An introduction to jQuery(zz)

Last Update
Updated: 09/8/2008

What's covered

Sample Download (ASP.NET)

www.west-wind.com/files/conferences/jQuery.zip

Ok, I admit it. For many, many years I hated JavaScript. I hated writing JavaScript code, and even more I hated the pain that goes along with dealing with different browsers using reams of script code. I still hate the same problems today, but thanks to a recently gained better understanding of JavaScript and a small JavaScript client library called jQuery I'm no longer dreading the days when I have to write client centric Ajax script code.

I started out as a JavaScript neophyte and stayed that way for a long time. Sure I used JS here and there for validation and few simple manipulation tasks, but it wasn’t until a few years ago when Ajax surfaced to even take a more serious interest. It took me some time after that to slowly get up to speed on modern JavaScript principles I had never even thought about up until then: Effective use of Closures, the Prototype model, class definitions, using functions as code, all of which require some getting used to when you are primarily dealing with static languages like C# as I do for most of my work.

But even knowing JavaScript reasonably well is not enough – these days it'd be silly to work with raw JavaScript for DOM programming and not use some sort of library to help bridge the browser specific quirks and provide utility functionality to make it easier to work in a browser agnostic environment. I've checked out a number of different libraries over the last couple of years, but the one that really grabbed me is jQuery. jQuery is a relatively small library that is based on a few very simple and intuitive principles. To me this library strikes the right balance between size, feature set and ease of use. Even originally after using it for just a few days I already felt like "this is what JavaScript and DOM programming should be!"

jQuery can bring tremendous productivity gains and it's easy to learn and work with. It's one of those tools that has drastically changed of how I think about client side development and frankly it has helped me improve my skill set significantly. It's also made me much more productive and more confident in being able to tackle complex UI and front end logic in JavaScript reliably.

Key features of jQuery

Let's take a look of why jQuery is a tool worth checking out. There's quite a bit of functionality provided in jQuery but here’s is a list of what I think are the key features:

DOM Element Selectors
jQuery Selectors allow you to select DOM elements so that you can apply functionality to them with jQuery’s operational methods. jQuery uses a CSS 3.0 syntax (plus some extensions) to select single or multiple elements in a document. Using CSS means that you use selector syntax you’re probably already familiar with from HTML styling and even if not it’s fairly easy to pick up the key CSS selector features. I’ll go as far as saying that jQuery is the reason I really started to grok CSS. Using CSS syntax you can select elements by id, CSS class, attribute filters, by relationship to other element and even filter conditions that can be chained together. A simple example: It's trivial to select all 2^nd column TD elements in a table with a simple selector like this: $("#gdEntries td:nth-child(2)").
The jQuery Object: The Wrapped Set
Selectors result in a jQuery object that is known as the Wrapped Set, which is an array like structure that contains each of the selected DOM elements. You can iterate over the wrapped set like an array or access individual elements via the indexer ( $(sel)[0] for example). More importantly though you can also apply jQuery functions against all the selected elements.
Wrapped Set Operations
The real power of the wrapped set comes from being able to apply jQuery operations against all selected DOM elements simultaneously. The jQuery.fn object exposes about 100 functions that can operate on the matched set and allow to manipulate and retrieve information from the selected DOM objects in batch. For example, you can easily manipulate all alternate rows in a table by adding a CSS class with $("#gdEntries tr:odd").addClass("gridalternate"). The .addClass() function is applied against each of the matched elements with one command. Intuitive methods allow you do to things like get and set .css() styles directly including smart logic that accounts for browser differences of assignment types (number and string translations mostly) and values (opacity does the right thing on all browsers). You can set and retrieve attributes with .attr(), or retrieve or set a value with .val(), .text() or .html(). You can clone selected DOM elements or create new elements from HTML text used as a selector and inject them into the document with methods like .appendTo(), .prependTo() or reversely use a parent element to .append() or .prepend() the new or selected element(s). There are basic but useful effects methods that can be applied to .show() and .hide() elements in a smart way that checks for opacity, display and visibility and adjusts all to show or hide elements. All of this and much more can be done against all of the selected elements.

Most wrapped set operations are also chainable so that they return the jQuery wrapped set object as a result. This means you can chain together many methods in a single command. Effectively this means you can select once, and operate many times against the same object and even filter or expand the wrapped set with methods like .find(), .filter() or .add().

The beauty of many of these functions is that they do things you actually want to do, and they are intuitively overloaded. Methods like .val() or .text() act both as retrieval and setter methods. Methods that deal with numeric values can take either text or numeric values. CSS assignments to browser dependent tags are automatically fixed up. Although the number of functions provided by jQuery is relatively small, many of the functions provide overloaded functionality to perform intuitive behaviors. The end result is that you have a relatively small API to learn, but a much broader range of functionality that is available on it.
Simplified Event Handling
Much of what is done in JavaScript code from DOM manipulation to AJAX calls are handled asynchronously and unfortunately the DOM implementations for event handling vary considerably between browsers. jQuery provides an easy mechanism for binding and unbinding events and providing a normalized event model for all supported browsers that makes it very easy to handle events and hook result handlers. All events are called in the context of the element that caused the event and they receive a fixed up and browser normalized event object that is consistent.

