Tampermonkey userscript prevents pages from loading

Question

I am currently making a userscript to interpret the APL programming language in a Stackexchange chat window. This is the code I have come up with:

// ==UserScript==
// @name     APL chat
// @version  2
// @grant    GM_xmlhttpRequest
// @grant    GM_listValues
// @match    https://chat.stackexchange.com/*
// @run-at   document-start
// @require  https://cdn.jsdelivr.net/npm/apl@0.1.15/lib/apl.min.js
// ==/UserScript==

// thanks to @cvzi for making it work correctly!

var MutationObserver = window.MutationObserver || window.WebKitMutationObserver;

var observer = new MutationObserver(function(mutations, observer) {
    // fired when a mutation occurs
    console.log(mutations, observer);
    let codes = document.getElementsByTagName("code");
        for (let elem of codes) {
            if(elem.innerText && !('interpreted' in elem.dataset) && elem.innerText[0] == '⋄') {
                elem.dataset.interpreted = true;  // see https://developer.mozilla.org/en-US/docs/Learn/HTML/Howto/Use_data_attributes#JavaScript_access
                let result = ''
                let color = 'red'
                // Catch apl error and show it in orange color
                try {
                    result = apl(elem.innerText).toString()
                } catch(e) {
                    result = e.toString()
                    color = 'orange'
                }
                let tmp = document.createElement("div");
                tmp.innerHTML = "<pre style=\"color:"+color+"\">"+result.replace("\n","<br>")+"</pre>";
                let parent = elem.parentElement;
                parent.appendChild(tmp.firstChild);
                //console.log(result);
            }
        }
});

window.addEventListener('DOMContentLoaded', (event) => {
    alert = function() {}; // Prevents ⎕← and ⍞← from trggering alerts
    window.alert = function(){};
      observer.observe(document, {
      subtree: true,
    ChildList:true,
      attributes: true
    });

});

When a code block is written to the body with a ⋄ character in it, it should be executed and displayed in red, underneath the code.

The apl() function takes a single string, and interprets in in the APL language, and it is imported from the @require statement, using ngn's javascript APL interpreter. After ngn's confirmation, I can say that it does not interfere with any globals.

It works perfectly on firefox(Greasemonkey), but on Chrome's Tampermonkey, any chat.stackexchange.com page I visit never stops loading. It gets stuck at this point:

And these are the console errors:

Uncaught TypeError: o.substr is not a function
    at HTMLLIElement.<anonymous> (master-chat-with-millinery.js?v=93a5b9100f35:1)
    at Function.each (jquery.min.js:2)
    at n.fn.init.each (jquery.min.js:2)
    at o (master-chat-with-millinery.js?v=93a5b9100f35:1)
    at Sidebar (master-chat-with-millinery.js?v=93a5b9100f35:1)
    at Vt (master-chat-with-millinery.js?v=93a5b9100f35:3)
    at StartChat (master-chat-with-millinery.js?v=93a5b9100f35:3)
    at HTMLDocument.<anonymous> (the-nineteenth-byte:182)
    at i (jquery.min.js:2)
    at Object.fireWith [as resolveWith] (jquery.min.js:2)
master-chat-with-millinery.js?v=93a5b9100f35:7 Uncaught TypeError: Cannot read property 'resolve' of undefined
    at Object.<anonymous> (master-chat-with-millinery.js?v=93a5b9100f35:7)
    at i (jquery.min.js:2)
    at Object.fireWith [as resolveWith] (jquery.min.js:2)
    at y (jquery.min.js:4)
    at XMLHttpRequest.c (jquery.min.js:4)
DevTools failed to load SourceMap: Could not load content for chrome-extension://dhdgffkkebhmkfjojejmpbldmpobfkfo/sm/66503b0e4799e2375dd4e11269326fc0fcb9a65474ff19f45c7f956fd381cea8.map: HTTP error: status code 404, net::ERR_UNKNOWN_URL_SCHEME

I have my suspicions on the MutationObserver and the @grant, but I'm not sure what exactly is causing this issue. It could be something else.

What is the correct fix for this problem?

Answer 1

You set @run-at document-start in your userscript, this makes the script run before the page loaded, if you remove it, the script is injected after DOMContentLoaded was fired.

If that does not help, you could try to use the original source from gitlab @require https://gitlab.com/n9n/apl/-/raw/master/apl.js you would need to change the apl call to result = apl.fmt(apl(elem.innerText)).toString()

I assume the conflict happens because the apl library (https://cdn.jsdelivr.net/npm/apl@0.1.15/lib/apl.js) has some coffee script helper functions included at the top of the file, notably there are some Array.prototype.* methods declared which will also be available in the page. These could break the page, see this question for more information: Why is extending native objects a bad practice?