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What are some empirical technical reasons not to use jQuery? [closed]

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Closed 9 years ago.
Context:
I am astounded by the number of front end developers that hack at HTML, Javascript and CSS all day long and that ignore tools like jQuery ( or other equivalent helper frameworks ) and refuse to use them. I am not talking about JavaScript gurus, I am talking about in the trenches every day Joe production developers. I get a lot of arguments that are more like excuses or personal opinions that I don't think have any technical merit, I want to make sure I am not missing something.
Question:
What are some empirical technical reasons not to use jQuery?
I am not looking for religious or dogmatic arguments or subjective opinions "like some other framework is better", consider jQuery the straw man for all comparable frameworks in the question.

Update 2015:
In this answer from 2011 I'm talking about libraries like jQuery, YUI
or Prototype. Today in 2015 that reasoning is still applicable to
frameworks like Angular, React or Ember. In those 4 years the
technology progressed tremendously and even though I see considerably
less prejudice against React or Angular than I saw against jQuery or
YUI, the same kind of thinking - though to a lesser extent - is still present
today.
Update 2016:
I highly recommend an article published few days ago:
Why jQuery? by Michael S. Mikowski, author of the Single Page Web Applications book
That article is basically a very detailed answer to this very
question. Had it been available when I was writing the answer below - I would have definitely quoted it.
Original answer:
I'll answer about jQuery but those are the same arguments that I've heard against using YUI, Prototype, Dojo, Ext and few others. Main arguments that I've heard:
file size, which in fact is 84.6 KB in case of jQuery 3.2.1 - probably smaller than the logo on an average website and can be served from Google's CDN which is likely to be already in the cache of most of your visitors. As using jQuery always means smaller file size of your own JavaScript files, it can actually mean smaller download, even if not already in the browser cache.
speed - writing pure JavaScript may be faster, but writing portable JavaScript seems to be impossible for most of the people. A website that is faster but doesn't work on every popular browser is useless in the real world. Besides jQuery uses some pretty heavy optimizations to actually be pretty damn fast and keeps getting even faster with every release, so it's actually not so easy to write faster code by hand for anything other than trivial examples.(*)
"intellectual property" - a company is scared using someone else's code - while in fact jQuery is open source and free software that is used everywhere from your grandma's blog to Amazon, from Twitter to Bank of America, from Google to Microsoft - if they can use it then any company can use it.
I can't remember hearing any other argument being used seriously.
(*) Here's a trivial example: getElementById('someid') vs. jQuery('#someid')
Is using getElementById faster? Yes. And of course everyone always checks the parentNode to catch when Blackberry 4.6 returns nodes that are no longer in the document, right? jQuery does. And everyone handles the case where IE and Opera return items by name instead of ID, right? jQuery does. If you don't do it then your code is not portable and you introduce subtle bugs that can be very difficult to find. And getElementById is the most trivial example that one could possibly find - don't even get me started on events and AJAX and the DOM...
Update:
There is actually a fourth result of asking why someone doesn't want to use jQuery. I forgot to put it on this list because it is not really an answer but rather the lack of any answer. The comment I got yesterday reminded me about it. This is hardly a "technical reason" to be added to the list but may be interesting nonetheless and may actually be the most common reaction.
What I personally suspect to be the main underlying reason to all of those reactions, though, is what I believe to be the biggest obstacle to progress in computer science: "I don't want to use it because I never did, therefore it must not be that important."
It was once the reaction to optimizing assemblers, compilers, structured programming, higher level languages, garbage collection, object oriented programming, closures or pretty much everything that we now take for granted — and today it's AJAX libraries. Maybe some day no one will remember that we once used to manually interact with the raw DOM API on the application level like now no one remembers that we once used to write programs using raw, unadorned, inscrutable hexadecimal numbers.

jQuery expresses everything in a DOM-centric paradigm which can be misleading and doesn't require any need to express things in an application pattern.
Many developers wind up programming themselves into a corner with this DOM-centric pattern and eventually realize they haven't created anything extensible or reusable.
Rebecca Murphey has a great write-up of her own switch to Dojo from jQuery - the blog post is more about why not jQuery versus why Dojo.