Small Footprint
jQuery is a fairly compact base library yet it’s feature packed with stuff you'll actually use. In my relatively short time of jQuery use I've gone through well over 85% of the jQuery functions with my code, which points at how useful the library is. All this functionality ends up in a compressed size of just around 16k (94k uncompressed with comments). For that you get selectors, a whole slew of operations that can be performed on the wrapped set, DOM normalization for most browsers, Ajax functionality, a host of utility functions for object/array manipulation and a number of basic effect functionality. Given the high utilization of jQuery in my use this 16k of script download provides a tremendous amount of “Bang for the Buck”.

Easy Plug-in Extensibility
jQuery is a language and DOM extension library and it provides a core set of useful features. It's small and tightly focused on providing core functionality and no more. For everything else jQuery provides a very easy plug-in API that has spawned hundreds of plug-ins for almost every conceivable common operation you might think up to perform on a set of DOM elements.

jQuery’s API allows extending the core jQuery object’s operations simply by creating a function and passing the jQuery Wrapped set as a parmeter. In this way plug-ins receive the wrapped set and can operate on it and participate in the jQuery chaining. This very simple but powerful plug-in model is very easy to work with and likely the key of why so many plug-ins exist and jQuery has become so popular so quickly. If you need some specialty functionality, chances are that a plug-in already exists with the functionality you're looking for. And if it doesn't, it's easy enough to creating it yourself with the help of jQuery or another plug-in as a baseline.

Ok so now that you have some idea of what jQuery provides let’s take it for a spin. In this article I’ll introduce you to core jQuery concepts of document manipulation purely from a client side perspective. In part 2 I’ll discuss how to use jQuery in combination with ASP.NET on the server for AJAX callbacks and how to integrate jQuery with server side controls and components.

Getting a feel for jQuery

Let's start by looking the basics of jQuery syntax and usage. Mind you this only covers a few features of jQuery, but it should give you an idea of the base principles as well as some of the power of this library.

Adding jQuery to your Page

jQuery is a client script library and so you have to add a script reference to your page. You can download the latest version of jQuery from the jQuery site at http://www.jquery.com/.

There are many ways that you can include jQuery into your page:

Reference a local copy via <script> tag in the page
Reference a local copy with ScriptManager
Embed script from Resource using the ClientScript object
Reference a remote copy from jQuery.com or Google Ajax API

Both jQuery.com and Google host the jQuery script files on their sites. Google’s repository includes both static file and Google API dynamic links to load libraries for both jQuery and jQuery.ui and the Google site tends to be faster and more reliable.

Personally I prefer to have local copies of my libraries available so I’m not dependent on an online resource if my connection is down or if I’m offline for example. I tend to use local scripts and sticking them into a /scripts folder of my Web application and that’s what I’ll use here in these samples which gets embedded in the header or via the client script manager from ASP.NET code:

<script src="scripts/jquery.js" type="text/javascript"></script>

</head>

<body>

If you’re interested in more info on the pros and cons of the various script inclusion methods check out this blog post.

Getting started with Selectors

Selectors are the key that make jQuery useful. You start by selecting content and then applying a jQuery function to it. Here are a few examples that hopefully are fairly self-explanatory.

You can select a single element:

$("#gdEntries").css("border", "solid 1px navy");

Or select by class (.):

$(".gridalternate").css("background", "lightsteelblue ");

or from a couple of classes (, like in CSS separates multiple selectors):

$(".gridrow,.gridalternate").attr("tag", "row");

You can also select elements (plain element) and apply filters to the elements. The following selects all buttons in a document and attaches a click handler:

$("input:button ").click(function(event) { alert("clicked"); });

A more complex example might select all rows in a table:

$("#gdEntries>tbody>tr").css("border", "solid 1px red");

If you’re at all familiar with CSS all of this should look familiar: You can use the same syntax in CSS stylesheets (with the exception of the :button filter – jQuery adds a host of custom filters) to find elements to style. The idea is simple: Use what you already know or at least use a standard mechanism rather than a custom selector syntax. CSS is quite powerful in selecting elements on a page and given a reasonably thoughtful HTML layout should make just about any element or group of elements easily selectable.

