A utility function, for Greasemonkey scripts, that detects and handles AJAXed content.

waitForKeyElements.js

/*--- waitForKeyElements():  A utility function, for Greasemonkey scripts,
    that detects and handles AJAXed content.

    Usage example:

        waitForKeyElements (
            "div.comments"
            , commentCallbackFunction
        );

        //--- Page-specific function to do what we want when the node is found.
        function commentCallbackFunction (jNode) {
            jNode.text ("This comment changed by waitForKeyElements().");
        }

    IMPORTANT: This function requires your script to have loaded jQuery.
*/
function waitForKeyElements (
    selectorTxt,    /* Required: The jQuery selector string that
                        specifies the desired element(s).
                    */
    actionFunction, /* Required: The code to run when elements are
                        found. It is passed a jNode to the matched
                        element.
                    */
    bWaitOnce,      /* Optional: If false, will continue to scan for
                        new elements even after the first match is
                        found.
                    */
    iframeSelector  /* Optional: If set, identifies the iframe to
                        search.
                    */
) {
    var targetNodes, btargetsFound;

    if (typeof iframeSelector == "undefined")
        targetNodes     = $(selectorTxt);
    else
        targetNodes     = $(iframeSelector).contents ()
                                           .find (selectorTxt);

    if (targetNodes  &&  targetNodes.length > 0) {
        btargetsFound   = true;
        /*--- Found target node(s).  Go through each and act if they
            are new.
        */
        targetNodes.each ( function () {
            var jThis        = $(this);
            var alreadyFound = jThis.data ('alreadyFound')  ||  false;

            if (!alreadyFound) {
                //--- Call the payload function.
                var cancelFound     = actionFunction (jThis);
                if (cancelFound)
                    btargetsFound   = false;
                else
                    jThis.data ('alreadyFound', true);
            }
        } );
    }
    else {
        btargetsFound   = false;
    }

    //--- Get the timer-control variable for this selector.
    var controlObj      = waitForKeyElements.controlObj  ||  {};
    var controlKey      = selectorTxt.replace (/[^\w]/g, "_");
    var timeControl     = controlObj [controlKey];

    //--- Now set or clear the timer as appropriate.
    if (btargetsFound  &&  bWaitOnce  &&  timeControl) {
        //--- The only condition where we need to clear the timer.
        clearInterval (timeControl);
        delete controlObj [controlKey]
    }
    else {
        //--- Set a timer, if needed.
        if ( ! timeControl) {
            timeControl = setInterval ( function () {
                    waitForKeyElements (    selectorTxt,
                                            actionFunction,
                                            bWaitOnce,
                                            iframeSelector
                                        );
                },
                300
            );
            controlObj [controlKey] = timeControl;
        }
    }
    waitForKeyElements.controlObj   = controlObj;
}

@BrockA I have a few scripts that modify YouTube and I use your function to detect new elements. They are small, non-commercial and have around 200-300 installs each.

Today they were all reported on GreasyFork as "unauthorized copy of https://gist.github.com/BrockA/2625891" with description being: "On 10 Nov 2015, the original author said that the license is Creative Commons, Attribution-NonCommercial-ShareAlike 4.0 International (CC BY-NC-SA 4.0)."

I didn't know you can @include a git io link so all of my scripts have a verbatim copy of yours at the bottom (I'd probably prefer to do that even if I knew, in case you ever took the gist down), with phrase "From: https://git.io/vMmuf" added in the top comment describing what they are. This is how the top two lines of your code look after this addition:

/*--- waitForKeyElements():  A utility function, for Greasemonkey scripts,
    that detects and handles AJAXed content. From: https://git.io/vMmuf

My questions:

1. have you reported all my YT scripts?
2. are you okay or not with this usage (copy paste + link to your gist)?

I've decided not to wait anymore and submitted a rebuttal to all the reports saying you didn't ever mind or report small, non-commercial and credited use or forks on GreasyFork when learning of them in the past and that my script is small, non-commercial and links to your gist. Please confirm that this is okay with you.

All my scripts that used this gist were were deleted by a moderator. The message I've received:

I consider https://gist.github.com/BrockA/2625891#gistcomment-1617026 the most clear statement by the author. Please comply with the terms of that license.

@FRex,

That wasn't me and I don't see how it applies to you, unless you are somehow selling your scripts.

I wonder if it was a YT bot; they're really going sick over there.

Anywho, I have now released this utility to the public domain.

Thank you for the help. I've submitted even more appeals. I will update here if they are accepted or rejected. I will also make a complaint about this on GreasyFork forum.

I wonder if it was a YT bot; they're really going sick over there.