One reason not to use a framework - and this is an extreme edge case - is when writing embeddable code for another website, such as a banner. Arbitrarily inserting some complicated library or another will be polluting the namespace and potentially breaking someone else's site. Not that I wouldn't put it past some advertisers to try anyway, the pond-sucking scum, but I digress...
I disapprove of adding a framework when one is already present and equally capable. I see it all too often and it's my pet hate, I see it as unwarranted bloat. That is another question entirely.
Other than that I cannot think of a justified reason not to.

filesize - but really, beyond that, it's an absolute god-send for cross-platform javascript and browser differences. You would have to have some very good reasons not to want it in your toolkit (or be a fundamentalist developer idiot).

They can't justify the filesize (even though it is probably less than script which doesn't utilise the abstractions provided).
They don't want to rely on third party tools.
Their business does not want to run any libraries (for whatever reasons).
Their business does not want to run any JavaScript code not written by their employees.

Learning: To actually code everything, and learn more. (Rather than using pre-coded stuff)
Size: jQuery has TONS of features you might not need, why make the user download so much code if it's not going to be used?
Alternatives: at this point, there are dozens of more powerful, well structured web frameworks out there.
Flexibility: Although jQuery is really flexible, you might need something it doesn't provide.

By all means, I like jQuery, but there are some reasons not to use jQuery:
Download size/bandwidth: jQuery 1.5 is now over 200K uncompressed, for some the size of the library is too much to justify the benefit.
Performance: Writing native JS will always be faster than abstracting it behind a library.
Added complexity: Many people will download the entire jQuery library when really they only need a few neat features from it.
Application Dependencies: Introducing dependencies always has its hang ups. If there is a bug in jQuery, you can debug and edit the file, but upgrading later introduces problems. You could leave the bug, but now you are dependent on jQuery's time table for a fix, not your own.

Because quite often it's just unnecessary.
if all I want to do is validate some input, or hilight some field then it's just as easy to write simple javascript / dom code. And jQuery doesn't really help in such simple cases, so the question should be why use it?
Clearly there are many cases where it is very useful, but sometimes people seem to use it for no real reason too.

I would prefer to use jquery for dom manipulation or traversing the dom , which is really easy with jquery . Moreover, attaching an event or event delegation are so easy using jquery or other framework otherwise you have to write custom event attachment for IE or non IE browsers etc.
But it has some performance penalty when you use $.each instead of vanilla JS for and array.push()...
other issues like if you bind an event and remove that without unbind it will have memory leak....
My conclusion is only use any framework for complex dom manipulation and rest use vanilla JS

Why not use jQuery?
I can't think of a good excuse to use vanilla JavaScript over jQuery (aside from the intimidation factor of learning something new), but some people prefer other JavaScript frameworks (like the excellent MooTools) due to the philosophical differences between them.
Some people simply don't like jQuery's DSL-ish syntax, but they recognize the importance of using a robust JavaScript framework.
Personally, I love jQuery, but I know people who use other frameworks and are no less productive.

What are pros and cons when choosing jquery as my primary javascript library? [closed]

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Closed 10 years ago.
I have started a new web application and have decided to use jquery as my primary javascript library... But still i want to get some pros and cons tips from SO users who used jquery ...

Pro's
Eliminates a lot of cross browser javascript issues
Can perform complicated Javascript operations in little code
Can easily add Ajax functionality to app
Has built in UI and effects libraries
Con's
Overhead of adding extra javascript to page
Learning curve may not be short for some developers

All pros, no cons.
The only thing will be the execution time overhead that jQuery adds to load its files and execute its functions, but compared to the Pro of coding cross browser compatible JavaScript it is nothing.
Also compared to other JavaScript libraries jQuery is one of the fastest and smallest out there. Their community is huge and you can easily find support and good documentation.