If you’re rusty on CSS, here are a few of the common jQuery/CSS selectors types that can be applied (full list):

Selector	Example	Description
Element	$("td")	Select an HTML element tag
#id	$("#divMessage")	Selects an element by its id
.cssclass	$(".gridalternate")	Selects a CSS style
selector,selector	$("input:button,input:text")	Multiple comma separated selectors can be combined into a single selection.
ancestor descendant	$("#divMessage a")	A space between selectors/elements/tags finds nested elements. This syntax is similar to Css ancestor descendant syntax.
parent > child > child	$("p > b")	Matches all immediate children of an element or selector expression that match the right element/selector.
prev ~ siblings ~ siblings	$("#row_11:nth-child(2)~td") $("~td")	Matches the next siblings at the sample level as the preceeding expression. Example, matches 3-nth columns of a table row. Best used as a find() or filter() against an existing jQuery instance.
prev + nextsibling + nextsibling	$("#tdMoneyCol+td") $("+td")	Matches the following sibling. Works best with find() or filter() against an existing jQuery object.
:filter	$("input:button")	: applies filters to the query. jQuery support CSS 3 filters plus a number of custom filters. Examples: :not,:button,:visible,:hidden,:checked,:first,nth-child(1),:has,:is,:contains,:parent
[@attribute]	$("p[class=gridalternate]	Selects an attribute of an element = equals string ^= startswith $= endswith *= contains

Selectors can get quite sophisticated. Lets say you want to look for any cells in a table that start have an ID that starts with the value Pk:

$("#gdEntries>tbody>tr>td[id^=Pk]").width(30);

Cool eh? You get a lot of control over exactly picking up elements you are looking for.

Let’s look at a simple, concrete example of an ASP.NET page and use jQuery on the client side to fix up the page to highlight some of jQuery’s features. The following page contains a GridView to display a plain table loaded from data. I’m going to add jQuery and a bit of code to manipulate the page. You can follow along if you like by downloading the code accompanying this article. Here’s the abbreviated ASP.NET page I’ll work with (a plain HTML is also provided in the sample download):

Simple ASP.NET markup used for these examples

<title>jQuery 101</title>

<script src="scripts/jquery.js" type="text/javascript"></script>

</head>

<body>

…

<asp:GridView ID="gdEntries" runat="server"

AutoGenerateColumns="false"

cssClass="blackborder"

CellPadding="4">

<asp:BoundField DataField="TimeIn" HeaderText="Time in" DataFormatString="{0:MMM dd hh:mm tt}" ItemStyle-CssClass="timeinField"/>

<asp:TemplateField ItemStyle-CssClass="descriptionField">

</ItemTemplate>

</asp:TemplateField>

<asp:BoundField DataField="ItemTotal" HeaderText="Total" />

</Columns>

</asp:GridView>

</form>
</body>

</html>

There’s only minimal formatting applied to the grid and if you run the page with the jQuery code commented out you get a very plain, black and white grid display.

Original Html Page

Figure 1: The original plain page to work from

So let’s use jQuery to demonstrate how we can select elements and make the table look a little nicer and add some interactivity. Let’s starte by applying an alternating row effect to the table rows.

Let’s add a script tag to the bottom page just above the </form> tag like this:

$(document).ready( function() {

$("#gdEntries tbody tr:even")

.addClass("gridalternate")

.css("border","solid 1px lightgrey");

});

</script>

Which gives you alternate row striping of rows:

Updated Web Page with alternating rows

Figure 2 – Alternating stripes applied with selectors and .addClass()

Let’s examine what makes this very simple jQuery command work.

The jQuery()/$() Object

Take a look back at the script block on the bottom of the page and notice the $ symbol in the script block, which is an alias to the jQuery object. $ is just a shortcut for the jQuery object and the two can be used interchangeably. So above I could have used

jQuery(document) or jQuery("#gdEntries") just as easily as $(document) or $("#gdEntries"). The jQuery object takes a selector as a parameter. Typically selectors are expressed as strings, but you can also pass DOM elements or another jQuery object.

For example $(document) passes a document DOM element and results in a single element wrapped set that contains the document object. $("#gdEntries") selects the gdEntries table on the page where # is the ID identifier just as in CSS.

The $(document).ready() Handler

Notice the use of the $(document).ready() handler. .ready() is a jQuery event handler and your first exposure to how to handle events with jQuery. This particular event handler lets you know when the Document is ready to be accessed and scripts have completed loading. This ensures that your code can access all DOM elements and script code reliably. A .ready() handler can be placed anywhere on the page and you can even have multiple ready handlers in the page.

Although a .ready() handler is optional and not always required, for consistency and reliability’s sake it’s best to always wrap any script ‘startup code’ into a .ready() handler to ensure reliable operation on all browsers.

The handler is implemented using an anonymous function which means the handler is declared right inline of the .ready() function call ($(document).ready( function {…}). This is a common practice with jQuery because inline functions are an easy way to write and read the short handler code that is common in JavaScript code. There are other advantages to inline functions, but for now just note that inline anonymous functions are a common way to write event handlers, so you’ll see more of them shortly. You could also pass a function pointer instead: The following is just as valid $(document).ready(onLoaded) where onLoaded is a function defined in script code externally.

