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;
}

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 :)