Develop Reference

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.

jQuery vs. javascript? [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 9 years ago.
Improve this question
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.

Does it make sense for software developer to use jQuery when learning Javascript?

It's often claimed that learning a lower-level language is a good foundation for any new developer. What about an experienced developer (say a C++ or Java guy) learning Javascript? in the same way should he learn using the raw language so he understands what's going on, and learn JQuery later, or use JQuery from the start as 'part of' Javascript?

I think it's worth spending a bit of time working on JavaScript without jQuery so you can appreciate just what jQuery brings you. But most of what you'll be learning in JavaScript (getElementById etc.) will be replaced by better jQuery functions, so don't spend too long in pure JavaScript.
I guess the important thing to realise is that a lot of what you do in JavaScript is actually just working with the DOM API. The JavaScript language is great, but the DOM API is RUBBISH. jQuery doesn't really abstract the language, just the API. So the distinction isn't between using JavaScript or using jQuery, it's more about using the DOM API vs using jQuery.

It's always good to understand the basics of 'raw' JavaScript. I would recommend learning basic JacvaScript first, then jQuery. It'll make jQuery easier once you know the basics of the language, and there are still quite a few scenarios where it's preferable to do parts of a code in regular JavaScript than the jQuery way.
You could still try to learn about both in parallel, but either way it is still important to understand JavaScript to really understand appreciate jQuery.

I am pretty new to JavaScript myself, and I had a few months to learn raw JavaScript before finding out about jQuery. I agree with GSto that learning raw is good, but don't spend to much time in it before looking at a framework.
Apart from that, I strongly recommend anyone building in Javascript to pick up a copy of Douglas Crockford's JavaScript: The Good Parts. Read it back to back and then keep it close to your computer!
JavaScript differs from C-like languages quite dramatically at some points...
EDIT: Crockford held a great talk at Øredev 2009. They have the whole talk in video on their site.

Learn Javascript first and about DOM, object literals, closures and currying. When you master these things you are ready to use a library.

If you want to be a good web-developer who knows javascript, then do not touch jquery for at least year. It will corrupt you. Seriously, I've seen too many people who claim that they know javascript but are unable to iterate through an array without using jquery.

I think you should learn raw language before jquery or another framework. If you do, cou can learn jQuery basis wery quickly.

I learned JavaScript before I did anything with jQuery and I would recommend the same to any one thinking to learn JavaScript, as this would allow you to move to another library or use raw JavaScript if you found that jQuery wasn't really suited to what you need or if you simply didn't like jQuery.
There may be some cases in which using raw JavaScript is better suited to what you want to do.

It's very important to learn the language itself, and somewhat important to become familiar with the issues faced by things like jQuery in making the programming environment more uniform across different browsers. However, except as exercises for people who have the time, attempting to re-implement the facilities that modern frameworks provide is a pretty bad idea.
A benefit of learning a modern framework is that they generally encourage the exploitation of the native power of Javascript, and avoid trying to make the language look and act like something it isn't. You have to develop a good feeling for what anonymous functions are and how they work, for example, in order to really use Prototype or jQuery or just about any other framework effectively.
Finally, reading the source code of a good modern framework is wonderfully enlightening.

Think of JQuery as being an abstraction that just happens to be built using Javascript.
Most things that you need to do can be done elegantly with JQuery.
To use JQuery effectively it is more important to understand the DOM, Events and CSS.
It helps me to think of JQuery and Javascript as being different paradigms even though we all know that JQuery is written in JS.
I came to Javascript from a C/C++ background and it took me a while to really get to grips with the object model first-class functions, inner functions and closures and prototypes.
JQuery is much clearer, easier to follow, and more powerful once you understand what it is that you are manipulation (i.e. DOM, Events, CSS).
So I'd learn JQuery first and fill in the Javascript gaps as they appear.

Don't forget that document.getElementById is still the fastest way of getting elements. So if you do simple stuff and that's sufficient so why use a library?

