How to read Referer and User-Agent HTTP request headers in client-side JavaScript?

Almost by definition, the client-side JavaScript is not at the receiving end of a http request, so it has no headers to read. Most commonly, your JavaScript is the result of an http response. If you are trying to get the values of the http request that generated your response, you'll have to write server side code to embed those values in the JavaScript you produce.

It gets a little tricky to have server-side code generate client side code, so be sure that is what you need. For instance, if you want the User-agent information, you might find it sufficient to get the various values that JavaScript provides for browser detection. Start with navigator.appName and navigator.appVersion.

This can be accessed through Javascript because it's a property of the loaded document, not of its parent.

Here's a quick example:

<script type="text/javascript">
  document.write(document.referrer);
</script>

The same thing in PHP would be:

<?php echo $_SERVER["HTTP_REFERER"]; ?>

One way to obtain the headers from JavaScript is using the WebRequest API, which allows us to access the different events that originate from http or websockets, the life cycle that follows is this: WebRequest Lifecycle

So in order to access the headers of a page it would be like this:

    browser.webRequest.onHeadersReceived.addListener(
     (headersDetails)=> {
      console.log("Request: " + headersDetails);
    },
    {urls: ["*://hostName/*"]}
    );`

The issue is that in order to use this API, it must be executed from the browser, that is, the browser object refers to the browser itself (tabs, icons, configuration), and the browser does have access to all the Request and Reponse of any page , so you will have to ask the user for permissions to be able to do this (The permissions will have to be declared in the manifest for the browser to execute them)

And also being part of the browser you lose control over the pages, that is, you can no longer manipulate the DOM, (not directly) so to control the DOM again it would be done as follows:

    browser.webRequest.onHeadersReceived.addListener(
        browser.tabs.executeScript({
        code: 'console.log("Headers success")',
    });
});

or if you want to run a lot of code

    browser.webRequest.onHeadersReceived.addListener(
        browser.tabs.executeScript({
        file: './headersReveiced.js',
    });
});

Also by having control over the browser we can inject CSS and images

Documentation: https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/webRequest/onHeadersReceived

Referer and user-agent are request header, not response header.

That means they are sent by browser, or your ajax call (which you can modify the value), and they are decided before you get HTTP response.

So basically you are not asking for a HTTP header, but a browser setting.

The value you get from document.referer and navigator.userAgent may not be the actual header, but a setting of browser.

I would imagine Google grabs some data server-side - remember, when a page loads into your browser that has Google Analytics code within it, your browser makes a request to Google's servers; Google can obtain data in that way as well as through the JavaScript embedded in the page.