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;
}

A better link to the example is here: http://stackoverflow.com/questions/8281441/fire-greasemonkey-script-on-ajax-request/8283815#8283815

@hellonearthis And the problem itself is clearly described here: https://stackoverflow.com/questions/12897446/greasemonkey-wait-for-page-to-load-before-executing-code-techniques/12899331#12899331

Not sure if this has already been accomplished, but I couldn't find an example of this without the need for JQuery. I forked this and removed reliance on JQuery, though I had to hobble it by disabling the iframe search. Might help some though, and it works for my project. Thanks for your work @BrockA

@mjblay What a crazy coincidence. I have been re-writing it as well within this past week to no longer rely on jQuery, and I also ended up removing the iframe feature until I had more time to re-write that part in vanillajs. I had published a gist a couple of days ago, but I removed it after encountering a few bugs. I checked out your Gist/Github account and couldn't find the fork. Are you planning on publishing it on Github?

@mjblay, you're welcome.

@insanid I inadvertently deleted it. I re-added the fork here: https://gist.github.com/mjblay/18d34d861e981b7785e407c3b443b99b. I haven't tested it thoroughly, but it seems to be working on a private site I maintain and use daily.

Can this act within an iframe that is itself within an iframe?

@Gallion, not directly but it can help. If you need to, open a question about that on some place like Stack Overflow and give details and a working test case or link to the target page.

It's a very useful script but recently I met an issue. If waitForKeyElements already finds the element at the first run and the element disappears. The timer cannot be cleared since the element is not found anymore.

@EliezerWu, Even when you set bWaitOnce to true?
Also, letting the timer keep running is not normally a problem.

Anyway, post a "Minimal, Complete, and Verifiable example" if you want me to investigate this further.

@BrockA,

I don't know if you still want/need this, but I consider it:

Creative Commons, Attribution-NonCommercial-ShareAlike 4.0 International (CC BY-NC-SA 4.0)

I'm not going to lawyer-up in any case.

Hi =)
How can i do for commercial use?
It would be used to dynamically generate buttons with javascript codes to automate tasks in my work =)

thanks.

@josepelupi, And anyone else who wants to use this commercially...

Consider this free to use commercially until your install base exceeds twenty-thousand users. At which point, any fee would probably consist of a modest (3 digits or less) donation to EFF or similar.

You could add a comment to your code: "Used by permission of github.com/BrockA until install base exceeds twenty-thousand users."

How can i do for commercial use?
It would be used to dynamically generate buttons with javascript codes to automate tasks in my work =)

@BrockA,

Many thanks for the reply. But i believe i was not clear enough in my question.

I have no intention of creating scripts to sell.

Im creating a script to share with my co-workers that automates parts of the activities we exercise while working (eg select options from a dropdown menu, fill out forms). So, my intention is to use waitForKeyElements to facilitate and allow the creation of dynamic buttons according to the elements that exist on the page.

So, i would like to know if it is allowed to include your code in my greasemonkey script to use it , "indirectly", for the purpose of commercial use.

thanks =)

Consider this free to use commercially until your install base exceeds twenty-thousand users. At which point, any fee would probably consist of a modest (3 digits or less) donation to EFF or similar.

@josepelupi, I did understand you. What you propose is still commercial use. If you get 20K people who use this, then let me know and we'll hash out a tiny donation, not to exceed $100, unless you cause me to incur costs in time or money.

But, I might suggest that if your script does approach 20K users, then some intranet team has not done their job well.

Until you get 20K people using the script, just comment in the code that you are legally covered.

This is a great script! I have modified it with what I feel are some important improvements (not requiring jQuery, not using setInterval(), allowing a selector function instead of a string).

That's good, @CoeJoder. You might also consider one of the other forks (almost 100 on Gist alone) that uses MutationObserver.

Yes, there are some quite fancy implementations among the forks. I would personally avoid using MutationObserver unless needed, and also wanted to get the script into a regular github repo so it can be used with common CDNs (for some reason they tend not to work with gists).

I want to move the function into an object in a separate file that I call libraries.js. The object is called singleton as in

singleton.waitForTargetElements()

However, it throws an undefined error for waitForTargetElements at this line:

let controlObj = waitForTargetElements.controlObj || {};

If I add waitForTargetElements to the list of parameters, then controlObj throws an undefined error.

So my question is how do I decouple the scope so that the function can be placed in another file but can still be called?

This is a great script! I have modified it with what I feel are some important improvements (not requiring jQuery, not using setInterval(), allowing a selector function instead of a string).

THANKS EXACTALY WHAT I NEEDED.
BrockA Version , doesn't work when userscript //@run-at is set to document-start

@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).