Pros:
Large community
Useful for selecting and iterating over sets of DOM nodes
Eliminates some inconsistencies between browsers
Makes it easy for inexperienced JavaScript developers to do relatively complex scripting tasks
Cons:
Regular updates that change existing behaviour
Some confusing fundamental API methods (e.g. attr() method: what exactly does it do? It certainly blurs the line between attributes and properties)
Encourages "chaining", which leads to code that is difficult to debug: in a long chain of jQuery method calls, there's nowhere to put logging statements, so you need a full-on debugger
Short list of supported browsers
A general point about general purpose libraries: they encourage developers not to attempt to understand the underlying problems that the libraries try to solve, leading to a tendency to think of the library as being magical and to use it for absolutely everything
Adds 70K of JavaScript to every page where it's used when often a few bytes of non-jQuery code would do
Performance is generally much slower than the equivalent (well thought-through) non-jQuery code
Quality of plug-ins is very variable

You've made a good choice, don't worry. jQuery is a very well designed library - it is powerful, clean, well documented, and extremely popular. Learn it well, and it will be a powerful tool in your arsenal.
That said, I think that to be really aware of its pros and cons versus other frameworks, you should first learn it well. Only a deeper understanding of a tool enables you real comparison with other tools.

The only con I can think of is the occasional memory leaks. It is a great framework on top of javascript and is not restrictive.
That said, jQuery UI is terrible to my taste and if your application requires highly interactive UI try Ext JS.

The pros and cons are really relative to what your new web application will be doing. If it is going to be JavaScript/Ajax-rich and requires cross-browser support then jQuery is the way to go - no doubt. However, if you are only going to use JavaScript very sparingly, then it may be over-kill to include a large-ish framework, and it would be more efficient (in terms of performance and page size) to code the JavaScript directly.

Pros: Lightweight, easy to use, good documentation, gets rid of nearly all cross-browser issues and normalizes the event model.
Cons: jQuery UI doesn't have much to offer, and the plugins are hit or miss.
If you're doing a very JavaScript-heavy Rich Internet Application, go with YUI or ExtJS. jQuery is an excellent DOM manipulation library, and is the best for highly custom UI work. But if you need lots of stock UI and a robust data management system behind it, you need a bigger framework to tie it all together.

jQuery vs. javascript? [closed]

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I recently stumbled upon some javascript forums (sadly, link is lost somewhere in the universe), where you could feel real hate against jQuery for not being... any good?
Most arguments seem to make sense actually.
Now, I really like jQuery, mostly for letting me concentrate on things I want to do rather on browser inconsistencies and it actually makes AJAXing with cool (or overused?) effects fun.
But if really is something rotten in the core of jQuery, I don't want to rely on it the way I actually... rely on it.
I don't want to start yet another argument over which framework is the best... but... Which framework is the best (joke)? As a case usage, think about small to medium web and it's administration.
I'm just trying to figure out, if stuff in some framework or pure javascript with few mine functions really makes difference.
Edit:
I actually tried to have a normal objective discusssion over pros and cons of:
Using a framework over pure javascript and
jQuery vs. others,
Since jQuery seems to be easiest to work with with the quickest learning curve. However, some people just don't understand it and think that I'm starting yet another flame (what I am not). I am actually voting to reopen this question.
Also I'm really interested in:
Does jQuery heavily rely on browser sniffing? Could that be a potential problem in the future? Why?
I found plenty JS-selector engines, are there any AJAX and FX libraries?
Is there any reason (besides browser sniffing and personal "hate" against John Resig) why jQuery is wrong?
jQuery actually, as most used, stands for other frameworks also.

It's all about performance and development speed. Of course, if you are a good programmer and design something that is really tailored to your needs, you might achieve better performance than if you had used a Javascript framework. But do you have the time to do it all by yourself?
My personal opinion is that Javascript is incredibly useful and overused, but that if you really need it, a framework is the way to go.
Now comes the choice of the framework. For what benchmarks are worth, you can find one at http://ejohn.org/files/142/ . It also depends on which plugins are available and what you intend to do with them. I started using jQuery because it seemed to be maintained and well featured, even though it wasn't the fastest at that moment. I do not regret it but I didn't test anything else since then.

Personally i think you should learn the hard way first. It will make you a better programmer and you will be able to solve that one of a kind issue when it comes up. After you can do it with pure JavaScript then using jQuery to speed up development is just an added bonus.
If you can do it the hard way then you can do it the easy way, it doesn't work the other way around. That applies to any programming paradigm.

jQuery like any other good JavaScript frameworks supplies you with functionality independent of browser platform wrapping all the intricacies, which you may not care about or don't want to care about.
I think using a framework is better instead of using pure JavaScript and doing all the stuff from scratch, unless you usage is very limited.
I definitely recommend jQuery!

