hilight = function( options ) { // Iterate and reformat each matched element. return this.each(function() { var elem = $( this ); // ... var markup = elem.html(); // Call our format function. markup = $.fn.hilight.format( markup ); elem.html( markup ); }); }; // Define our format function. $.fn.hilight.format = function( txt ) { return "<strong>" + txt + "</strong>"; };

We could have just as easily supported another property on the options object that allowed a callback function to be provided to override the default formatting. That's another excellent way to support customization of your plugin. The technique shown here takes this a step further by actually exposing the format function so that it can be redefined. With this technique it would be possible for others to ship their own custom overrides of your plugin – in other words, it means others can write plugins for your plugin.

Considering the trivial example plugin we're building in this article, you may be wondering when this would ever be useful. One real-world example is the Cycle Plugin. The Cycle Plugin is a slideshow plugin which supports a number of built-in transition effects – scroll, slide, fade, etc. But realistically, there is no way to define every single type of effect that one might wish to apply to a slide transition. And that's where this type of extensibility is useful. The Cycle Plugin exposes a "transitions" object to which users can add their own custom transition definitions. It's defined in the plugin like this:

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$.fn.cycle.transitions = { // ... };

This technique makes it possible for others to define and ship transition definitions that plug-in to the Cycle Plugin.

linkKeep Private Functions Private

The technique of exposing part of your plugin to be overridden can be very powerful. But you need to think carefully about what parts of your implementation to expose. Once it's exposed, you need to keep in mind that any changes to the calling arguments or semantics may break backward compatibility. As a general rule, if you're not sure whether to expose a particular function, then you probably shouldn't.

So how then do we define more functions without cluttering the namespace and without exposing the implementation? This is a job for closures. To demonstrate, we'll add another function to our plugin called "debug". The debug function will log the number of selected elements to the console. To create a closure, we wrap the entire plugin definition in a function (as detailed in the jQuery Authoring Guidelines).

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// Create closure. (function( $ ) { // Plugin definition. $.fn.hilight = function( options ) { debug( this ); // ... }; // Private function for debugging. function debug( obj ) { if ( window.console && window.console.log ) { window.console.log( "hilight selection count: " + obj.length ); } }; // ... // End of closure. })( jQuery );

Our "debug" method cannot be accessed from outside of the closure and thus is private to our implementation.

linkBob and Sue

Let's say Bob has created a wicked new gallery plugin (called "superGallery") which takes a list of images and makes them navigable. Bob's thrown in some animation to make it more interesting. He's tried to make the plugin as customizable as possible, and has ended up with something like this:

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jQuery.fn.superGallery = function( options ) { // Bob's default settings: var defaults = { textColor: "#000", backgroundColor: "#fff", fontSize: "1em", delay: "quite long", getTextFromTitle: true, getTextFromRel: false, getTextFromAlt: false, animateWidth: true, animateOpacity: true, animateHeight: true, animationDuration: 500, clickImgToGoToNext: true, clickImgToGoToLast: false, nextButtonText: "next", previousButtonText: "previous", nextButtonTextColor: "red", previousButtonTextColor: "red" }; var settings = $.extend( {}, defaults, options ); return this.each(function() { // Plugin code would go here... }); };

The first thing that probably comes to your mind (OK, maybe not the first) is the prospect of how huge this plugin must be to accommodate such a level of customization. The plugin, if it weren't fictional, would probably be a lot larger than necessary. There are only so many kilobytes people will be willing to spend!

Now, our friend Bob thinks this is all fine; in fact, he's quite impressed with the plugin and its level of customization. He believes that all the options make for a more versatile solution, one which can be used in many different situations.

Sue, another friend of ours, has decided to use this new plugin. She has set up all of the options required and now has a working solution sitting in front of her. It's only five minutes later, after playing with the plugin, that she realizes the gallery would look much nicer if each image's width were animated at a slower speed. She hastily searches through Bob's documentation but finds no animateWidthDuration option!

linkDo You See The Problem?

It's not really about how many options your plugin has; but what options it has!

Bob has gone a little over the top. The level of customization he's offering, while it may seem high, is actually quite low, especially considering all the possible things one might want to control when using this plugin. Bob has made the mistake of offering a lot of ridiculously specific options, rendering his plugin much more difficult to customize!

linkA Better Model

So it's pretty obvious: Bob needs a new customization model, one which does not relinquish control or abstract away the necessary details.

