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dynamically generate content for a page when clicking on product

everyone. I am making a website with t-shirts. I dynamically generate preview cards for products using a JSON file but I also need to generate content for an HTML file when clicking on the card. So, when I click on it, a new HTML page opens like product.html?product_id=id. I do not understand how to check for id or this part ?prodcut_id=id, and based on id it generates content for the page. Can anyone please link some guides or good solutions, I don't understand anything :(.

It sounds like you want the user's browser to ask the server to load a particular page based on the value of a variable called product_id.
The way a browser talks to a server is an HTTP Request, about which you can learn all the basics on javascipt.info and/or MDN.
The ?product_id=id is called the 'query' part of the URL, about which you can learn more on MDN and Wikipedia.
A request that gets a page with this kind of URL from the server is usually a GET request, which is simpler and requires less security than the more common and versatile POST request type.
You may notice some of the resources talking about AJAX requests (which are used to update part of the current page without reloading the whole thing), but you won't need to worry about this since you're just trying to have the browser navigate to a new page.
Your server needs to have some code to handle any such requests, basically saying:
"If anybody sends an HTTP GET request here, look at the value of the product_id variable and compare it to my available HTML files. If there's a match, send a response with the matching file, and if there's no match, send a page that says 'Error 404'."
That's the quick overview anyway. The resources will tell you much more about the details.

There are some solutions, how you can get the parameters from the url:
Get ID from URL with jQuery
It would also makes sense to understand what is a REST Api and how to build a own one, because i think you dont have a backend at the moment.
Here some refs:
https://www.conceptatech.com/blog/difference-front-end-back-end-development
https://www.tutorialspoint.com/nodejs/nodejs_restful_api.htm

How to collect client data when we redirect using 301 redirect?

There is a system like Clickmeter that allows people to create a smart link for their banner ads. Here is a short explanation of the system. You can enter a URL as the landing page and system gives you back a special URL to put instead of the original. Now if someone clicks on the special link, he will be redirected to the landing page that you wanted to.
I am developing something like that but here is the trouble. I must use 301 redirect because of some SEO things, and 301 redirect is only available in server side as I know. But I want to get some client data such as browser name, operating system model and browser language before redirecting the client. And I am doing this part in javascript, absolutely client side.
I dont know what to do or if I am wrong about something else. But I know that Clickmeter is doing exactly the thing that I want to. They get some client data and then do 301 redirect. Here is a sample link of CLickmeter: http://9nl.it/vz0d

You don't get the client's data in the client's side. You do it in the server side by reading and processing the Request.UserAgent before doing the 301 redirection:
// get the User Agent
string userAgentText = HttpContext.Current.Request.UserAgent;
// Process the User Agent, and extract the information you want: browser, OS, version...
...
// Make the 301 redirection to the target URL programmatically
HttpContext.Current.Response.Status = "301 Moved Permanently";
HttpContext.Current.Response.AddHeader("Location", TARGET_URL);
You could get similar information in JavaScript by using window.navigator.userAgent (and AJAX to send the data on click), but I would not recommend this solution for two reasons reasons:
If you are providing just a URL (your service sounds like a URL shortener), you cannot inject the JavaScript code.
If the user has JavaScript disabled, this solution will not work at all.
To find more information on how this could be done, read these questions:
How do I get just the OS from Request.UserAgent?
What is the difference between Request.UserAgent and Request.Browser?
301 redirect in asp.net 4.0
How can you detect the version of a browser? (if you still decide to go with a JS approach)

JavaScript: 404 page, how to get the requested url?

I'm hosting few static web pages on GitHub (gh-pages). If the user tries to access a page which isn't available, he/she is moved to a custom 404.html.
What I'm wondering is if is it possible to access the original requested URL from the custom 404.html, using just JavaScript? There's no PHP nor any other server side technology available.
I've looked at the JavaScript's Location-object but that seems to give only the access to the current URL (in this case the 404.html) but not to the original requested URL. What I'm trying to achieve is a 404.html which gives suggestion like "Did you mean to access url ..." to the user but in order to do so, I need the access to the original URL.

your only hope would be document.referrer but of course GH would need to set it, which is highly unlikely for any page returning a HTTP 404 out of a request ...

You need to look at the url in document.referrer

Because the user is moved by the server to a 404 page, JavaScript cannot know abot the requested url.
It may be posible if you add in .htaccess to redirect the user to a page with the url: page.php?url=requested_url , then the requested_url appears in the address bar, which can be read by javascript.

I've tested this with a custom domain and location.href will actually give the current url, which in this case is the faulty one. So, while document.referrer will only give empty string, location.href will give the url you want.
I'm wondering if this has to do with what kind of GH pages you're hosting as well as if you're using a custom domain. My understand was, however, that it was only possible to serve a custom 404.html using a custom domain.

