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is using javascript document ready twice in one document ok?

I have a header on my webpage that contains the some javascript for some jquery plugins i am using. These plugins are used on a bunch of pages so I just include them in the header document that is included in everyone one of my pages.
There is one page however that I would like to include some other javascript but is only needed on this one page. can i use the document ready function a second time on the same page or is that poor form?
I don't want to include the javascript on everypage as it is not needed and would be a waste to load on every page.

YES
The jQuery docs are clear that this is fine.

Yes, you can use it multiple times. It binds each function to the triggered event of the document being ready.

no nothing wrong it will be executed in the order it is specified
here is a demo http://jsfiddle.net/LKuz2/7/

Yes, keeping in mind that:
Your code will be less readable.
You will end up with multiple function scopes with each block. So variables you create in one closure will not be visible from other ones.
Recommendation: If you think you need multiple $(document).ready( blocks, you are probably wrong. Take time to re-work your solution to something more maintainable.
Erm, and this has been covered here before. More than once. By Me. How embarrassing :)
What are the side effects (if any) of multiple $(document).ready() in an HTML page?
Can you have multiple $(document).ready(function(){ ... }); sections?

Is it safe to substitute html <script src="xxx.js"> 'includes' with $.getScript(...)?

(I am still very new to Javascript and JQuery) I am trying to keep my code clean by creating one js file per html file. I also have a couple of common js file containing code used by page-specific js files.
I was searching for solutions to import/include other js files in a given js file and I came accross this one. It refers to $.getScript().
Instead of using multiple <script src="xxx.js"></script> as imports/includes in my html pages, could I move them to my js files and use $.getScript(...) instead? I mean, is it safe? Is it a good practice?
EDIT
Is it safe regarding cyclic references?
EDIT II
To make it more clear, let's imagine I have a1.js, a2.js and a3.js.
For a1.js:
$.getScript("a2.js");
$.getScript("a3.js");
...
For a2.js:
$.getScript("a3.js");
...
For a3.js:
$.getScript("a2.js");
...
There is a potential infinite loop between a2.js and a3.js. Should I handle it with something like this in each js file:
var vari;
if (typeof vari === 'undefined') {
vari = 1;
}

Regarding good Practice that answer is as always... it depends.
Your Page Style
Contra:
If you for example use a script to substitute some page fonts, the effect will be that the font change will be even more visible for the user.
If your script changes the height of some elements, this will be very noticeable.
PRO:
If you load a Script to handle a specific form, that the user first has to edit, there is no problem at all
If you load a Script that starts animation that can be substituted with a single loading animation you can do this.
If your page is a application with many views and models, you can use getScript like getJson, in this case your page speed will greatly improve.
Your Coding Style
Not every Page and Script is structured to be used this way. JQuery's $(document).ready() fires every registered handler once, even after the event occurred. This does not necessarily mean every handler works this way, certainly not the DOM2 Events.
If you have anywhere inline Scripts it will no longer work.
You can no longer guarantee a certain order your initialization code will have, so you can run in problems have to add more checks, that e.g. a expectedly visible container is still visible.
Whats the reward?
On high performance pages, you gain some thats clear. But script tags at the end of the page can do the same thing with half the work (mainly: remove inline scripts). In my opinion getScript is something like a last reward, you should not overuse it, because the potential to not only scare other developers but also your customers away is clearly there. I would only use it in the environment of a web application, because here are the real benefits
UPDATE response to your comment
Using getScript on your page should look like this:
//since you need it there will be some kind of wrapper
var initClosure = function() {...}
if(typeof optionalNamespace == 'undefined') {
$.getScript('/foo.js', initClosure);
} else {
initClosure();
}
All depending code is in initClosure, and you check a namespace, or variable name (even something like window['blub'] or simply blub will work). You will need this, since the on getScript depending function, wich typically sets default values or appends something to the dom should only be called once.
Nevertheless I don't really see the point in cyclic references, because this would mean:
load script 1 -> wait -> loaded ->load script 2 -> wait ->loaded -> [...] ->load script 1
This situation should be avoided for at least 2 reasons
The browser can not predict this. If there are several script tags, your browser will take care of parallel downloads, so the overall speed (simplified & rough) is the time the biggest file will need to load. In my Example it will take the sum of the script loads.
Initialization of your scripts will be handled twice, so any state will get lost.

unobtrusive Javascript, should I use it? what is the best way to manage and organize events? how do I prevent inefficiencies?

