Develop Reference

Is it better to have a single .js file or copy the script in every page?

I'm building a website which uses three, identical JavaScript functions in a lot of HTML pages.
Is it better to link all the pages to a single .js file or copying the script to every HTML page which uses it?

I recommend putting the script into a separate file and link to it. If you want to make changes to the code, you only have to do it once. => Removing redundancy. On the other hand, you reduce the amount of requests to the webserver, because each script tag will create an extra GET request. But I gladly pay this price for low redundancy!

I prefer to use one file and link to it.
But if the functions are pretty small google will be happy if you put them in the footer of your page directly above the body tag. I would do this only if you use a template where you have one footer for all pages and don't need to edit every single page.

Load pages via AJAX and execute javascript and CSS

I've been searching for a while now, but I can't figure out how to load an entire page via AJAX and still execute all javascript and css.
Mostly I just end up with the plain text without any CSS.
Is there a way to do this? I tried jQuery.get, jQuery.load and jQuery.ajax, but none really work like that.

I have a different solution. You may try it with an iframe. Use jQuery to append an iframe script including all relevant codes into some part of your page (like some div). This may do it for you including CSS, like;
$('<iframe src="your_page.html"/>').appendTo('#your_div');
Or you may try something like;
$('<iframe src="your_page.html"/>').load(function(){
alert('the iframe is done loading');
}).appendTo('#your_div');

I have solved similar problem as following.
Download the webpage over ajax
Iterate it over and find any <script> and </script> tags
Get content from within these tags as text
Create new <script> element and insert there the code
Append the tag to your webpage
Another thing is you will need to somehow call the script..
I have done it this way:
I set standardized function names like initAddedScript callback which I am calling after appending the script to the page. Same as I have deinitScript called when I do not need the code (and its variables,..) anymore.
I must say this is awful solution, which likely means you have bad application architecture so as I have had:)
With css is it the same, but you do not need any handlers. Just append the style tag to your documents head.

If the page you load doesn't have any style data, then the external stylesheets must have relative paths that are not correct relative to the invoking document. Remember, this isn't an iFrame - you aren't framing an external document in your document, you're combining one document into another.
Another problem is that loading your complete page will also load the doctype, html, head, and body tags - which modern browsers will cope with most of the time, but the results are undefined because it's not valid HTML to jam one document into another wholesale. And this brings me to the third reason why it won't work: CSS links outside of the head section aren't valid, and the misplaced head section caused by your haphazard document-in-document collage.
What I'd do for compliance (and correct rendering) is this, which would be implemented in the Success callback:
Copy all link elements to a new jQuery element.
Copy the contents of all script in the head section
Copy the .html() contents from the loaded document's body tag
Append the link elements (copied out in step 1) to your host document's head
Create a new script tag with your copied script contents and stick it in the head too
Done!
Complicated? Kind of, I guess, but if you really want to load an entire page using AJAX it's your only option. It's also going to cause problems with the page's JavaScript no matter what you do, particularly code that's supposed to run during the initial load. There's nothing you can do about this. If it's a problem, you need to either rewrite the source page to be more load-friendly or you could figure out how to make an iFrame suit your needs.
It's also worth considering whether it'd work to just load your external CSS in the host document in the first place.

I suppose you are looking for something like this:
your page div --> load --> www.some-site.com
After a quik search the closest solution seems to be the one by "And": Load website into DIV
You have to run a web server and create a proxy.php page with this content:
Then your JQuery load() function should be like this:
$("#your_div_id").load("proxy.php?url=http://some-site.com");
NB. I have tested this solution and it should not load all the CSS from the target page, probably you'll have to recreate them. For example the image files stored on the remote server will not loaded, I suppose due to authentication policy.
You will be also able to view only the target page without the possibility to browse the target site.
Anyway I hope this could be a step forward to your solution.

Get your entire webpage as text using ajax
document.open();
document.write(this.responseText);
document.close();
OR
document.documentElement.outerHTML = this.responseText;
But you need to change the path of css and js pages in original webpage if the resulting webpage is in another directory.