Read what Joel Spolsky says about abstractions. What happens when your abstraction layer leaks? Do you know why the problem happened, or what you can do to fix it?

I would start learning the javascript syntax and usage first, but don't focus too much on the different functions, as a lot of them will have better jQuery alternatives.

The answer - both.
If you are working on something professionally and have a deadline... JQuery. Then go home and learn the internals. JQuery is very powerful and there is no need to reinvent the wheel. Sometimes the best code you write... is no code at all.
Now that being said, its always a good idea to understand your code and the "black majic" that happens under the hood. This helps to decide which JQuery method or selector to use for the best job... measure the pros vs cons.

You should do both at the same time. Learn javascript loops, types, prototypes and just forget the DOM methods because jQuery is more elegant for that.

You should learn basic js first, variables, loops, functions, closures, inheritance, and js design patterns. If you're already a programmer, you can skip a lot of stuff. jQuery is not a language. Once you know that, then jquery is a tool, that mostly helps you abstract differences between browsers (and provides another dialect of js). I don't use jquery, I use ext-js (another tool, another dialect), so I would caution strongly against learning jquery as if it were a language.

If you're going to work with the DOM (as admittedly most do, since most Javascript is used for client-side web development), then JQuery will be helpful.
However, don't confuse it with the language Javascript - JQuery is just another library for DOM access, albeit a very ubiquitous and imho great one.
I'd say it's somewhat analogous to asking if it's wrong to use Win32 or some other platform API while learning C++.

jQuery is just one of several libraries out there. What are you going to do if you don't use jQuery or the company you work for doesn't? Using this library does carry some weight, too. Are you going to always use it even if you only needed one minor feature?

Is it worth it to use jQuery for Ajax instead of building your own JavaScript?

Aside from the framework, is jQuery worth using rather than creating your own javascript? I've always debated if the framework was better to use than to create your own calls. Are their disadvantages of using it?
Sorry for beginner question, I'm trying to feel out if it would be better to use this and create some of the ajaxish workings of my site rather than develop it from scratch.
Are there other frameworks out there that would be better to use to create an ajaxish website?

Yes, jQuery is worth it. I speak as someone who resisted using any library for a long time, then finally saw the light.
I do recommend that you build some hand-rolled Ajax interactions before you dive into using jQuery for Ajax, so that you understand exactly what is happening with Ajax. Once that's achieved, though, let the library do the dirty work.

jQuery (and most other framework) are for making difficult things simple. It keeps you from having to write cross-browser compatabile code. It keeps you from having to write recursive methods to update multiple dom-elements. It basically cuts your development time down substantially, and saves you a lot of frustration.
Stackoverflow Archive:
Which Javascript framework (jQuery vs Dojo vs … )?
Great discussion (with lots of involvement) over various javascript frameworks. It will benefit you to browse this in depth, or even at a cursory level.
When should I use a javascript framework library?
Which Javascript Framework is the simplest and most powerful?
What JavaScript library would you choose for a new project and why?
Which Javascript Ajax Framework is most powerful and very lightweight?
Which javascript framework can be used for all browsers?

If you like reinventing the wheel, write your own. For me, I prefer to spend my time focusing on solutions rather than lines and lines of javascript code. I'll use JQuery to save resources and frustration

One thing I want to add is for the past couple of months, I was trying to fill a web developer position for my team, and it proved to be a difficult task.
Lots of folks knew how to use a JS framework (JQuery, ExtJS, YUI...), but once outside of the framework, they found themselves in an unfamiliar ground. I had a self-claimed senior JS developer with over 10 years of experience couldn't tell me how to use function.call() or function.apply() or how to implement simple inheritance in JavaScript.
Framework is great and definitely useful, but you will also want to make sure to know how to implement the basic OO concept and DOM manipulation without the framework.
BTW, we use ExtJS and if you are starting to pick a JS framework, you may want to look around to see which one suits you the most.

jQuery is a big project with many users. By that nature, it will be better tested than anything you write by yourself.
It is a great framework for most uses I've come to need.
Is there anything in particular you are trying to do?

jQuery helps you avoid worrying too much about cross-browser JavaScript issues, and helps you get things done quickly. As far as I'm concerned, the key reason for using it vs your own hand-rolled JavaScript probably is the "don't re-invent the wheel" cliché.