I have been struggling with choosing unobtrusive javascript over defining it within the html syntax. I want to convince my self to go the unobtrusive route, but I am having trouble getting past the issues listed below. Can you please help convince me :)
1) When you bind events unobtrusively, there is extra overhead on the client's machine to find that html element, where as when you do stuff, you don't have to iterate the DOM.
2) There is a lag between when events are bound using document.ready() (jquery) and when the page loads. This is more apparent on very large sites.
3) If you bind events (onclick etc) unobtrusively, there is no way of looking at the html code and knowing that there is an event bound to a particular class or id. This can become problematic when updating the markup and not realizing that you may be effecting javascript code. Is there a naming convention when defining css elements which are used to bind javascript events (i have seen ppl use js_className)
4) For a site, there are different pieces of javascript for different pages. For example Header.html contains a nav which triggers javascript events on all pages, where as homepage.html and searchPage.html contains elements that trigger javascript on their respective pages
sudo code example:
header.html
<script src="../myJS.js"></script>
<div>Header</div>
<ul>
<li>nav1</li><li>nav2</li>
</ul>
homepage.html
<#include header.html>
<div class="homepageDiv">some stuff</div>
searchpage.html
<#include header.html>
<div class="searchpageDiv">some other stuff</div>
myJS.js
$(document).ready(function(){
$("ul.li").bind("click",doSomething());
$(".homePageDiv").bind("click",doSomethingElse());
$(".searchPageDiv").bind("click",doSomethingSearchy());
});
In this case when you are on the searchPage it will still try to look for the "homepageDiv" which does not exist and fail. This will not effect the functionality but thats an additional unnecessary traversal. I could break this up into seperate javascript files, but then the browser has to download multiple files, and I can't just serve one file and have it cached for all pages.
What is the best way to use unobtrusive javascript so that I could easily maintain a ( pretty script heavy) website, so another developer is aware of scripts being bound to html elements when they are modifying my code. And serve the code so that the client's browser is not looking for elements which do not exist on a particular page (but may exist on others).
Thanks!

You are confused. Unobtrusive JavaScript is not just about defining event handlers in a program. It's a set of rules for writing JavaScript such that the script doesn't affect the functionality of other JavaScript on the same page. JavaScript is a dynamic language. Anyone can make changes to anything. Thus if two separate scripts on the same page both define a global variable add as follows, the last one to define it will win and affect the functionality of the first script.
// script 1
var add = function (a, b) {
return a + b;
};
// script 2
add = 5;
//script 1 again
add(2, 3); // error - add is a number, not a function
Now, to answer your question directly:
The extra overhead to find an element in JavaScript and attach an event listener to it is not a lot. You can use the new DOM method document.querySelector to find an element quickly and attach an event listener to it (it takes less than 1 ms to find the element).
If you want to attach your event listeners quickly, don't do it when your document content loads. Attach your event listeners at the end of the body section or directly after the part of your HTML code to which you wish to attach the event listener.
I don't see how altering the markup could affect the JavaScript in any manner. If you try to attach an event listener to an element that doesn't exist in JavaScript, it will silently fail or throw an exception. Either way, it really won't affect the functionality of the rest of the page. In addition, a HTML designer really doesn't need to know about the events attached any element. HTML is only supposed to be used for semantic markup; CSS is used for styling; and JavaScript is used for behavior. Don't mix up the three.
God has given us free will. Use it. JavaScript supports conditional execution. There are if statements. See if homePageDiv exists and only then attach an event listener to it.
Try:
$(document).ready(function () {
$("ul.li").bind("click",doSomething());
if (document.querySelector(".homePageDiv")) {
$(".homePageDiv").bind("click",doSomethingElse());
} else {
$(".searchPageDiv").bind("click",doSomethingSearchy());
}
});
Your question had very little to do with unobtrusive JavaScript. It showed a lack of research and understanding. Thus, I'm down voting it. Sorry.