Selectors Again

Let’s come back to the grid example code that performs the selection of the rows in the example. $("#gdEntries tbody tr:even") selects all the even rows in the grid view’s table output. In plain English the selector means: select the gdEntries element (#) and find any child tbody elements (space=child) find any tr elements below it (space=child) and then return a filtered set(:expression=filters) that includes only the even elements.

The result of the selector is a wrapped set of all even rows in the table. The .addClass("gridalternate") call then applies the .gridalternate CSS style to the each matched element which gives it its new highlighted appearance. Finally the .css function applies some custom styling to each matched element. Both .addClass() and .css apply their operations against all the matched elements at once.

Selectors often select more than you expect

If you’re following along in the sample, you’re probably noticing that while the above works to highlight even rows, there’s also a funky side effect in the table: If you look at the pager – which ASP.NET renders as a table – you’ll find that its alternating rows have also been marked up with the .gridalternate class. The problem is that ASP.NET’s GridView rendering is, uhm rather imprecise. It doesn’t render proper tbody and tfoot tags to delineate table headers and footers properly and so our selector query is too broad as the selector also matches the child elements of the selected rows which includes the child Pager table in the footer.

There are a number of ways to fix the selector to make this work properly. First we can apply a filter to the selection by excluding certain elements.

$("#gdEntries tbody tr")

.not(":first,:last")

.filter(":even")

.addClass("gridalternate")

.css("border","solid 1px lightgrey");

So instead of filtering on all odd rows we’ll filter the rows and omit the first and last rows (the header and the footer). We then filter then list and grab only the odd items out of that set. Another even easier way is to use only direct children using the > operator rather than spaces which select all children:

$("#gdEntries>tbody>tr:even")

Another common way to filter elements that’s efficient and offers the developer the most control is via CSS class assignments to elements that are to be matched. You can use either real CSS style classes or ‘marker’ classes that can identify elements.

Take another look at the way the GridView’s rows are rendered:

This adds a class=”griddatarow” to each row rendered in the HTML. There’s no griddatarow CSS class, but jQuery can still match on it in the selector expression. So the following works:

$("#gdEntries tr.griddatarow:odd")

Or even as simple as:

$(".griddatarow:odd")

The latter works, but the first is preferable even if it is more verbose. It’s best to be as specific as possible in order to optimize selection for jQuery in the document. The former finds the gdEntries table and only searches for children in it, while the latter has to parse the entire document. The fewer elements matched by the first selector filters the less elements jQuery has to iterate over. Be precise if possible.

For kicks let’s say you wanted to select the second cell of the third row in the grid:

$("#gdEntries>tbody>tr:nth-child(3)>td:nth-child(2)")
.addClass("gridalternate");

Or maybe all second columns?

$("#gdEntries>tbody>tr>td:nth-child(2)")

.addClass("gridalternate")

Easy! Think about how cool this is: If you had to do this manually via script code this would be a fairly complex task. Selectors make it very easy to get access to just about any element or any sets of elements in the page and this feature is key.

The Wrapped Set

The result of $("#gdEntries>tbody>tr:even") is a jQuery object which is an array like structure that has a length property and an array containing each of the matched elements. You can reference the DOM elements in the wrapped set directly:

var res = $("#gdEntries>tbody>tr:even");

var len = res.length; // match count. 0 = no matches

var el = res[0]; // first element

el = res.get(0); // same but 'official' syntax

The jQuery constructor always returns an object result, so even if the selector finds no matches an object is always returned and you can check the .length property to determine if the selection returned matches.

Once you have a set of matches you iterate manually with a for loop or use the .each() wrapped set function to iterate over the list. What’s nice about .each() is that it assigns the this pointer to the selected element so you can do this:

$("#gdEntries tbody tr:even").each( function(index) {

alert(this.id + " at index: " + index);

});

Wrapped Set Functions

Manual iteration is useful at times, but more commonly you'll use one or more jQuery’s over 100 operational functions to operate on the resulting wrapped set’s elements. Both the .addClass() and .css() functions we saw above are applied against all the elements matched by the jQuery selector created with the jQuery() or $() function – in this case each of the rows selected. You can find all the jQuery functions on the API reference page or if you’re into terse lookup sheets you can check out this jQuery cheat sheet you can print out.

Function Chaining

jQuery lets you use a fluent interface for most function calls which means that you can chain many jQuery function calls together in single command. We’ve already seen examples of this earlier:

$("#gdEntries tbody tr")

.not(":first,:last")

.filter(":odd")

.addClass("gridalternate")

.css("border","solid 1px lightgrey");

This syntax chaining works because most jQuery functions return a matched set as a result. For example, .addClass, .css both return the original matches they acted on. Other functions like .not and .filter are actually filter functions – they modify the original matched set and return a new filtered matched set instead. There’s also an .end() function that is used to remove any filters and return to the original matched set specified at the beginning of the chain.