I don't know what a YT bot means in this context. The reports all linked to your comment about CC-BY-SA-NC and it was upheld by a GreasyFork moderator who ignored my wall of text about how you didn't disapprove/attack copies and forks of your gist.

@BrockA my scripts were all restored. I have no further issues.

Thank you again for your quick reply, the CC0 relicensing and the original function.

@FRex,
You're welcome; glad the script is useful to others.

I want to paste it inside my UserScript file but I got errors like $ is not defined

I want to paste it inside my UserScript file but I got errors like $ is not defined

You need jquery to be installed, e.g.
// @require https://code.jquery.com/jquery-latest.min.js

Or you can use version provided by CoeJoder https://gist.github.com/BrockA/2625891?permalink_comment_id=3279773#gistcomment-3279773

I want to paste it inside my UserScript file but I got errors like $ is not defined

You could do something like this:

// Loads jQuery and triggers a callback function when jQuery has finished loading
function addJQuery(callback) {
    let script = document.createElement('script');
    script.setAttribute('src', '//code.jquery.com/jquery-latest.min.js');
    script.addEventListener('load', function() { callback(); }, false);
    document.body.appendChild(script);
}
 
// The main script
function main() {
 // YOUR CODE GOES HERE
}
 
// Load jQuery and then execute the main function
addJQuery(main);

Is it possible to make waitForKeyElements work with async calls at intervals?

I use waitForKeyElements on a page with paginated lists, with list element as the selector. In my userscript, I have a function that returns a promise after making GM_xmlhttpRequest and fetching some data. I need to run it at sparse intervals to be gentle with the api. But if I use waitForKeyElements on a class corresponding to multiple elements, requests go all at once. I tried using async/await and setTimeout inside my callback function, but I couldn't get the desired results. Is this somehow possible?

I know that async functions do not properly work with forEach() loop, but work with for/of. But in this case I need something like waitForKeyElements to detect that items updated after a pagination.

@enchained,

Yes, that is possible. It sounds as if you are calling the API from within WFKE and it's spamming the server?

In that case, maybe you can use a different selector that only changes once per page? If not, wrap debouncing techniques around the API call. Something like https://www.freecodecamp.org/news/javascript-debounce-example/

Also, depending on your situation, WFKE may not be the best choice. Don't be afraid to just use a plain old setInterval or MutationObserver.

Can't help much more without details but this is not the place for that. (Unless you have a very short MCVE).

@BrockA - Thank you for this waitForKeyElements script. I was making user scripts by trial and error and sometimes my code didn't work and someone suggested using this script and it made my code work. So most of the time I use waitForKeyElements for everything.

There is only one problem though. It won't let me add !important. The following won't work. I have to remove the !important part for it to work, and I often do need the !Important on some elements and websites.

waitForKeyElements ("ELEMENT", Example);
function Example (jNode) {jNode.css({"border-radius" : "24px !important"});}

@gaveitatry2 - I'm glad the script is useful to you.

But that CSS thing is a quirk of jQuery. You need to adjust your code slightly.
See https://stackoverflow.com/a/11963008 for how to set the !important flag.

@gaveitatry2 - I'm glad the script is useful to you.

But that CSS thing is a quirk of jQuery. You need to adjust your code slightly. See https://stackoverflow.com/a/11963008 for how to set the !important flag.

Thank you so much! This is a problem that I have had for many years, and I never knew that there was a solution!

I changed this (doesn't work):

waitForKeyElements ("ELEMENT", Example);
function Example (jNode) {jNode.css({"border-radius" : "24px !important"});}

To this. And now it works:

waitForKeyElements ("ELEMENT", Example);
function Example (jNode) {jNode.attr("style", "border-radius: 24px! important").css({"SOME OTHER CSS"});}

@BrockA would you mind to make a typescript version?

@peanutive00 Show me how to use Typescript with Tampermonkey or Violentmonkey -- in Firefox -- and I will consider it.

@BrockA I am using vite-plugin-monkey this package to do the hmr deployment in Chrome, I never tested in Firefox.

@peanutive00 If you're just trying to benefit from static typing while using this script as a dependency in a TypeScript project, you only need to have the JavaScript annotated with JSDoc type hints. See JSDoc Reference for TypeScript. For example, see my fork of this script. Or, you could produce a .d.ts file for the original version and avoid modifying the .js file altogether.

@peanutive00 Thanks, I'll check that out but I don't do much TM coding these days so it may be a long while. Meanwhile CoeJoder seems to have a workaround.

@BrockA

Could you convert the following code into waitForKeyElements, please? When I use it as is, it only works on some of the span.rightActionButton elements on each page, but not all of them. So I want to see if it will work if I use wFKE, but I don't know the proper format. Hope you don't mind. Thanks.