"I actually tried to had a normal objective discusssion over pros and
cons of 1., using framework over pure javascript and 2., jquery vs.
others, since jQuery seems to be easiest to work with with quickest
learning curve."
Using any framework because you don't want to actually learn the underlying language is absolutely wrong not only for JavaScript, but for any other programming language.
"Is there any reason (besides browser sniffing and personal "hate"
against John Resig) why jQuery is wrong?"
Most of the hate agains it comes from the exaggerated fanboyism which pollutes forums with "use jQuery" as an answer for every single JavaScript question and the overuse which produces code in which simple statements such as declaring a variable are done through library calls.
Nevertheless, there are also some legit technical issues such as the shared guilt in producing illegible code and overhead. Of course those two are aggravated by the lack of developer proficiency rather than the library itself.

Does jQuery heavily rely on browser sniffing? Could be that potential problem in future?
Why?
No - there is the $.browser method, but it's deprecated and isn't used in the core.
I found plenty JS-selector engines, are there any AJAX and FX libraries?
Loads. jQuery is often chosen because it does AJAX and animations well, and is easily extensible. jQuery doesn't use it's own selector engine, it uses Sizzle, an incredibly fast selector engine.
Is there any reason (besides browser sniffing and personal "hate" against John Resig) why jQuery is wrong?
No - it's quick, relatively small and easy to extend.
For me personally it's nice to know that as browsers include more stuff (classlist API for example) that jQuery will update to include it, meaning that my code runs as fast as possible all the time.
Read through the source if you are interested, http://code.jquery.com/jquery-1.4.3.js - you'll see that features are added based on the best case first, and gradually backported to legacy browsers - for example, a section of the parseJSON method from 1.4.3:
return window.JSON && window.JSON.parse ?
window.JSON.parse( data ) :
(new Function("return " + data))();
As you can see, if window.JSON exists, the browser uses the native JSON parser, if not, then it avoids using eval (because otherwise minfiers won't minify this bit) and sets up a function that returns the data. This idea of assuming modern techniques first, then degrading to older methods is used throughout meaning that new browsers get to use all the whizz bang features without sacrificing legacy compatibility.

Jquery VS javascript, I am completely against the OP in this question. Comparison happens with two similar things, not in such case.
Jquery is Javascript. A javascript library to reduce vague coding, collection commonly used javascript functions which has proven to help in efficient and fast coding.
Javascript is the source, the actual scripts that browser responds to.

What are the arguments against using a JavaScript Framework for a Web site development company? [closed]

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Our company builds websites and web applications. We are a small firm and our team of developers are always building the javascript functions from scratch or copying from other websites built by us. Every time I bring to the table the word standardization and using a JS framework like JQuery, Prototype or any other, I am told Frameworks have the three points below as arguments against them:
Mainly for people that don't know enough JS
Frameworks limit Javascript developers
Frameworks bloat the actual development code with a lot of things that are not used.
We don't use enough Javascript in our applications for us to need JS framework
In my mind it seems that Frameworks, give our team a good starting point, documentation, a community and always the option to grow on top of the framework. Could some Framework users elaborate further?
EDIT 1:
Thanks to all of you for your great responses. I really did not think that this was going to be such a hot topic. I am glad I asked the question. I posted another similar question in the following link in case you might think you want to add something. The topic of the new question is CSS related. Thanks.

By your coworkers point of view, .NET and JAVA are for people who don't know enough assembly.
Frameworks exist for a reason. They allow you go focus on the problem instead of dealing with repetitive code. They allow you to be confident (assuming you use well tested frameworks) that certain pieces of your code are reliable and well tested.
If your coworkers are against frameworks, I would seriously consider moving on.

Since no one has mentioned it - a Javascript framework rapidly becomes one more project dependancy, and in general terms, dependencies are bad as they represent points of failure.
As for this:
Mainly for people that don't know
enough JS
Without elaborating, I will say that if one of our team said something like that in my presence, I would try to shrug it off as a joke. If I thought they were being serious, I would probably have to kill them.
And as for this:
Frameworks limit Javascript
developers
That could translate to "Frameworks make it marginally harder to write spaghetti code, and that's what I do best"
Those are not arguments, they are excuses.