Script tags contained in jQuery AJAX response not evaluated when inserted into the DOM?

According to the jQuery documentation for $.ajax, "included script tags are evaluated when inserted in the DOM.". When I use ajax to grab the content of a dialog box, which in turn contains a script tag for displaying a ReCaptcha box, the ReCaptcha box does not appear when added to the DOM. According to FireBug, the script tag is also now missing from the added content. Navigation directly to the dialog content displays the ReCaptcha just fine.
Does anyone know why this may be occuring and/or know a work around? Any help would be greatly appreciated.
Code in action can be viewed here:
https://dustinhendricks.com/
Then click "Register Now".

Does the script try to do things like document.write()? That won't work when loaded dynamically, only on the initial page load.
In general, scripts contained in HTML that is innerHTML'd is not evaluated consistently across browsers. To fix that, jQuery actually looks for script tags and executes them manually.
But that doesn't really matter. Even if jQuery didn't do that, the script would be running in a different type of context than it normally is when a page is loading for the first time. It's not really an 'inline script' anymore, and a lot of 3rd party scripts were written with it being 'inline' as an assumption. You'll have to figure out what the script is doing and find a way to call it with dynamic content.

Take a look at the Google reCAPTCHA AJAX API. Using the CAPTCHA this way should fix your problem.
I hope it helps!

Refrashing the Javascript File without page refresh.

I am working on a very complex web site which is wizard based and have many JavaScript files included.
Now problem is if I pass through many step and at some later stage I find a JavaScript problem, as I fix it I have to load the page again and as I load the page again the Wizard will be started from step one again.
What I want is if I make JavaScript change in a file, there should be a possibility to refresh the JavaScript file through Firebug or something like it.
Thanks a lot.

No, not really. You could try to change the <script>'s src attribute to something like /your/script.js?timestamp=1234567890, but this doesn't give a reliable guarantee that the script will be loaded again.
You could add another script element into the DOM, but then you'd have two very similar scripts loaded at the same time, which may be problematic.

Programmatically remove <script src="/unwanted.js".. /> reference

I have partial control of a web page where by I can enter snippets of code at various places, but I cannot remove any preexisting code.
There is a script reference midway through the page
<script src="/unwanted.js" type="text/javascript"></script>
but I do not want the script to load. I cannot access the unwanted.js file. Is there anyway I can use javascript executing above this refernce to cause the unwanted.js file not to load?
Edit: To answer the comments asking what and why:
I'm setting up a Stack Exchange site and the WMD* js file loads halfway down the page. SE will allow you to insert HTML in various parts of the page - so you can have your custom header and footer etc. I want to override the standard WMD code with my own version of it.
I can get around the problem by just loading javascript after the original WMD script loads and replacing the functions with my own - but it would be nice not to have such a large chunk of JS load needlessly.
*WMD = the mark down editor used here at SO, and on the SE sites.

In short, you can't. Even if there is a hack, it would heavily depend on the way browsers parse the HTML and load the scripts and hence wouldn't be compatible with all browsers.

Please tell us exactly what you can and cannot do, and (preferably; this sounds fascinating) why.
If you can, try inserting <!-- before the script include and --> afterwards to comment it out.
Alternatively, look through the script file and see if there's any way that you could break it or nullify its effects. (this would depend entirely on the script itself; if you want more specific advice, please post more details, or preferably, the script itself.

Could you start an HTML comment above it and end below it in another block?
What does the contents of unwanted.js look like?

You can remove a script from the DOM after it is called by using something simple such as:
s = document.getElementById ("my_script");
s.parentNode.removeChild(s);
This will stop all functions of the script but will not take it out of user's cache. However like you wanted it can't be used.

Basically you can't unless you have access to the page content before you render it.
If you can manipulate the HTML before you send it off to the browser, you can write a regular expression that will match the desired piece of code, and remove it.