Animating CSS Properties

We have mastered some valuable examples of animation so far—sliding, fading, and some fancy hiding and showing—but we haven’t had a lot of control over what exactly is animated and exactly how it happens. It’s time to introduce a very exciting jQuery function, helpfully called animate, which lets you animate a whole host of CSS properties to fashion some striking effects of your own. Let’s have a look at an example of animate in action:

$('p').animate({
padding: '20px',
borderBottom: '3px solid #8f8f8f',
borderRight: '3px solid #bfbfbf'
}, 2000);

This code will animate all paragraphs on the page, changing the padding from its initial state to 20px and adding a beveled border over a period of 2 seconds (2,000 milliseconds).
To use animate, we pass an object literal containing the properties we would like to animate specified as key/value pairs—much the same as when you assign multiple properties with the css function. There’s one caveat that you’ll need to remember: property names must be camel-cased in order to be used by the animate function; that is to say, you’ll need to write marginLeft, instead of margin-left and backgroundColorinstead of background-color. Any property name made up of multiple words needs to be modified in this way.

Even more excitingly, the values you define can be relative to the element’s current values: all you need to do is specify += or -= in front of the value, and that value will be added to or subtracted from the element’s current property. Let’s use this ability to make our navigation menu swing as we pass our mouse over the menu items using the hover function:
Licensed to

$('#navigation li').hover(function() {
$(this).animate({paddingLeft: '+=15px'}, 200);
}, function() {
$(this).animate({paddingLeft: '-=15px'}, 200);
});

Mouse over the navigation menu, and you’ll see the links wobble around nicely.

Making Sure the Page Is Ready

Before we can interact with HTML elements on a page, those elements need to have been loaded: we can only change them once they’re already there. In the old days of JavaScript, the only reliable way to do this was to wait for the entire page (including images) to finish loading before we ran any scripts.
Fortunately for us, jQuery has a very cool built-in event that executes our magic as soon as possible. Because of this, our pages and applications appear to load much faster to the end user:

$(document).ready(function() {
alert("TEST");
});

The important bits here (highlighted in bold) say, “When our document is ready, run our function.” This is one of the most common snippets of jQuery you’re likely to see. It’s usually a good idea to do a simple alert test like this to ensure you’ve properly included the jQuery library—and that nothing funny is going on.

Adding and Removing Classes

If we need to remove the CSS from inline style rules, where should we put it? In a separate style sheet, of course! We can put the styles we want in a rule in our CSS that’s targeted to a given class, and use jQuery to add or remove that class from targeted elements in the HTML. Perhaps unsurprisingly, jQuery provides some handy methods for manipulating the class attributes of DOM elements. We’ll use the most common of these, addClass, to move our zebra stripe styles into the CSS file where they belong.
The addClass function accepts a string containing a class name as a parameter. You can also add multiple classes at the same time by separating the classnames with a space, just as you do when writing HTML:
$(‘div’).addClass(‘class_name’);
$(‘div’).addClass(‘class_name1 class_name2 class_name3′);

Selecting: The Core of jQuery

jQuery borrows the conventions from CSS for referring to id and class names. To select by id, use the hash symbol (#) followed by the element’s id, and pass this as a string to the jQuery function:
$(‘#celebs’)
You should note that the string we pass to the jQuery function is exactly the same format as a CSS id selector. Because ids should be unique, we expect this to only return one element. jQuery now holds a reference to this element.
Similarly, we can use a CSS class selector to select by class. We pass a string consisting of a period (.) followed by the element’s class name to the jQuery function: $(‘.data’)

The jQuery Alias

Including jQuery in your page gives you access to a single magical function called (strangely enough) jQuery. Just one function? It’s through this one function that jQuery exposes hundreds of powerful tools to help add another dimension to your web pages.
Because a single function acts as a gateway to the entire jQuery library, there’s little chance of the library function names conflicting with other libraries, or with your own JavaScript code. Otherwise, a situation like this could occur: let’s say jQuery defined a function called hide (which it has) and you also had a function called hide in your own code, one of the functions would be overwritten, leading to unanticipated events and errors.
We say that the jQuery library is contained in the jQuery namespace. Namespacing is an excellent approach for playing nicely with other code on a page, but if we’re going to use a lot of jQuery (and we are), it will quickly become annoying to have to type the full jQuery function name for every command we use. To combat this issue, jQuery provides a shorter alias for accessing the library. Simply, it’s $.
The dollar sign is a short, valid, and cool-looking JavaScript variable name. It might seem a bit lazy (after all, you’re only saving five keystrokes by using the alias), but a full page of jQuery will contain scores of library calls, and using the alias will make the code much more readable and maintainable.

Cross-browser Compatibility

Aside from being a joy to use, one of the biggest benefits of jQuery is that it handles a lot of infuriating cross-browser issues for you. Anyone who has written serious JavaScript in the past can attest that cross-browser inconsistencies will drive you mad. For example, a design that renders perfectly in Mozilla Firefox and Internet Explorer 8 just falls apart in Internet Explorer 7, or an interface component you’ve spent days handcrafting works beautifully in all major browsers except Opera on Linux. And the client just happens to use Opera on Linux. These types of issues are never easy to track down, and even harder to completely eradicate.
Even when cross-browser problems are relatively simple to handle, you always need to maintain a mental knowledge bank of them. When it’s 11:00 p.m. the night before a major project launch, you can only hope you recall why there’s a weird padding bug on a browser you forgot to test!
The jQuery team is keenly aware of cross-browser issues, and more importantly they understand why these issues occur. They have written this knowledge into the library—so jQuery works around the caveats for you. Most of the code you write will run exactly the same on all the major browsers, including everybody’s favorite little troublemaker: Internet Explorer 6.
This feature alone will save the average developer a lifetime of headaches. Of course, you should always aim to keep up to date with the latest developments and best practices in our industry—but leaving the task of hunting down obscure browser bugs to the jQuery Team (and they fix more and more with each new version) allows you more time to implement your ideas.