Apart from added productivity, JQuery's main plus is that it takes all (most) of the pain out of cross-browser testing.

There is no reason you cannot write your own code and splash in JQuery where needed. JQuery is incredibly lightweight for what it provides and you will quickly find that you can do most takes easier in JQuery (especially once you start using the plugins for it) than by hand. I am a big believer in not reinventing the wheel. The JQuery code is tested by tons of people and there are so many plugins to chose from I could almost guarantee that what I want to do has already been done before.
If you are starting with a clean site, now is the perfect time to take JQuery for a spin though, because if you do get hooked on it your code will be more consistent if you solve similar tasks using the same technique. Go give it a try :)

I would highly recommend using jQuery (or any comparable framework) over straight JavaScript. It just makes common programming tasks easier, most notably cross-browser development.

jquery makes js related work easier and more important standardised/compliant.
If you need re-assurance as to its use:
MS & Nokia are bundling it in their dev platforms.
Its compatible with many of the newest web technologies.
It has a large developer community supporting it.
There are many plug-ins available to aid RAD.
and more...
Give it a go!!

Definutly! Jquery wil save you a lot of time struggling over code and makes your code easier to read to non-experts.

Should I learn/become proficient in Javascript?

I am a .NET webdev using ASP.NET, C# etc... I "learned" javascript in college 5+ years ago and can do basic jobs with it. But I wonder if it is useful to become proficient in it.
Why should I learn Javascript?
Is it more advantageous then learning JQuery or a different library?

Yes, definitely learn Javascript before you learn one of the libraries about. It's the whole walk-before-you-can-run thing.

Make sure you add these sites to your bookmarks:
Mozilla's developer site: This contains the reference to the Javascript API in Mozilla. This will help you make sure you're writing code that Firefox understands.
IE's site in Microsoft Developer Network: The same, for IE.
W3's reference of DOM for HTML: In most web applications today, the Javascript code manipulates the DOM, which is an internal keeping track of the objects displayed on screen (but you already knew that, right ?) This is the reference to the DOM API. It is language neutral, which means it does not target Javascript, but these methods exist in Javascript too.
Douglas Crockford' site: Doug Crockford is THE MAN when it comes down to Javascript. The articles in his page are a must read. Because Javascript has closures and first-class functions, he believes it is closer to Lisp and Scheme than to other languages. And he teaches you how to greatly improve your code with these language features.
Yahoo Developer network: You may also want to check this. I'm not a regular visitor to this site, though, so I can't really say much about it.

Yes, absolutely you should learn JavaScript if you are doing web development. I highly recommend JavaScript: The Good Parts, by Doug Crockford. And, JQuery is a great framework to use (this site uses it) -- it kind of depends on what you are trying to do -- YUI and ExtJS are also very nice.

The answer is simple.

Hands down yes. There's a reason that Google have made such a big fuss about the V8 JS engine for Chrome, why Mozilla are working on TraceMonkey for Firefox and why Webkit have been working on Squirrelfish for a while (now Squirrelfish extreme). It's because JS is becoming more popular by the day.

Javascript is one of those languages that spending a few hours learning will probably teach you 99% of what you will ever really use. I would imagine you are at the point in your learning of javascript that you know more than enough now and just learn one or more of the frameworks now.