Just because jQuery.ready() executes does not mean that the page is visible to the end user. This is a behaviour defined by browsers and these days there are really 2 events to take into consideration here as mootools puts it DomReady vs Load. When jQuery executes the ready method it's talking about the dom loading loaded however this doesn't mean the page is ready to be viewed by the user, external elements which as pictures and even style sheets etc may still be loading.
Any binding you do, even extremely inefficient ones will bind a lot faster than all the external resources being loaded by the browser so IMHO user should experience no difference between the page being displayed and functionality being made available.
As for finding binding on elements in your DOM. You are really just fearing that things will get lost. This has not really been my actual experience, more often than not in your JS you can check what page you are on and only add javascript for that page (as Aadit mentioned above). After that a quick find operation in your editor should help you find anything if stuff gets lost.
Keep in mind that under true MVC the functionality has to be separate from the presentation layer. This is exactly what OO javascript or unobtrusive javascript is about. You should be able to change your DOM without breaking the functionality of the page. Yes, if you change the css class and or element id on which you bind your JS will break, however the user will have no idea of this and the page will at least appear to work. However if this is a big concern you can use OO-Javascript and put div's or span's as placeholders in your dom and use these as markers to insert functionality or tell you that it exists, you can even use html comments. However, in my experience you know the behavior of your site and hence will always know that there is some JS there.
While I understand most of your concerns about useless traversals, I do think you are nickle and dime'ing it at this point if you are worried about 1 additional traversal. Previous to IE8 it used to be the case that traversing with the tag name and id was a lot faster than my selector but this is no longer true infact browsers have evolved to be much faster when using just the selectors:
$("a#myLink") - slowest.
$("a.myLink") - faster.
$("#Link") - fastest.
$(".myLink") - fastest.
In the link below you can see that as many as 34 thousand operations per second are being performed so I doubt speed is an issue.
You can use firebug to test the speed of each in the case of a very large dom.
In Summary:
a) Don't worry about losing js code there is always ctrl+f
b) There is no lag because dom ready does not mean the page is visible to start with.
Update
Fixed order of speed in operations based on the tests results from here
However keep in mind that performances of IE < 8 are really had if you don't specify the container (this used to be the rule, now it seems to be the exception to the rule).

How to organize JS loading?

I have a serious project with big JS file with many functions, and now I have some questions about it.
Of course, I will stick together all my js files and gzip it, but I have a question about functions initialisations. Now I have many, many simple functions, without any classes. I heard that it is better to separate my code into many classes, and I want to do it, and on page generation step mark in my script, which classes I need on specific page. For example, I don't need a WYSIWYG editor at the main page, and so forth, so I will create special var with array of js classes which I need. Is this the best practice? Will clients get performance and memory savings?
Suppose that I have many links with onclick actions. What is better: leave them as <a href="#" onclick="return smth();"> or rewrite as <a href="javascript:void(0);"> and create jquery .bind in document.ready section? But what if the client wants to click something before the document is ready?
After a click on something I must create a div with specific static html. Where is it better to store this content? In some var in my class? Or maybe in some template?

I usually organize my JavaScript using namespacing to prevent function name clashing and further add each object into it's own .js file for organization purposes
var MyObject = function() {
return {
method_1 : function() {
// do stuff here
},
method_2 : function() {
// do stuff here
}
};
}();
MyObject.method_1();
MyObject.method_2();
I then import whatever is needed on my pages. Having a few extra global functions that aren't used on a page won't make a noticeable change in performance.
Some people turn the whole onclick in HTML into a grand philosophical debate... for me, it's dirty, yet easy to read and quick to implement. Binding events works great and completely separates JavaScript from HTML but sometimes can be difficult to maintain if your HTML being maintained by many developers gets very sloppy
Keeping the HTML bits in JavaScript has worked fine for me

You might also be interested in looking at the Google Closure JavaScript compiler.
Also: If your JS is large enough to affect performance, you may actually end up getting better performance out of separate files and load depending on what your page initially needs or load-on-demand (via jQuery getScript).
You might also want to look into what kinds of actions you expect users to be using before the DOM is ready.
As for HTML in JS (or vice-versa). It depends. How much HTML are you building? I usually don't put more than one line of HTML in a .html() statement. If it's really more than that, I look at a different solution (like pulling it from another source on-demand or putting it somewhere, hidden, in the page and yanking it over).

What is the best way to add an event in JavaScript?