Chaining is a great way to keep code compact, but it’s optional. If you prefer you can also write more traditional code like this:

var matches = $("#gdEntries>tbody>tr>td:nth-child(2)");

matches.addClass("gridheader");

matches.removeClass("gridhighlight");

Chained statements can sometimes be difficult to debug so this explicit method is easier to step through with a debugger and see the individual results, then one huge chained statement.

Not all jQuery functions can are chainable. Some operations result in non-matched sets. For example functions like .val(), .text(), .html() return string values, .width() and .height() return numbers, .position() returns a position object. If you look at the jQuery function list though it should be pretty clear what the result types of most functions are. Typically functions that you would expect to return a void result return a matched set. Any functions that return ‘data’ or ‘values’ don’t return a matched set.

Event Handling

Event handling is one of the nicest aspects of jQuery because it makes the process easy and consistent across browsers. jQuery provides the high level .bind() and .unbind() functions to generically attach and detach event handlers on matched sets. In addition most of the common events like click, key and mouse events have dedicated handler functions like .click(), .mousedown(), change() and .keydown(). jQuery event handlers simply take a function as a parameter and jQuery tracks these handlers so that they can also be unbound later. It can’t be much easier than this. In addition there’s .one() which fires an event exactly once then disconnects the handler and .toggle() which toggle between alternating clicks, and .trigger() which can trigger events on elements.

jQuery also provides a common model for event handlers:

The this pointer always equals the element the event fired on
An event object is always passed as a parameter
Event object is a cross-browser ‘normalized’

Let’s build on top of the earlier grid example by highlighting grid rows when you hover over them regardless of browser. The following block of code uses jQuery's .hover() event function which fires as an element is hovered over. .hover takes both an entry and exit event function handler as parameters.

Let’s change the previous command to:

$("#gdEntries>tbody>tr")
.not(":first,:last")

.hover( function() {

$(this).addClass("gridhighlight");

}, function() {

$(this).removeClass("gridhighlight");

})

.filter(":even")

.addClass("gridalternate");

Note that I re-arranged the original query a little so that I can apply the hover behavior to all rows and then filter the list afterwards to include only the even rows and apply the alternating style to those.

The .hover() function is an example of a jQuery event handler and it happens to be a special one because unlike most it takes two callback functions as parameters. The first for the mouseenter and the second for the mouseout operation. jQuery handles .hover() smartly detecting only ‘real’ mouseout events when leaving the container unlike the mouseout event which also fires when entering child elements. In other words, jQuery provides the hover behavior you would expect by abstracting mousenter and mouseout.

If you run the page now you should get an orange highlight for each row that the mouse hovers over now.

Now let’s also handle clicks on the row and pop up a dialog that shows the value of the selected third column, which can be easily done with the following .click() handler:

$("#gdEntries>tbody>tr")

.not(":first,:last")

.hover( function() {

$(this).addClass("gridhighlight");

function() {

$(this).removeClass("gridhighlight");

})

.click( function(e) {

alert("Amount: " + $(this).find("td:nth-child(3)").text());

})

.filter(":even")

.addClass("gridalternate");

This code hooks up a click handler to every row in the matched set and responds to a click. Event handlers are always called in the context of the element that caused the event – in this case the row and so this points at the row DOM element. You can turn this into a jQuery object $(this) and then apply further filtering to this single element to find the third child cell and display the contents of this cell.

Adding selection and clicking to the grid

Figure 3 – The grid with hovering and click event handling added.

Displaying the amount is not all that useful though. A more common scenario for this type of setup is for navigation where you click or perform some AJAX request based on the row selection. Unfortunately the ASP.NET Gridview doesn’t provide any sort of id or context on the client side so it’s not easy to take a context based action. If you want to navigate or post back data in some sort of useful context we have to take matters into our own hands. It’s pretty easy to fake context though knowing that you can use jQuery to easily access document content. So, we can embed an id value into the generated content of one of the cells. The grid already has a template column so I’ll change it to:

<asp:TemplateField ItemStyle-CssClass="descriptionField">

<div style="display:none"><%# Eval("pk") %></div>

</ItemTemplate>

</asp:TemplateField>

Which embeds the pk into the generated output. You can then handle the .click() event by retrieving that PK value in the 2^nd column and looking at the first <div> tag and getting its text:

.click( function(e) {

var pk = $(this).find("td:nth-child(2) div:first-child").text();

window.location = "someotherpage.aspx?id=" + pk;

})

Using jQuery and its flexible selector syntax it becomes possible to use HTML and the DOM as data storage by allowing you to store state content in the HTML and get at it via selection as in this example. And it makes it possible even to use an inflexible component like the GridView and make it more dynamic. Approaches like this can completely change the way you think about the purpose of client UI and interactivity. For example, you can do in place editing of content in the browser and then pick up that content later to post to the server via AJAX or even a Postback. The possibilities are endless.