How does LinkedIn know or keep track of where I embedded its widget?

When you try to integrate with LinkedIn's Apply Now button, you first sign up for an API key. The form asks you to enter the Javascript Domain API, which is the Fully-qualified domain name of all pages that will call the JavaScript API with this key. In return, it produces an API key and some HTML code for you which you can copy n paste to your web page and get started.
This is the code their wizard produced:
<script src="http://platform.linkedin.com/in.js" type="text/javascript">
api_key: 7a4ghb12agvda4552da
</script>
<script type="IN/Apply"
data-companyname="Asd"
data-jobtitle="Software Developer"
data-joblocation="Istanbul"
data-email="abc#xyz.com">
</script>
Now, how does one keep track of where this script is embedded? I first entered http://example.com as the my Javascript Domain API. It turned out that I can only use this widget on the example.com domain.
What's inside in.js that tells LinkedIn where it is embedded?
The reason I'm asking is because I am also building a widget myself, and I want to make sure only the signed-up domains can use my widget.
Edit: As a bonus, what if I download in.js, remove the part where it does the domain check and include my own version of in.js in my page? How do they prevent that?
A LinkedIn employee mentions that both client-side and server-side checks are done. But what kind of a check would that be? I am looking for some deep insight into the issue. How can I produce such a widget? On the client-side, how do you check the current page that hosts your .js file? And how do you get which domain is hosting the js file?
Any help appreciated. Thanks.

The LinkedIn Javascript framework won't work if you make a local copy of in.js - the backend server (which in.'s calls) checks to make sure that the in.js is coming from the correct server as well as checking to make sure that the framework will only work on the specified domain(s).
This question was asked/answered here:
https://developer.linkedin.com/forum/security-prevent-impersonations

in.js has a script which adds another script tag into the DOM. It passes the API key (probably as a GET parameter in the script's URL), then the server checks the HTTP referer (which is a standard HTTP header browsers send indicating the website which sent them to get that page) and checks if it matches the API key in the database.
A simpler version would contain something like this:
document.write('<script src="http://mysite.com/api.js?key="' + api_key + '></' + 'script>');
Then on the server, something like this pseudo-code:
var expectedDomain = queryTable('apikeys').equal('key', GET('key')).field('domain').run();
if (expectedDomain === parseDomain(http.referer)) {
respond(myscript);
}

how to cross direct user with google analytics

After a user fills in my "new" user form on "example-one.com", the "create" controller creates the record in the db. Then it does a redirect_to to an external site "payment-checkout.com". I have setup the Google Analytics code on both sites.
Google provides two functions _link and _linkByPost for use to use in any links or forms that go to your external domains. The problem is the user is being redirected by the controller action outside of the view and I cant use those two javascript functions to pass on the relevent G.A. info - what do i do?
Can anyone help?

The way _link works is by passing the Google Analytics cookies from your first domain via a query string to your second domain. The second domain, if configured correctly, will accept those URL parameters and apply them as cookie values for the purposes of tracking.
So, it shouldn't be difficult for you to apply your own version of the _link function.
Specifically, the _link function passes the following cookies:
__utma, __utmb, __utmc, __utmx, __utmz, __utmv and __utmk
Into a query string as such: ?__utma=87278922.614105561.1288923931.1294376393.1298325957.6&__utmb=87278922.1.10.1298325957&__utmc=87278922&__utmx=-&__utmz=87278922.1288923931.1.1.utmcsr=(direct)|utmccn=(direct)|utmcmd=(none)&__utmv=-&__utmk=72493274
So, all you need to do to replicate the _link function is, before you apply the server side redirect, grab the cookie values, and apply them as a query string on the URL you're redirecting to.
Now, that's not the only thing you'll need to do to get this working. The Google Analytics configuration on the payment site will need to be configured with _setAllowLinker set to true, as well as potentially disabling the domain hash and setting a particular domain name for the tracking cookies; it depends on your configuration. You can find out more about that in Google Analytics Cross Domain Tracking Guide.

#yc's approach looks like the best bet but if that doesn't work, I would suggest having your controller redirect the user to a "temp" page on your site itself and show some text like "Checking out....Please wait..." and using Javascript trigger the call to the "_link" function to redirect the user to the "payment-checkout.com" (again using Javascript).

I assume you're also tracking the page the user returns to and want to measure how many users you lose in the process in between?
My knowledge of the Google Analytics API is fairly limited, so maybe there's a better solution, but you could consider rendering a page containing the GA code and triggering the _link() function from there?
It might also be possible to perform an AJAX call on submitting the form (maybe using remote_form_for) and handling the GA redirect in an RJS-response:
page << "_gaq.push(['_link', 'http://example.com/test.html']);"
However, I'm not sure how well that would fit into your application.