I see 2 main ways to set events in JavaScript:
Add an event directly inside the tag like this:
do foo
Set them by JavaScript like this:
<a id="bar" href="">do bar</a>
and add an event in a <script> section inside the <head> section or in an external JavaScript file, like that if you're using prototypeJS:
Event.observe(window, 'load', function() {
$('bar').observe('click', doBar);
}
I think the first method is easier to read and maintain (because the JavaScript action is directly bound to the link) but it's not so clean (because users can click on the link even if the page is not fully loaded, which may cause JavaScript errors in some cases).
The second method is cleaner (actions are added when the page is fully loaded) but it's more difficult to know that an action is linked to the tag.
Which method is the best?
A killer answer will be fully appreciated!

I think the first method is easier to read and maintain
I've found the opposite to be true. Bear in mind that sometimes more than one event handler will be bound to a given control.
Declaring all events in one central place helps to organize the actions taking place on the site. If you need to change something you don't have to search for all places making a call to a function, you simply have to change it in one place. When adding more elements that should have the same functionality you don't have to remember to add the handlers to them; instead, it's often enough to let them declare a class, or even not change them at all because they logically belong to a container element of which all child elements get wired to an action. From an actual code:
$$('#itemlist table th > a').invoke('observe', 'click', performSort);
This wired an event handler to all column headers in a table to make the table sortable. Imagine the effort to make all column headers sortable separately.

In my experience, there are two major points to this:
1) The most important thing is to be consistent. I don't think either of the two methods is necessarily easier to read, as long as you stick to it. I only get confused when both methods are used in a project (or even worse on the same page) because then I have to start searching for the calls and don't immediately know where to look.
2) The second kind, i.e. Event.observe() has advantages when the same or a very similar action is taken on multiple events because this becomes obvious when all those calls are in the same place. Also, as Konrad pointed out, in some cases this can be handled with a single call.

I believe the second method is generally preferred because it keeps information about action (i.e. the JavaScript) separate from the markup in the same way CSS separates presentation from markup.
I agree that this makes it a little more difficult to see what's happening in your page, but good tools like firebug will help you with this a lot. You'll also find much better IDE support available if you keep the mixing of HTML and Javascript to a minimum.
This approach really comes into its own as your project grows, and you find you want to attach the same javascript event to a bunch of different element types on many different pages. In that case, it becomes much easier to have a single pace which attaches events, rather than having to search many different HTML files to find where a particular function is called.

You can also use addEventListener (not in IE) / attachEvent (in IE).
Check out: http://www.quirksmode.org/js/events_advanced.html
These allow you to attach a function (or multiple functions) to an event on an existing DOM object. They also have the advantage of allowing un-attachment later.
In general, if you're using a serious amount of javascript, it can be useful to make your javascript readable, as opposed to your html. So you could say that onclick=X in the html is very clear, but this is both a lack of separation of the code -- another syntactic dependency between pieces -- and a case in which you have to read both the html and the javascript to understand the dynamic behavior of the page.

My personal preference is to use jQuery in external js files so the js is completely separate from the html. Javascript should be unobtrusive so inline (ie, the first example) is not really the best choice in my opinion. When looking at the html, the only sign that you are using js should be the script includes in the head.
An example of attaching (and handling) events might be something like this
var myObject = {
allLinkElements: null,
init: function()
{
// Set all the elements we need
myObject.setElements();
// Set event handlers for elements
myObject.setEventHandlers();
},
clickedLink: function()
{
// Handle the click event
alert('you clicked a link');
},
setElements: function()
{
// Find all <a> tags on the page
myObject.allLinkElements = $('a');
// Find other elements...
},
setEventHandlers: function()
{
// Loop through each link
myObject.allLinkElements.each(function(id)
{
// Assign the handler for the click event
$(this).click(myObject.clickedLink);
});
// Assign handlers for other elements...
}
}
// Wait for the DOM to be ready before initialising
$(document).ready(myObject.init);
I think this approach is useful if you want to keep all of your js organised, as you can use specific objects for tasks and everything is nicely contained.
Of course, the huge benefit of letting jQuery (or another well known library) do the hard work is that cross-browser support is (largely) taken care of which makes life much easier

Libraries like YUI and jQuery provide methods to add events only once the DOM is ready, which can be before window.onload. They also ensure that you can add multiple event handlers so that you can use scripts from different sources without the different event handlers overwriting each other.
So your practical choices are;
One. If your script is simple and the only one that will ever run on the page, create an init function like so:
window.onload = function () {
init();
}
function init() {
// actual function calls go here
doFoo();
}
Two. If you have many scripts or plan to mashup scripts from different sources, use a library and its onDOMReady method to safely add your event handlers