Creating new HTML and embedding it into the DOM

jQuery objects can also be created directly by simply passing an HTML string as the parameter to the jQuery object. To create Html and add it to the document is as easy as:

$("<div id='_statusbar' class='statusbar'></div>")

.appendTo(document.body)

Html can be injected anywhere in the document with .appendTo() or .prependTo() which inserts into the child items of the selector element(s). .append(), prepend() inserts to the current selected element instead.

You can also .clone() existing nodes easily. Try this on the example above:

$("#gdEntries").clone()

.appendTo(document.body)

.attr("id","gdEntries2");

And you’ll see the entire GridView duplicated in the page. Not terribly useful, but there are a number of situations where cloning is very useful. For example, you can use cloning() for a sort of templating mechanism. To insert a new row with new content in the table above you can use code like this:

var row = $("#gdEntries>tbody>tr:nth-child(2)").clone();

row.find("td:first")
.text(new Date().formatDate("MMM dd - hh:mmt"));

row.find("td:nth-child(2)")
.html("<b>" + new Date().formatDate("hh:mm:ss") +
"</b><br />More text second item here");

row.find("td:last")
.text( 199.99.formatNumber("c"));

row.insertBefore("#gdEntries>tbody>tr:nth-child(2)")
.show();

(note the format functions are a few helper functions that can be found ww.jquery.js provided with the source download). The nice thing with this approach is that you don’t have to create HTML on the client – you’re only injecting data and maybe minimal markup rather than HTML strings because you are cloning an existing node and only ‘filling in the blanks’.

Content doesn’t have to pre-exist either. You can load ‘template’ content from non-visible HTML elements on the page or embedded inside of scriptblock for example to clone or create a new jQuery object from. This example is simplistic because I’m injecting static content, but you could retrieve content from an AJAX callback or captured form input fields and load that into the cloned ‘template’.

Templating is a powerful topic and I’ll come back to it later in Part 2 when we look at a couple of different templating tools you can use in combination with jQuery (if you’re eager to check it out now look at jTemplates or John Resig’s cool Micro-Template engine).

jQuery Effects Functions and some jQuery UI Features

One nice touch of jQuery is that it includes a few simple yet very effective effects methods. For example, if I wanted to add a little pizzaz to those new elements added in the last example, instead of calling .show() I could fade in the elements with simple code like this:

row.insertBefore("#gdEntries>tbody>tr:nth-child(2)")

.css("background", "paleyellow")

.hide()

.fadeIn(1000);

The .fadeIn() and .fadeOut() functions do exactly as their name implies and provide a very easy way to give a visual clue that something is updating on the page. Other methods include .slideUp() and .slideDown() which slide content from the top or bottom to the current absolute position again with speed indicators. For lower level animation there’s the .animate() function which allows you animate numeric CSS properties to a new value over a timeout period to provide custom animations. In addition there are a host of animation plug-ins available that can provide all sorts of fade and transitions effects if you’re into the pompous kinds of effects. It’s pretty impressive what you can do with very little effort, but even the basic built-in function do a lot to liven up a Web page. Just remember to keep it simple and not just add noise to the page.

Another Simple Example showing a Popup Window

jQuery can also help to create utilitarian UI features. Let’s add a pop up window to the example, and allow dynamically typing selectors into it that select and highlight matched elements in the page. The end result should look something as shown in Figure 4.

Displaying a Pop up Window for selection

Figure 4 – Displaying a dynamic pop up window with some effects.

The dialog is popped up by clicking on the Show Query Dialog window, which ‘slides in’ the dialog and displays with a slightly transparent background ontop of the content. You can then type a jQuery selector and basically query the document. When you click show the selected elements are highlighted in dark red in the document, then removed after a few seconds.

The dialog is stored on the page as a simple <div> tag element that’s styled and it looks like this:

<div class="gridheader">Enter a jQuery Expression</div>

Expression: <span id="lblExpressonMsg"
class="errormessage"></span><br />

</div>

There’s also some styling for the dialog that makes it transparent, initially hidden and absolutely positioned on the page:

#divjQueryDialog

{

width: 380px;

position: absolute;

left: 100px;

top: 70px;

display: none;

}

The code that makes the panel visible applies opacity and handles the matching and selection is relatively simple too:

Code to pop up a jQuery Selector test window

function showQueryDialog()

{

var dialog = $("#divjQueryDialog");

dialog.hide()

.slideDown("slow")

.css("opacity",.90);

$("#btnExpression").bind("click", function(event)

