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jQuery .load() with Relative Path - XMLHttpRequest Cannot Load - Workaround?

I have a locally-stored project whose directory structure is the following (I minimized non-relevant folders):
What I want to do is that in an HTML file, like index.html, to add a <header> such that its contents would be loaded from an external HTML file, so all of what I'll have to write in index.html would be <header>, and my solution would load the content automatically.
To do this, I'd like to use JavaScript (preferably jQuery, but I'll accept other solutions if they work and jQuery doesn't, or if they work and executed faster than jQuery).
I don't think that I should use an <iframe> due to the fact that it'd probably increase loading times more than using jQuery/JavaScript (which, like I said, is what works now, when the website is live).
Right now, I'm using the jQuery .load() function. I don't know much about jQuery, but I've been told that it should work locally - and it doesn't, for me.
My browser's console shows me the problem:
jquery-3.1.1.min.js:4 XMLHttpRequest cannot load file:///C:/Users/GalGr/Desktop/eiomw/header.html. Cross origin requests are only supported for protocol schemes: http, data, chrome, chrome-extension, https, chrome-extension-resource.
And I'm trying to overcome it.
This code works on my live website - it might not be updated to the code of the files that I linked to below, but it doesn't matter - their code matters.
This is the index.html file:
index.html
This is the header.html file:
header.html
This is `main_script.js:
main_script

The reason you're having a problem with this locally is mainly down to security measures in your browser.
Essentially whenever you're using jQuery's load() function it makes a separate HTTP request (approach known as AJAX) for the file or URL you give it.
Modern browsers enforce that the URL you request using AJAX methods is from the same origin (server) as a security feature to stop pages randomly loading content from anywhere on the internet in the background. In your case it seems like this shouldn't affect you because you're browsing your pages locally and the request you're making using load() is also for a local file (header.html).
However, I am assuming you're just opening up the page directly in your browser, so your browser's URL will look something like 'file:///C:/Users...' (similar example in the error message you gave). This means your browser is directly reading the file from disk and interpreting it as HTML to display the page. It seems likely you don't actually have a local HTTP server hosting the page, otherwise the URL would start with 'http://'. It is for this reason that the browser is giving the security error, even though your AJAX request for header.html is technically from the same source as the page it is executed on.
Your server will have an HTTP server which it's using to host the pages, and so everything works fine as you're then using HTTP as normal, and this security feature does not get in your way.
I would suggest that you simply install an HTTP server locally on your dev machine. You don't even need to 'install' one per-se, there are loads of development HTTP servers that just run standalone, so you start them up when you want to browse your local HTML files. As you appear to be on Windows, I'd check out either IIS (Windows' HTTP server) or IIS Express (like IIS but runs standalone). There are also many others available like Apache, Nginx, etc. etc.
If you do this, you can host your pages on something like 'http://localhost/index.html'. Then, any AJAX requests you make for local files will work fine, just like your server.
Hope that makes sense, and I'm not telling you something you already know?

Why not using something more straight foreword like mustache.js ?

I found a solution:
Using phpStorm's built-in localhost, I was able to emulate a server that handles my requests and responses.

Receive then implement .js files from server in a chrome extension

I have a chrome extension that, when clicked, needs to display a modal to the user. For convenience we would like to pull all the necessary code for the modal from a server and then execute the JavaScript in the context of the extension, before displaying anything to the user.
(running a Node.js server)
Is there a way to do this?
If not, any suggestions on alternative routes we could take?
cheers!

Have a read through https://developer.chrome.com/extensions/contentSecurityPolicy
In short - it's possible, but you need to serve the script over HTTPS, or else Chrome will reject the origin. You also need to explicitly write the CSP in the manifest.
"Convenience" is a strange word to use here though. What if the network is flaky or slow?

Open local html pages using http to test

I am coding a webapp and i get this kind of errors with chrome :
XMLHttpRequest cannot load file:///C:\Users\Tordah\Desktop\foobar.xml. Cross origin requests are only supported for HTTP.
&
Uncaught NetworkError: Failed to execute 'send' on 'XMLHttpRequest': Failed to load
I believe i get this error because the HTML file is accessed with file: protocol and not http: protocol which creates compatibility issues with XMLHttpRequest (That's a guess). I would like to know if there's any ways of testing my pages using http protocol only with my local machine (because only later in this project i will have access to a server, and therefore the app will only work using http, but for now i would like to be able to test it properly).
Is there any workarrounds that i could do on a local machine and that could with with the three browsers IE, Firefox and Chrome?
Thanks.

