Develop Reference

What is the utility of a jQuery object or most common methods associated? [duplicate]

Closed. This question is opinion-based. It is not currently accepting answers.
Want to improve this question? Update the question so it can be answered with facts and citations by editing this post.
Closed 5 years ago.
Improve this question
I'm a student and also new to JQuery. I want to know the difference between JQuery vs JScript. How we decide what to use and when to use those two technology.
Considering
Performance
Technology

jQuery is JavaScript. It is a JavaScript library, so it operates on top of JavaScript. It cannot exist on its own, so you can't use one over the other. You can use just JavaScript or JavaScript and jQuery.
jQuery is designed to make many JavaScript development tasks much easier.
Use jQuery when it will significantly reduce your development time, and you can afford the extra overhead of downloading the library.
A lot of people automatically include jQuery without considering the fact that it might not make the particular development task at hand much easier. This is a bad habit to get into...
Personally my most common uses are:
Complex element selection
Animation
Event handling
There is a humorous Photoshopped screen shot of a "Stack Overflow" post that pokes some fun at the overuse of jQuery. Enjoy:

I was initially against jQuery and, for new programmers, I still am. It is a pure JavaScript framework that has several benefits; here are some of the major ones:
crossbrowser support
easy element selection
customizable plugins
large support community
very popular
Some would argue that it is large, which could affect your web performance... and they would be right. However, because of its popularity, it is likely to have already been used on some site you've visited before. Most browsers these days are good about caching scripts/images, so that download hit is reduced over time.
On the other hand, many new programmers that rely on the framework become very dependent on it, so much so that they lose their ability to evaluate the best tool for the job, and use the most familiar tool that they're used to. These same programmers lose their ability to properly debug and become reliant on an API that has no standard.
What I like using it for is it's cross browser support, which is great for event handling; and whenever you apply dynamic styling to elements on the page (or really anything that requires you needing a more advanced set of elements). Element selection is it's #1 tool, where you can select elements by id, type, attribute, classname, tagname, hierarchical relationship,... all sorts of ways.

I want to know the difference between JQuery vs JScript.
JavaScript is a scripting language for the web. Microsoft refers to their implementation of it as "JScript" but in terms of syntax it's pretty much the same.
jQuery is a software library, written in JavaScript, whose intention is to help JavaScript developers when writing code that is to be run in a web page.
How we decide what to use and when to use those two technology.
JavaScript (including jQuery) can be used to add interactivity to a web page beyond that which is possible merely with HTML and CSS. This allows for a more "application-like" experience for the user. Many see it as a bad idea to make a web page which depends on JavaScript, and won't run without it, and insist that JavaScript should only enhance the user experience. However, websites which won't work without JavaScript do exist and are becoming more common.
jQuery runs in JavaScript, and is written in JavaScript. If you are writing JavaScript for a web page, it is often a good idea to also use a library (or framework) like jQuery to make the task of browser-compatibility that much easier. It is like a pre-packaged set of JavaScript routines that you may have otherwise needed to write yourself, packaged in an easy-to-use way.
A drawback of using jQuery is that it is a relatively large file size, which does matter on the web. Some would argue that if you are not using enough of jQuery to justify its file size on your site, you should consider something else (such as a more modular framework, or not using a library/framework at all).

It's perhaps a little redundant now but 'JScript' was originally the name of the Microsoft implementation of Javascript in old versions of Internet Explorer. Thankfully it's long since disappeared although if you find references to it in older literature just replace with 'Javascript'.
Back to the questions, if you're new to this, I'd strongly recommend going for JQuery (or another Javascript library) instead of raw Javascript. You can do very complex things with one line of JQuery that would take a lot of coding and debugging if you were to use raw Javascript. Ultimately you will need an understanding of the underlying language to take full advantage of JQuery but if you want something you can deploy today, start with JQuery rather than Javascript.

I have been recently struggling with the same question, jQuery or JavaScript. Coming from a coding background that includes several old school programming languages (C, perl, ksh, and more recently Java and C#), I am finding that JavaScript is MUCH easier to quickly understand than jQuery. From a support perspective, the ease of understanding the code is more important than less typing when creating the code. The one answer here, that does have me reconsidering the use of jQuery, is browser compatibility. This one significantly impacts future application support. Based on my experience (over 20 years as a software engineer) the application life cycle spends a lot more time in support than it does in development.

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

As it currently stands, this question is not a good fit for our Q&A format. We expect answers to be supported by facts, references, or expertise, but this question will likely solicit debate, arguments, polling, or extended discussion. If you feel that this question can be improved and possibly reopened, visit the help center for guidance.
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.