{

// Grab the selector entered as text

var exp = $("#txtExpression").val();

// Execute the selector through jQuery

var matches = $(exp);

// *** check result

var len = matches.length;

if ( len < 1)

{

$("#lblExpressionMsg").text("No matching elements");

showStatus("No matches",3000,true);

return;

}

showStatus( len.toString() + " matches",3000,true);

// No errors clear error message

$("#lblExpressionMsg").text("");

// Highlight all matches

matches.addClass("selectionhighlight");

// Set up timer to remove the highlight

setTimeout(function()

{

$(exp).removeClass("selectionhighlight");

},5000);

// Unbind event and hide dialog

$("#btnExpression").unbind("click");

dialog.slideUp();

});

}

The code initially makes the div visible and applies opacity. It then hooks up a click handler to the button and an anonymous function to handle the processing of the click. That code captures the input text and then uses jQuery to ‘evaluate’ the selector applying a special class to the matched set. A timeout is also set up the removes the highlight class after 5 seconds.

I mentioned nesting anonymous functions earlier and the above code shows another example – the code above is quite asynchronous, yet it is written in a single block of code that keeps context. The inner click handler has access to dialog without having to reselect it. This is due to the way JavaScript handles closures or function blocks that are nested within each other which can see private variables defined higher up in the function definition stack – even if the code is asynchronous. In this example this is trivial but if the out code (or ‘class’) contained more state it’s quite convenient to have the state passed to the handler merely by virtue of being declared inside of the parent closure. This type of coding can often be easier to read as it keeps relative logic together in one place.

A couple of other things to highlight here: Notice the code to make the dialog transparent:

dialog.hide()

.slideDown("slow")

.css("opacity",.90);

jQuery is smart about how it handles certain CSS tags like opacity that browsers handle differently. Internet Explorer doesn’t support opacity (because it’s not officially part of the CSS 2.1 standard), but jQuery automatically does the right thing applying a filter for Internet Explorer. Similar rules are applied for other CSS properties and certain functions. For example, .text() retrieves text consistently, .scrollLeft/Top retrieves scroll position safely. So jQuery acts as a normalizer between different browser implementations.

One last enhancement: If you want to make the window draggable so you can move it around the page this is also easy to using the draggable plug-in from jQuery UI. To use this plug-in download jQuery UI dump the library into your scripts folder (relevant portions provided in the sample). Then add:

To the page and simply add:

var dialog = $("#divjQueryDialog");

dialog.draggable();

That’s it. You’ve just made your ‘window’ a lot more window-like with a single line of code through this simple plug-in. There are a number of options you can apply to draggable – for example to set the drag handle to the window’s header you can use:

dialog.draggable({ handle: $("div.gridheader") });

which makes only the header initiate a drag operation which is a more natural drag behavior for a window.

Plug-ins for Everything Else

Plug-ins are one of the main reasons that jQuery has become so popular. Part of the reason for this is that jQuery has a super simple API for plug-ins. There is a jQuery.fn property which hosts any operational functions that can operate on the matched set. By simply implementing a new function on jQuery.fn you effectively create a new matched set method. Here’s a very simple pulse plug-in that pulses an element by fading it out partially and then fading back to full visibility:

$.fn.pulse = function(time)

{

if (!time)

time = 2000;

// this == jQuery object that contains selections

this.fadeTo(time,0.30,

function() {

$(this).fadeTo(time,1);

});

return this;

}

To use this plug-in you can now simply use jQuery and use the pulse function on it for the matched set of elements. For example, earlier we added a new item to the grid – you can pulse this item maybe instead of fading it in:

row.insertBefore("#gdEntries>tbody>tr:nth-child(2)")

.css("background", "paleyellow")

.pulse(2000);

When you run this now you should see the new row pulse for 2 seconds as it is added. This plug-in also works against many matched elements:

$("#gdEntries>tbody>tr")

.filter(":even")

.addClass("gridalternate");
.pulse(1000);

which pulses only the even rows.

A plug-in receives a this pointer that is the jQuery matched set. So you can apply further functionality to this matched set in the plug-in code. If the plug-in doesn’t return a distinct result value you should always pass the jQuery result set back as a return value so that the function is chainable for additional jQuery functions.