Arguments against:
Frameworks prevent you from re-inventing the wheel
Frameworks generally contain well tested code
Frameworks are well supported by the community
Frameworks force you to focus on the business problem you're trying to solve
</sarcasm>
Frameworks may have a license you don't agree/can't work with

A few positives for javascript frameworks (like JQuery).
They provide standardization in ui
elements.
Reduce time to develop complex
interfaces and effects.
Normalize efforts by providing
functions that are already
cross-browser compatible.
Due to efforts in cross
compatibility documentation is more
useful in a framework as you can use
the framework's api as canon
instead of searching for obscure
support for various/proprietary
javascript functions.
Reduced learning curve for new
developers making them productive on
your software quicker.
I completely disagree that a framework limits javascript developers. Quite the opposite actually. Most frameworks provide extensive plug-in mechanisms where the framework can be extended using raw javascript utilizing hooks in the framework itself.

I'll use jQuery as an example, but what I'm saying here could apply to most JavaScript frameworks.
Many frameworks (notably jQuery) are far too monolithic and not modular enough.
While depending on well-tested 3rd party software is often more than justified, "frameworks" tend to give you a lot more functionality than you need at the moment.
In many projects, I very much like the convenience that jQuery gives me for selecting sets of elements (using $(".classname"), for example). But, if I'm not using any significant amount of AJAX, I don't need the AJAX utilities provided by jQuery.
Software should do one thing and do it well, and software written in JavaScript is no exception. Most of the frameworks you refer to try to do everything, resulting in unnecessary complexity.
One place this can bite you is when you're considering upgrading to the next version of the framework. That involves crawling through jQuery's changelogs for backwards-incompatible changes and searching your project for areas where that code is used. This can be quite a nightmare, especially if you don't necessarily have a comprehensive list of which jQuery features you use and which ones you don't.
Also, jQuery (and other frameworks) tends to cause developers to start depending on new features of jQuery without even thinking about it, making it harder to determine which features of jQuery your project uses and which it doesn't.
If you use a utility which does one thing, then you know exactly which features of that utility you're using. There's only one. (If you aren't using that utility at all, it's easy to determine. Such a determination would mean you could safely remove it from your project.)
I'm all for using well-tested 3rd party code. But if it tries to do too much, (that is, if it's a framework rather than a utility), you should probably look for an alternative. If it tries to do too much (like jQuery tries to do too much), then it's got some serious, foundational design flaws that will probably come back to bite you.

I'm surprised no one has already mentioned it:
A lot of web developers default to using JQuery without considering the alternatives
And end up including it on a web page to do a few trivial tasks which could easily be done in pure JavaScript
The result is that users have to wait for the whole library to download and it slows down web browsing
Also:
Some web developers get carried away with the design of web pages, and end up developing unnecessarily complex web pages because of the power of JQuery
Just because JQuery enables you to create scripts with good cross-browser compatibility it doesn't mean that the end result is usable on different devices / interfaces
I'd also argue the cross-browser compatibilty because I've seen instances of webkit not playing well with JQuery
JQuery encourages "fast" scripting - but if you rush it you are likely to have missed something out
Writing in JavaScript from scratch is slower - but I believe that you end up with a more complete solution which more closely matches the users needs
Using JQuery can shift the focus of the web developer to creating web sites which are highly graphical and visually appealing, whereas the focus should be on functionality and usability
JQuery is not a silver bullet for web development
I am biased here because I don't use JQuery, but it is because I haven't found a need for it yet - maybe it's because I focus more on usability and functionality rather than making the user interface look pretty (sorry I know JQuery can do more than that).

An argument against libraries is BROWSER SUPPORT most libraries support only a subset of browsers out there .
Here is an example of BBC rolling out their own instead of using something like jquery .

I liked the answer of pb +
Mainly for people that don't know
enough JS
I believe it is too complicated for them, so they use this excuse. FW allows you to build much more complex applications.
Frameworks limit Javascript developers
bullshit
Frameworks bloat the actual
development code with a lot of things
that are not used.
what is it today extra 100k-200k? especially if you use the CDN versions (at google for instance). And this is assuming you use nothing in the FW.