I would recommend brushing up on your non-frameworked javascript first. Refreshing/learning basic concepts of dom manipulation and what not. Like learning how to build a linked list, stack or queue in C++ before learning how to use the STL (standard template libraries).
In addition to brushing up on straight javascript, it might be good to get into a framework that doesn't abstract and change the way things work so much, for instance Prototype. You code with it very much the same way you code with straight javascript. Read through the Prototype code, learn how to make classes, and do some fancy stuff. From experience, I can say reading through the Prototype.js helped me learn alot.
After messing around a bit, then I'd say go for jQuery. If jQuery didn't, literally, change the way you write code I'd say go for it first, but learning how to build classes and js inheritance and what not can be a very important lesson for someone who wants to become fluent in JS.

Learning javascript is recommended for any web application developer. Why?
You will better understand the possibilities, limitations and dangers related to developing a web application
It is a boost for your career, if you are working on a web application that has a user interface.
However, learning javascript is usually a trade-off between a programming language and another. You should consider whether javascript is relevant for your career or project.

Unless you want to really get into javascript, I think you'd be better off learning enough JS to leverage one of the tried and tested javascript libraries out there.

One thing nice about JavaScript is that it is quite different from mainstream languages such as C#, VB.NET or Java. Learning it, especially if you have occasions to use it, will give you another insight on programming, and that's always good. I think it's worth learning it.

If you are doing web development then at some point you are going to get exposed to Javascript or ECMAScript at some point in your career for any one of a number of reasons. At a minimum you should know enough Javascript to be able to be able to validate user input; however, the web is moving in the direction of using more an more Ajax so you should also know enough Javascript to properly leverage one of the major libraries out there such as jQuery.
As some of the other users have noted, you can learn most of what you need of Javascript on a day to day basis in a single day or a couple of afternoons. If you want to get more advanced with Javascript then you are going to have to invest much more time in learning the language but odds are that unless you seek out this type of work that you are not going to encounter something that a preexisting library doesn't already exist for.

If all you want is to do some simple UI-effects and the like, I suggest you just pick a library and go for it!
Using libraries eliminates all the flawed implementations of JavaScript and provides you with an API which is the same across all browsers. And if you're working together with others it is also a great way of implementing code-standards and best practices.

Learning a second programming language is always good.
By the sound of it, JavaScript is a language that you use, to it will be of practical use too. As a web dev, it has been recommended to me in a review that i learn at least basic JavaScript.
A library such as jQuery is essential for web development thse days, so you could learn that too.

I don't think a lot of deliberate learning makes sense (but of course you need some basic knowledge), but I also think after some years of web development you'll become pretty proficient in the language anyway :)

If you are a webdev then yes, you should be proficient with Javascript. Javascript is a major part of making web apps as interactive as desktop apps.
With that being said, learn to use one of the cross-browser compatible libraries like JQuery, Prototype, etc. We do not need to have any more single browser crud created using Javascript, just because any real man/woman rolls their own.
A few things to learn in Javascript:
1. Basic syntax
2. The various flavours of function declaration.
3. Passing functions around and how to use passed in functions.

I recommend Jeremy Keith's books: DOM Scripting and Bulletproof Ajax. After you become more fluent in JS I would recommend a JS library(I use jQuery, but that is not important).
JS is important to learn. You cannot use a framework without the proper understanding of how it works. That is doing things backwards.

i thing you should have a good knoloedge base of language specification and DOM (Document Object Model). it means,you shoud know how find/create "page object" an edit properties. Also you should have an idea of "object oriented" javascript tecniques, which the starting point of a lot of framework. you don't need learn specific framework if you don't use it. simple keep in mind generic base concept!

I'll go with the opposite answer most are putting out there. Learning javascript as a developer these days is almost pointless. The language is similar enough to java/C# that it's syntax and semantics shouldn't be lost on you.
What you should learn is jQuery.
As you use jQuery you'll pick up the most common things you'll ever need from javascript anyway.

If you're involved with the Web in anyway then the answer is "Yes, always". Maybe an embedded or system's programmer could get by without JavaScript, but not a webdev.
Most of the libraries are designed to alleviate some of the pain of interacting with a multitude of browsers. They will not abstract away core JavaScript functionality.

Yes, you should learn JavaScript. Sooner or later you will need to use it!