$(document).ready(function(){
$('span.rightActionButton').each(function(){
$(this).css("background-color", "red");
var oldUrl = $(this).attr("onclick");
var newUrl = oldUrl.replace("window.location='", "window.open('https://www.somewebsite.com");
$(this).attr("onclick", newUrl + ", target='_blank')");
});});

@gaveitatry2 ,

That entire code block would be replaced with:

waitForKeyElements ("span.rightActionButton", rewriteSpanLinks);

function rewriteSpanLinks (jNode) {
    jNode.css ("background-color", "red");
    var oldUrl = jNode.attr ("onclick");
    var newUrl = oldUrl.replace ("window.location='", "window.open('https://www.somewebsite.com");
    jNode.attr ("onclick", newUrl + ", target='_blank')");
}

Thank you.

This code is so bad. It should be using a MutationObserver or DOM/script load events instead of running a jQuery selector on the entire document every 300 ms.

@cow1337killer3 by all means, rewrite it :)

bug report:
cant't wait the same element, the callback can't be called

// ==UserScript==
// @name         New Userscript
// @namespace    http://tampermonkey.net/
// @version      2025-02-21
// @description  try to take over the world!
// @author       You
// @match        http://127.0.0.1:5500/playground.html
// @icon         https://www.google.com/s2/favicons?sz=64&domain=0.1
// @require      https://z.chaoxing.com/js/jquery-3.5.0.min.js
// @grant        none
// ==/UserScript==

(function() {
  'use strict';

  waitForKeyElements(".child", (item) => {
    console.log(item[0]); // Will be executed
  });

  waitForKeyElements(".child", (item) => {
      console.log(item[0]); // Never be executed
  });


})();

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>playground</title>
</head>
<body>
  <button onclick="addComment()">addComment</button>

  <ul class="comments">
    <li class="comment">第1条评论</li>
  </ul>

  <div class="app"></div>

  <script>
    setTimeout(() => {
      const appElement = document.querySelector('.app');

      const pElement = document.createElement('p');

      pElement.className = 'child';

      pElement.textContent = 'child';

      appElement.appendChild(pElement);
    }, 1000);

    function addComment() {
      const commentsEl = document.querySelector('.comments');

      const liElement = document.createElement('li');

      liElement.className = 'comment';

      liElement.textContent = `第${commentsEl.children.length + 1}条评论`;
      
      commentsEl.appendChild(liElement);
    }
  </script>
</body>
</html>

@HHsomeHand , This is not really a bug. It's an artifact of the purpose and expected use of WFKE. This version of the function marks nodes with a flag that can be cleared with $(".child").removeData('alreadyFound'); IF-AND-ONLY-IF you understand and use the asynchronous timing correctly.

Anyway, there are workarounds depending on the exact details of what you're attempting, but the easiest is to just put all code for ".child" into only one waitForKeyElements call.

bug report: cant't wait the same element, the callback can't be called

No one's bothered to link the mystical rumored fork that uses MutationObserver so here's the link: https://gist.github.com/double-beep/c4d4ec3866e5e54ae514c0aab60af242

No one's bothered to link the mystical rumored fork that uses MutationObserver so here's the link: https://gist.github.com/double-beep/c4d4ec3866e5e54ae514c0aab60af242

Thanks, it's interesting to compare the two approaches. Maybe this is out of the scope of this page, but...what would be the advantage of using this script over Mutation Observer (and vice versa)?

For a modern dynamic webpage, you normally wouldn't want to naively attach a MutationObserver to the root of the document body (callback thrashing!). It's often a good idea to first poll for a particular parent element e.g. comment section container, and then attach a MO to that element. I like using ViolentMonkey's utility instead of MutationObserver directly. Here is an example userscript: Turn rumble comment timestamps into clickable links (here I'm using GM_wrench.wait() for more general purpose polling, not WFKE)

No one's bothered to link the mystical rumored fork that uses MutationObserver so here's the link: https://gist.github.com/double-beep/c4d4ec3866e5e54ae514c0aab60af242

Thanks, it's interesting to compare the two approaches. Maybe this is out of the scope of this page, but...what would be the advantage of using this script over Mutation Observer (and vice versa)?

Properly implemented, MutationObserver is now faster, more efficient, and more responsive. I do not think that linked script is the best example and it is not as flexible or robust as this WFKE. Nor is it a drop-in replacement. But if it, or any of the thousands of now available alternatives, works for you, then awesome. ;)

To be fair to me, this WFKE was written before MutationObserver was widely available and while it was particularly buggy and resource intensive. It performs well enough that I almost never need anything else and haven't felt a pressing need to refactor it yet.