Stop Execution of JavaScript Function (Client Side) or Tweak It

Remove a javascript function with Greasemonkey

It's not clear that update is a global function. If it isn't then that approach won't work.

But you can override the keypress handler with:

unsafeWindow.document.onkeypress = function(){};

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For a general, high-powered way to selectively block, or replace, any JS (on Firefox), use @run-at document-start and the checkForBadJavascripts function, like so:

// ==UserScript==
// @name        _Replace select javascript on a page
// @include     http://YOUR_SERVER.COM/YOUR_PATH/*
// @require     https://gist.github.com/raw/2620135/checkForBadJavascripts.js
// @run-at      document-start
// @grant       GM_addStyle
// ==/UserScript==
/*- The @grant directive is needed to work around a design change
    introduced in GM 1.0.   It restores the sandbox.
*/

checkForBadJavascripts ( [
    [   false,
        /document\.onkeypress\s*=\s*update/,
        function () {
            addJS_Node (myKeypressFunction.toString() );
            addJS_Node ('document.onkeypress = myKeypressFunction;');
        }
    ]
] );

function myKeypressFunction (evt) {
    /*  DO WHATEVER HERE BUT USE NO GREASEMONKEY FUNCTIONS INSIDE
        THIS FUNCTION.
    */
    console.log ("Keypress function fired.");
}

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See this answer, for more information on checkForBadJavascripts.

Disabling a page's focus-checking function using GM

In general, to surgically disable or alter a page's javascript, use checkForBadJavascripts as shown in "How to alter this javascript with Greasemonkey?".

However, in this case, the page is using document.hasFocus(), so it should be enough to just hijack that:

// ==UserScript==
// @name     _YOUR_SCRIPT_NAME
// @include  http://YOUR_SERVER.COM/YOUR_PATH/*
// @run-at   document-start
// @grant    none
// ==/UserScript==

document.hasFocus = function () {return true;};

How to block url or .js file execution on a specific using greasemonkey script? (client side)

The second method below usually works but something seems to be interfering with it on your page for some reason. An ugly workaround is to put an empty hljs property on the window in advance, so that the page script, when run, thinks it already exists and does nothing:

window.hljsLoader = {};

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You can add a MutationObserver to the document at the beginning of pageload and watch for the addition of a script tag which has that URL as a src, and remove it. Make sure you're using @run-at document-start, and then do:

const observer = new MutationObserver((mutations) => {
  mutations.forEach(({ addedNodes }) => {
    addedNodes.forEach((addedNode) => {
      if (addedNode.nodeType === 1 && addedNode.matches('script') && addedNode.src === 'https://cdn.jsdelivr.net/gh/s9e/hljs-loader@1.0.16/loader.min.js') {
        addedNode.remove();
        observer.disconnect();
      }
    });
  });
});
observer.observe(document.documentElement, { childList: true, subtree: true });

Although the script tag will get temporarily added to the DOM, it will be removed in the observer microtask before it gets a chance to run.

Live snippet of the script working:

<script>
const observer = new MutationObserver((mutations) => {
  mutations.forEach(({ addedNodes }) => {
    [...addedNodes].forEach((addedNode) => {
      if (addedNode.nodeType === 1 && addedNode.matches('script') && addedNode.src === 'https://cdn.jsdelivr.net/gh/s9e/hljs-loader@1.0.16/loader.min.js') {
        console.log('Script tag addition intercepted and removed');
        addedNode.remove();
        observer.disconnect();
        console.log('hljsLoader has not been loaded:', typeof hljsLoader);
      }
    });
  });
});
observer.observe(document.documentElement, { childList: true, subtree: true });
</script>

<script async="" crossorigin="anonymous" data-hljs-style="github-gist" integrity="sha384-TB2DTH77ndX7xwCHAtxD7BZqyn4r429ZSoGL7vcrb5x0bFSvLGAMoiriMUcIqeTu" onload="console.log(hljsLoader)" src="https://cdn.jsdelivr.net/gh/s9e/hljs-loader@1.0.16/loader.min.js"></script>

Replace Execution Of Javascript File Loaded From Website With My Own

Currently I've found only a solution to download and install Fiddler web debugging proxy and change Firefox proxy settings to those specified in Fiddler. To execute edited file I've used Fiddler's rule to autorespond with local file when there is a request to specified URL.

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