The reason Bob is so drawn to this high-level simplicity is that the jQuery framework very much lends itself to this mindset. Offering a previousButtonTextColor option is nice and simple, but let's face it, the vast majority of plugin users are going to want way more control!

Here are a few tips which should help you create a better set of customizable options for your plugins:

linkDon't Create Plugin-specific Syntax

Developers who use your plugin shouldn't have to learn a new language or terminology just to get the job done.

Bob thought he was offering maximum customization with his delay option (look above). He made it so that with his plugin you can specify four different delays, "quite short," "very short," "quite long," or "very long":

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var delayDuration = 0; switch ( settings.delay ) { case "very short": delayDuration = 100; break; case "quite short": delayDuration = 200; break; case "quite long": delayDuration = 300; break; case "very long": delayDuration = 400; break; default: delayDuration = 200; }

Not only does this limit the level of control people have, but it takes up quite a bit of space. Twelve lines of code just to define the delay time is a bit much, don't you think? A better way to construct this option would be to let plugin users specify the amount of time (in milliseconds) as a number, so that no processing of the option needs to take place.

The key here is not to diminish the level of control through your abstraction. Your abstraction, whatever it is, can be as simplistic as you want, but make sure that people who use your plugin will still have that much-sought-after low-level control! (By low-level I mean non-abstracted.)

linkGive Full Control of Elements

If your plugin creates elements to be used within the DOM, then it's a good idea to offer plugin users some way to access those elements. Sometimes this means giving certain elements IDs or classes. But note that your plugin shouldn't rely on these hooks internally:

A bad implementation:

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// Plugin code $( "<div class='gallery-wrapper' />" ).appendTo( "body" ); $( ".gallery-wrapper" ).append( "..." );

To allow users to access and even manipulate that information, you can store it in a variable containing the settings of your plugin. A better implementation of the previous code is shown below:

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// Retain an internal reference: var wrapper = $( "<div />" ) .attr( settings.wrapperAttrs ) .appendTo( settings.container ); // Easy to reference later... wrapper.append( "..." );

Notice that we've created a reference to the injected wrapper and we're also calling the .attr() method to add any specified attributes to the element. So, in our settings it might be handled like this:

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var defaults = { wrapperAttrs : { class: "gallery-wrapper" }, // ... rest of settings ... }; // We can use the extend method to merge options/settings as usual: // But with the added first parameter of TRUE to signify a DEEP COPY: var settings = $.extend( true, {}, defaults, options );

The $.extend() method will now recurse through all nested objects to give us a merged version of both the defaults and the passed options, giving the passed options precedence.

The plugin user now has the power to specify any attribute of that wrapper element so if they require that there be a hook for any CSS styles then they can quite easily add a class or change the name of the ID without having to go digging around in plugin source.

The same model can be used to let the user define CSS styles:

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var defaults = { wrapperCSS: {}, // ... rest of settings ... }; // Later on in the plugin where we define the wrapper: var wrapper = $( "<div />" ) .attr( settings.wrapperAttrs ) .css( settings.wrapperCSS ) // ** Set CSS! .appendTo( settings.container );

Your plugin may have an associated stylesheet where developers can add CSS styles. Even in this situation it's a good idea to offer some convenient way of setting styles in JavaScript, without having to use a selector to get at the elements.

linkProvide Callback Capabilities

What is a callback? – A callback is essentially a function to be called later, normally triggered by an event. It's passed as an argument, usually to the initiating call of a component, in this case, a jQuery plugin.

If your plugin is driven by events then it might be a good idea to provide a callback capability for each event. Plus, you can create your own custom events and then provide callbacks for those. In this gallery plugin it might make sense to add an "onImageShow" callback.

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var defaults = { // We define an empty anonymous function so that // we don't need to check its existence before calling it. onImageShow : function() {}, // ... rest of settings ... }; // Later on in the plugin: nextButton.on( "click", showNextImage ); function showNextImage() { // Returns reference to the next image node var image = getNextImage(); // Stuff to show the image here... // Here's the callback: settings.onImageShow.call( image ); }

Instead of initiating the callback via traditional means (adding parenthesis) we're calling it in the context of image which will be a reference to the image node. This means that you have access to the actual image node through the this keyword within the callback:

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$( "ul.imgs li" ).superGallery({ onImageShow: function() { $( this ).after( "<span>" + $( this ).attr( "longdesc" ) + "</span>" ); }, // ... other options ... });

Similarly you could add an "onImageHide" callback and numerous other ones. The point of callbacks is to give plugin users an easy way to add additional functionality without digging around in the source.