Can I be an expert on jQuery without knowing a lot about Javascript? [closed]

As it currently stands, this question is not a good fit for our Q&A format. We expect answers to be supported by facts, references, or expertise, but this question will likely solicit debate, arguments, polling, or extended discussion. If you feel that this question can be improved and possibly reopened, visit the help center for guidance.
Closed 10 years ago.
I'm a backend programmer and I wonder about one thing. I know just a little about Javascript. I know the basics when I look at some Javascript code, I understand what code is trying to do. But I have never used it to create animations.
Then I heard about jQuery. Can I be an expert on jQuery without knowing a lot about Javascript?
And how about Ajax? Can I do Ajax animations (search suggestions) just with jQuery and don't have to learn Javascript?

JQuery provides a framework that allows you to create a lot of great features relatively simply but it's still javascript and doesn't release you from the need to have at least a reasonable understanding of javascript to use it.
Javascript is one of those misunderstood languages because it's a scripting language, it's a very powerful and flexible language... learn it and you'll be grateful you did.

Javascript, jQuery and Ajax - misconceptions
Javascript is a language. All modern desktop web browsers support Javascript. But their support differs. That's where jQuery comes in. It's built on top of Javascript and its functions are written in a way so they work with most of these browsers in exactly the same way. So developers don't have to write all-browser-supported javascript code by themselves.
Nontheless, jQuery is still build on top of Javascript. jQuery is a Javascript library. And when you use jQuery, you still write Javascript code. Think of it as additional functions to the existing Javascript functionality. jQuery calls work the same on all browsers and you still use Javascript.
What about Ajax. There's no thing like Ajax animations. There are Javascript/DOM/jQuery animations. They may be triggered by Ajax calls/responses, but they are not Ajax animations. Ajax in itself is a communication facade between your client side (Javascript code) and server side (whatever platform and language is used there).
Animations are done by manipulating HTML DOM elements using Javascript + time. Libraries like jQuery provide the functionality to do this kind of stuff the easy way. Again to make it easier on you. So you don't have to do it by yourself.
Answer to your question
So if you want to develop client apps using jQuery it's imperative you become familiar with Javascript. And HTML. And DOM. Ajax is not mandatory. To become really good with jQuery one has to become really profficient in Javascript. Because it's the client language you'll build your app. Using upgrades in the form of jQuery library. jQuery itself uses rather complex parts of Javascript that you'll have to understand to some extent to be able to debug problematic situations in your app.

That's a bit like asking if you can use ASP.NET without learning C# or VB, or if you can use the java.math library without learning Java. jQuery is a library to make make JavaScript programming easier. You still have to write JavaScript to use it.

Ajax and animation are two different things. AJAX (Asynchronous Javascript And XML) is a communication protocol. Animation is the art of making still things appear to move. ;-) Jquery and its associated plugins, Dojo, Script.a.licious and other such libraries include built in animations that you can plug in without understanding the underlying javascript -- however, as everyone else here has said, understanding javascript will make your coding experience much more pleasant and effective.
Don't forget that there are a very large number of answers already available on Stack Overflow that will help you become acquainted with the language (not to mention a large number of experts willing to give even more help.)
I'd recommend starting with some of these questions:
The Hidden Features of Javascript
How Does Javascript's Prototype work
Object Construction with Prototyping

i am a backend programmer
So there is your answer: no, because jQuery is javascript for designers.
edit whoever voted this down really is too serious in life :-)

No
While you can certainly use jQuery without knowing JavaScript, there's absolutely no way to become an "expert" at jQuery without knowing JavaScript.
You'll be fine without a JavaScript background if you just need basic animations, event handlers, and DOM traversal. But being an "expert" at jQuery presumably requires more than that, in which case you'll need to learn JavaScript in order to write clean, elegant, and maintainable jQuery code.

Learning the ins and outs of Javascript will make your jQuery experience much more pleasureable. Specifically I would recommend looking at Javascript's unusual model for OOP, closures, scoping, namespacing - that sort of thing. It will make life much easier for you.

jQuery is a JavaScript library the way stdlib is a C library, so yes you'll need to know minimal JavaScript as well as basic understanding of CSS/HTML.
However, learning jQuery is probably the best way to learn JavaScript these days, as it takes care of the big hurdles that trip up those new to JavaScript (the DOM and the event model) and makes AJAX a breeze. It also encourages best practices such as unobtrusive JavaScript, as opposed to putting "onclick" everywhere, and forces you to learn about closures. If you're already a programmer, you'll have no problem picking it up.

Yes you can but you will not be able to do complicated stuff which will use pure javascript, because jquery is not everything.
But with the help of the internet and seeing examples, you can do alot.

jQuery in its core is javascript. learning javascript would be good but with jQuery it is not really required.

You need to learn JavaScript, but you don't need to learn the DOM or raw XHR. This is what jQuery saves you from.

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

Closed. This question is opinion-based. It is not currently accepting answers.
Want to improve this question? Update the question so it can be answered with facts and citations by editing this post.
Closed 3 years ago.
Improve this question
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]

Closed. This question is opinion-based. It is not currently accepting answers.
Want to improve this question? Update the question so it can be answered with facts and citations by editing this post.
Closed 2 years ago.
Improve this question
(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.
:)