There are plenty of good reasons to be suspicious of frameworks in general, balanced of course by lots of reasons why they are worthwhile.
I use jquery now, and frankly within an hour of learning it realised that it fits the job so well that if it didn't exist I'd only end up reimplementing something very similar myself, only it wouldn't be as good or as cross platform.
There isn't much bloat there, it's very small and well designed and does nothing at all that stops you writing any javascript you want for specific cases that don't fit your needs.

Why would I want to use jQuery? [closed]

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(I understand that someone else asked a similar question and it was closed as 'argumentative', but I'm really interested in understanding the arguments around this.)
I know JavaScript really well. I've been writing it professionally for years. I've internalized a lot of the cross-browser incompatibilities and sketchiness, know DOM manipulation like the back of my hand, have worked with some of the best web developers in the industry & picked up a lot of their mojo.
I've been checking out jQuery. I understand the point of a javascript library (how many times have I written animation, getElementsByClass, and hide/show functions?). But to be honest, it seems like a waste of time to learn an entirely new syntax that isn't less complex. It seems like I'd be bashing my head against a wall to learn an entirely new interface to the same old JavaScript.
I'm not technically an engineer, so maybe I'm missing something. Could someone spell out the tradeoffs of jQuery? Is it really faster to learn and understand jQuery syntax than to just learn JavaScript?

There are a few big benefits to using a framework over homegrown/handwritten code:
Abstractions. I'm sure you're very proud of the fact that you've slung enough JS to be able to write animations from scratch. You should be! However, abstracting common functionality away from yourself is actually very liberating. You just call the method and know the innards will be executed, and executed well. Even if you're very fast at writing the low-level stuff yourself, that's still time you've spent on that instead of solving today's problems.
Common language. Using a common framework is like speaking a common language. You can collaborate with other developers very easily, and each can pick up where others left off without friction. (Compared to stepping into an application which uses a homegrown library for the things jQuery can do.)
Experts. The people working on jQuery are JavaScript gods. I am really, really good at JavaScript, and you probably are too, but there's a big difference between normal good and jQuery good. A team of insanely good people are constantly poring over a small set of common functionality - tuning it, tweaking it, enhancing it to make it the best it can possibly be. That represents a huge number of man-hours a single person like you or me simply cannot reproduce, no matter how good we are. And if you are as good as the jQuery guys, you can only benefit by combining your talent with theirs and contributing to the jQuery codebase. It's a melting pot of insane talent.

I'm not technically an engineer, so maybe I'm missing something. Could someone spell out the tradeoffs of jQuery? Is it really faster to learn and understand jQuery syntax than to just learn JavaScript?
jQuery is more than "just another interface" to Javascript. It allows you to express yourself in ways that are more compact and succincter than the corresponding Javascript implementation. At the same time, it's clearer and much more powerful. Specifically, the benefits you get include:
Expressiveness. jQuery is essentially a DSL for DOM manipulation and querying. This specificity is a major source of its utility and effectiveness.
Cross-browser. To a very large extent, jQuery is "write once, run anywhere". Even in 2009, this is still a surprisingly rare feat for web-based platforms. It's gratifying and relieving to know that you won't have to waste time debugging obscure problems on IE6 (well, most of the time).
Highly complete documentation. As a developer, I prize APIs and frameworks that have taken the time to spell out what all the moving pieces are supposed to be doing. Nothing is more encouraging than knowing that (1) things are stable enough that writing documentation isn't an attempt to hit a moving target and (2) things are useful enough that they've attracted enough people to flesh out the documentation fully.
In sum, the difference between Javascript and jQuery is analogous to the difference between assembly and higher-order programming languages. Although they're both technically both "an interface to machine language" and can both do similar things, the latter is considered far more powerful because of how easy it is to represent things and develop rapidly.

One thing I don't see mentioned is that the library is written to work cross browser on a wide range of popular browsers and platforms: IE6+, Firefox 2+, Safari 3+
That alone is reason enough to use jQuery then to write your own JavaScript and have to worry about cross browser issues yourself.