I assume that you didn't install a local server...i suggest you XAMPP, is the best for doing this kind of stuffs...
https://www.apachefriends.org/download.html
here is the url where you can download it. when you already install it, you have to copy your work into the directory
C:/xampp/htdocs/(here you paste your work)
open your browser and write in the url like this:
http://localhost:8080/(name of your work directory)/(what archive you want to see, better if it has index)

You need to install a web server. You have a bunch of different choices, but since you didn't mention an operating system, I'll suggest XAMPP from https://www.apachefriends.org/download.html
That will give you an Apache distribution containing MySQL, PHP, and Perl and will run on Windows/Mac/Linux

JavaScript and CouchDB - How do I avoid cross-origin policy errors on GET/POST/PUT/DELETE requests

I am posting this question on Super User as well. In my opinion this question overlaps the two...
I am creating a simple JavaScript wrapper for CouchDB's REST-ful interface, but I am stuck on same-origin policy issues.
So far I've been developing my code to work locally - and only as a proof of concept - on Mozilla FireFox. My server is running on localhost, port 5984.
To disable cross-origin policy in Mozilla FireFox you can use the PrivilegeManager, but it only gets me half-way in the sense that I can't do PUT requests against my server...
/*
* Including this in my JavaScript file only seems to disable cross-origin
* policy checks for POST and GET requests in Mozilla FireFox.
* PUT requests fail.
*/
netscape.security.PrivilegeManager.enablePrivilege(
"UniversalBrowserRead UniversalBrowserWrite"
);
Is there any way that I can configure my server to hide it's location so I won't have to implement browser-specific work-arounds to avoid same-origin policy issues? If not: what browser work-arounds exist to disable same-origin policy completely?

Unfortunately, any browser workarounds to disable same-origin policies are likely to be treated as serious security bugs and fixed as soon as possible.
See if you can come up with a way to work within the same-origin policy without trying to bypass it.
Can you serve your example scripts on the target server? Could you build a reflection script that would load the target script on your server after a local script on the users computer uploaded whatever they modified?
There should be a good solution that doesn't involve bypassing the same-origin policy. Trying to hack your way around it is a good way to ensure that your code doesn't work properly in future browsers.

I strugled with that issue too, trying to run automated tests on a local html file connecting to a virtualized CouchDB server, here's my solution:
I created a small implementation (and open sourced it) of the simplest solution when you can't enable CORS on the server,
you need to upload a .js and an .html file to the target server, (you can use any security mechanism to restrict access to this file if you want). Or you can change some simple parameters on the html file to restrict by domain.
On your page you use the same script to create an invisible iframe where the hosted .html is loaded, and proxy certain methods (sort-of RPC) thru that iframe using window.postMessage(), by default jQuery ajax methods can be proxied without extra configuration.
All this with one line of js code :)
FrameProxy at GitHub
(fell free to use it and fork it!)

javascript not working on localhost

Ok so I'm lost here, frustrated and pulling my hair and out. Plus probably about to be fired or take a pay cut.
I moved Files from a development server to my local machine. The files are consistent (used diff tool), all the dependencies are there. It works for the most part. The problem is that the some of the javascript (not all) is just not working. We're using jquery and a lot of plugins for it. I've checked with the web developer plugin in firefox and all the js files are loading. I cleared the cache in both firefox and chrome multiple times to no avail. The development server is a windows server running wamp. My local machine is running ubuntu. Somebody tell me what I missed.

Download firebug as a Firefox extension and view the http request and responses.
Easiest may be from within the 'net' tab to determine if your script is making a request.
Very likely that it is a source domain issue. There are no work-around for this issue. The ajax request and the source data must be on the same domain.

It may have something to do with JavaScript's security limitations. (In certain circumstances) You can only operate on URLs or pages from the current domain, which most likely changed when you moved the files off the other server. More here.

Are you running the files via a webserver, or just opening the files directly? If it's the latter, you'll want to set up a server on your local machine for local testing, and serve the files using it. Otherwise, you'll very likely run into the domain restrictions others have mentioned above.

You may need to host the site using a local server. VS IDE has an add-on called live server. You need to set up a workspace in order for it to work. The port used on my machine was 5500.
You need to make sure any dependencies for javascript are running on your server or the javascript will not be executed. These dependencies are listed in the json file.
ex. If you require express, you need to be running node or the javascript won't execute in your web browser.
In the terminal:
node app.js
Any dependencies that are not installed and running on the server will not execute.

Are you accessing the html web pages through the webserver and not simply double clicking the file to open it?
Also if you have WebDeveloper toolbar installed the click "Disable", "Disable Javascript" and make sure "All Javascript" isn't ticked.