jQuery is so easy to use that sometimes we just forget that it's not CSS. While using CSS, we don't have to give much thought to performance, because it's so fast that it's not worth the effort to optimize it. But when it comes to the real world, jQuery can drive developers crazy with performance issues. Sometimes you lose precious milliseconds without even noticing it. Also, it's so easy to forget about some functions and we keep using the old (and not-so-good) ones.

Let's take a look at a few of the most-common-and-easiest-to-fix mistakes while using jQuery in your projects.

1. You aren't using the latest jQuery version
Each version update means higher performance and several bug fixes. The current stable release is 1.7.2, and I'm pretty sure you know about plenty of sites developed using 1.6 and below. Ok, ok, you can't just update every old site for every jQuery update (unless your client is paying you to do so) but you should at least start using it for your new projects. So, forget about this local copy and just grab the latest release every time you start a new project.

2. You aren't using a CDN-hosted copy of jQuery
How many unique visitors you`ve got last month? I bet the number is still under 1 billion, right?
So you'd better use Google's copy of jQuery instead of yours. If your user still has the cached file of Google's website (or from many other sites that uses its CDN) his browser will just get the cached version, improving a lot your website's performance. You can use it by copying & pasting this HTML:

<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.7.2/jquery.min.js"></script>

3. You aren't using a fallback for CDN version
I know I said Google is awesome and stuff, but they can fail. So, every time you rely upon external sources, make sure you have a local fallback. I've seen this snippet in the HTML5 boilerplate source code and just found it amazing. You should use it too:

<script src="http://ajax.googleapis.com/ajax/libs/jquery/1.7.2/jquery.min.js"></script>
<script>window.jQuery || document.write('<script src="js/libs/jquery-1.7.2.min.js"><\/script>')</script>

4. You aren't chaining stuff
While doing common operations, you don't need to call the element every time you want to manipulate it. If you're doing several manipulations in a row, use chaining, so jQuery won't need to get the element twice.

Instead of this:

$(“#mydiv”).hide();
$(“#mydiv”).css(“padding-left”, “50px”);

Use this:

$(“#mydiv”).hide().css(“padding-left”, “50px”);

5. You aren't caching stuff
This is one of the most important performance tips. If you'll call an element at least twice, you should cache it. Caching is just saving the jQuery selector in a variable, so when you need to call it again you'll just reference the variable and jQuery won't need to search the whole DOM tree again to find your element.

/* you can use it this way because almost every jQuery function returns
the element, so $mydiv will be equal to $(“#mydiv”); also it's good to
use the $mydiv so you know it's a jQuery caching var */

var $mydiv = $(“#mydiv”).hide();
[.. lot of cool stuff going on here …]
$mydiv.show();

6. You aren't using pure js
Specially for attributes modification, we have several built in methods to get stuff done with pure javascript. You can even “convert” jQuery objects back to DOM nodes to use them with simpler methods, like this:

$mydiv[0].setAttribute('class', 'awesome'); //you can convert jQuery objects to DOM nodes using $jqObj[0]

7. You aren't checking plugins before including in your site
You know, jQuery is not the hardest thing in the world to code. But good JS (and jQuery), that is pretty hard. The bad news is that while you aren't a good programmer, you'll have to rely on trial and error to find out what is good and what isn't. A few points you must be aware of while including a plugin in your project:

File Size (the easiest to check!) – if a tooltip plugin is bigger than jQuery source, something is really wrong;
Performance – You can try it with firebug and others. They give you easy to understand charts to you'll know when something is out of place;
Cross-browsing – Test, at least on IE7, but Chrome, Firefox, Safari and Opera are good ones to try also
Mobile – Don't forget that everything is getting mobile. See if the plugin works, or at least doesn't crash your mobile browser
8. You aren't open to remove jQuery
Sometimes it's just better to remove it and use simple ol' CSS. Actually if you want, for instance, an opacity hover, or transition can be done with CSS along with good browser support. And there's no way jQuery can beat plain CSS.

9. You are using jQuery for server side tasks
I know that masking and validating are good, but don't rely just on jQuery for those. You need to recheck everything on the server side. That is especially important if you are thinking about using AJAX. Also, make sure everything will work with JS disabled.

So, it's your turn!

Do you have anything you think should be on this list? Share your thoughts!

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