You can't learn jQuery without learning JavaScript, and you can't be a jQuery guru without being a JavaScript guru.
That said, it really is much faster to do things with jQuery than with "bare metal" JavaScript. Moreover, the way one works with jQuery is at a far more abstract level than the way one works with "bare metal" JavaScript. In addition, the jQuery syntax is very basic and not at all hard to learn, although the way you think about jQuery is very different from the way you think about "bare metal" JavaScript but enables you do to much more, much more rapidly.

community support
other developers writting and testing code, make it possible for you to do things you simply do not have time to do. Its not about doing anything you do not know how to do, its about doing things quickly, efficiently, and of very high quality (its already been tested).

From the sounds of it, you know much of the same stuff that jQuery knows about the DOM, and I can only assume you've built-up a nice DOM toolset for yourself over the years that incorporates all of this knowledge. In short, No, you don't need to use jQuery. However, some of the rest of us do not know as much about the DOM, and jQuery levels the playing field so we can get on with getting work done for our clients.
That is not to say you should not learn jQuery (as opposed to using it). You may pickup a few things you didn't know. The 3 main features that distinguish it from other DOM libraries (including your own probably) are:
built-in support for CSS selector syntax, useful for finding elements
methods that operate on the set of wrapped elements
chaining. Every method in jQuery.prototype (with "setter" behaviour) returns this
Personally, 1) and 3) don't hold a high premium. But 2) turns out to be huge for me. I never realized until jQuery how much my code tended towards operating on a set of nodes rather than a single node (if someone would have asked me I would have guessed the opposite tendency). 2) virtually eliminated all for loops from my code (or .forEach or however else I tended to abstract it), which was actually quite liberating. Now I notice it's the rare occasion where my code must operate on some single element.

Speed of development and turnaround of code.

One thing I haven't seen mentioned in the other answers is that, eventually, someone else is going to have to maintain your code. If it's all custom, they'll have a rougher time of it than if a more standard library is used. If that's not of any concern to you, and you don't think you're likely to need any of the really slick jQuery modules, then keep on with what you're doing.

For me, there are several benefits, like speed of development, etc.
But at the end of the day, it boils down to one thing:
It removes a HUGE amount of the cross-browser BS that can eat up so much time and resources, so I don't have to deal with it

jQuery allows you to write shorter, cleaner code that's easy to understand. Many people use it, so there are many plugins that work along with it, minimizing bloat. The same may not be true of your own personal library.

If you know JavaScript really well you should also know that jQuery's syntax isn't different from JavaScript's syntax.
What you might be referring to is the chaining idiom (used everywhere in jQuery) which seems to make things look very different from other sort of code. Well there's a reason for that, it's because it bonds really well with how the DOM works.

If you truly know JavaScript, you'll be at running speed with jQuery in under a week's time. If you don't already know your way around JavaScript, adding a layer on top might do more harm than good. But that's just like anything else!
You're going to be writing better looking and easier to understand JavaScript at a faster pace and only once — not once per browser. Downside? You're going to tack on another ~50KB of JavaScript. That's what caching is for :)

Jquery is javaScript in the hand of the designer.
It is as easy as writing CSS styles on an element.
Also Jquery is the easiest to understand, write maintain, add plugins.

I was working on a gallery program for a client (which is now exhibiting inexplicable behavior in IE6 and 7 - surprise, surprise - but I found that when I switched from bare-metal Javascript to jQuery a lot of the work got much easier. To me, it makes available the CSS view of the DOM while writing Javascript - which makes traversal and manipulation much more intuitive. Also, the cross-browser compatibility and brevity is wonderful.

It doesn't really matter if jQuery is great or awful. It's the powerhouse JS library now, used on a huge number of pages, and future browsers will have to accommodate it or be seen as themselves buggy.
What I don't like about jQuery: I'm not that thrilled about how it ignores mobile phone browsers in its test suites. It gets pulled in on all kinds of pages on mobile phones, and yet it makes no effort to ensure it works well on them.

jQuery degrades gracefully and it's one of the fastest (not THE fastest anymore though):
http://mootools.net/slickspeed/
There are many other frameworks out there, and whichever you choose, you probably wouldn't go wrong, but having uses jQuery myself, I'd definitely recommend it.
:)