$("#gdEntries>tbody>tr")

.filter(":even")

            .addClass("gridalternate");
            .pulse(1000)
            .addClass("small");

The above is an overly simplistic plug in. More commonly you need a little bit of set up data for a plug-in as well as needing to loop through elements. Here’s another plug in that centers any elements in the browser window or inside of another element:

A jQuery centerInClient Plug that centers content in the Window

$.fn.centerInClient = function(options) {

var opt = {

container: window, // selector of element to center in

completed: null

};

$.extend(opt, options);

return this.each(function(i) {

var el = $(this);

var jWin = $(opt.container);

var isWin = opt.container == window;

// have to make absolute

el.css("position", "absolute");

// height is off a bit so fudge it

var heightFudge = 2.2;

var x = (isWin ? jWin.width() :
jWin.outerWidth()) / 2 - el.outerWidth() / 2;

var y = (isWin ? jWin.height() :
jWin.outerHeight()) / heightFudge - el.outerHeight() / 2;

el.css({ left: x + jWin.scrollLeft(),
top: y + jWin.scrollTop() });

var zi = el.css("zIndex");

if (!zi || zi == "auto")

el.css("zIndex", 1);

// if specified make callback and pass element

if (opt.completed)

opt.completed(this);

});

}

This plug-in includes a couple of common patterns. First it has a set of option parameters that can be passed in as an object map (ie. {container: "#divContainer"} ). Notice that opt is defined as a private variable and then extended with the option parameter object. $.extend() is a wonderful utility function that extends an object with the content of another object and that makes parameter processing easy for the user, by only requiring to pass parameters she’s interested in. Most plug-ins accept options parameters in this fashion and it’s a good idea to implement this in your own plug-ins.

The next common plug-in pattern is to return this.each( function() {…}); which ensures the jQuery matched set is returned .The .each function iterates through each of the matched DOM elements individually and lets you operate against each of them one at a time. Inside of the .each() function handler the this pointer is always the DOM element iterated over. You can use $(this) to get a jQuery instance for that element which is typically what you want to manipulate the element.

Because it’s so easy to create plug-ins I’ve ended up creating quite a few plug-ins myself both for my own and shared use. The simple plug-in model is a genius concept: It allows for jQuery to stay small with only a core set of features while still exposing all of the functionality that the core library has to any extensions. It’s a model more API vendors should embrace and execute as nicely as jQuery has.

What's not to like?

If all of this sounds like a glowing commercial of a tool, you can be assured that I'm just a happy user who's stumbled onto this library some time ago and fell in love with it the first time I used it. I tend to be very critical of the tools I use, but I have yet to have any serious complaints about jQuery. This library just works for me and it works the way I like to work.

But the library is not "the perfect tool" and doesn't solve every JavaScript and DOM problem for you, but in my time of over a year with jQuery I haven't run into a showstopper problem. You may still need a set of a few helper functions to help with non-DOM related functionality. For example, I still use my old JavaScript library for a number of UI and JavaScript utility functionality like date and number formatting, providing windowing support and a host of other features. Bottom line: You'll probably still have a utility library with helper functions to provide a few custom routines especially when it comes to formatting and manipulating the standard JavaScript types which jQuery doesn't cover.

The biggest concern that you might run into is versioning or possible interference with other libraries. Although jQuery supports a simple mechanism for isolating its own jQuery namespace (either via closure wrapping or using the $.noConflict() function), you may still run into issues with other libraries that also expose the $() function. There's also the issue of versioning of the jQuery library itself which is rev'd frequently. If you use the library names as downloaded (which include the version numbers) you'll end up renaming files and references frequently as the library is updated. If you use a fixed name you may run into version issues.

But all of this is minor and even expected. To be honest I'm grasping to find fault with jQuery for balance here because my editor mentioned I should mention downsides. A quick search around the Web seems to confirm my feelings though – there’s not a lot of negative content that digs up dirt on jQuery. But you can judge for yourself by trying it out and my bet is you'll dig it because it's quick to pick up and highly practical.

Ajax and ASP.NET Server Side Next Time

In this article I’ve introduced jQuery’s core functionality for DOM manipulation and JavaScript extensions. I’ve focused on the client side functionality without any thought given to the server side because ultimately this is what jQuery excels at most.

In the next installment I’ll look at jQuery’s AJAX functionality and how you can use ASP.NET as a backend to receive these AJAX calls in a variety of ways from plain Page callbacks, to REST calls to using WCF. I’ll also look at integration on how to create ASP.NET controls that use jQuery or jQuery plug-ins in combination with server side components and how to manage jQuery scripts in general in your ASP.NET applicications.

Until then check out jQuery for yourself and get excited about the possibilities it offers for improving your client side Web development. You won’t be disappointed.

posted @ 2008-10-11 16:36 stu_acer 阅读(550) 评论(0) 编辑收藏举报

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stu-acer

An introduction to jQuery(zz)

Key features of jQuery

Getting a feel for jQuery

Adding jQuery to your Page

Getting started with Selectors

The jQuery()/$() Object

The $(document).ready() Handler

Selectors Again

Selectors often select more than you expect

The Wrapped Set

Wrapped Set Functions

Function Chaining

Event Handling

Creating new HTML and embedding it into the DOM

jQuery Effects Functions and some jQuery UI Features

Another Simple Example showing a Popup Window

Plug-ins for Everything Else

What's not to like?

Ajax and ASP.NET Server Side Next Time

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