Javascript – Detect if any JavaScript function is running


I know it may sound very strange, but I need to know if there is any active/running javascript in the page.

I am in situation in which I have to run my javascript/jquery code after everything on the page is rendered and all other scripts have finished.

Is it possible to detect this?


Thank you all for the answers. Unfortunately, I was not able to find a solution, because I have no full control of what is going on the page.

Even, I was able to put my javascript in the end of the page, I think it will not be solution again. The reason is that when the page is rendering a function is triggered, it calls other functions and they calls other and so on. As a result, some of the data is incorrect and that's why i need to run my code to correct it.

I use setTimeout with 2 seconds to ensure that my code will be executed last, but this is ugly…

So, thank you all, but this is more problem with the system, not the js.

Best Solution

JavaScript on web browsers is single-threaded (barring the use of web workers), so if your JavaScript code is running, by definition no other JavaScript code is running.*

To try to ensure that your script occurs after all other JavaScript on the page has been downloaded and evaluated and after all rendering has occurred, some suggestions:

  • Put the script tag for your code at the very end of the file.
  • Use the defer and async attributes on the tag (they'll be ignored by browsers that don't support them, but the goal is to make yours the last as much as we can).
  • Hook the window load event via a DOM2 style hookup (e.g., addEventListener on browsers with standards support, or attachEvent on older IE versions).
  • In the load event, schedule your code to run after a setTimeout with a delay of 0ms (it won't really be zero, it'll be slightly longer).

So, the script tag:

<script async defer src="yourfile.js"></script>

...and yourfile.js:

(function() {
    if (window.addEventListener) {
        window.addEventListener("load", loadHandler, false);
    else if (window.attachEvent) {
        window.attachEvent("onload", loadHandler);
    else {
        window.onload = loadHandler; // Or you may want to leave this off and just not support REALLY old browsers

    function loadHandler() {
        setTimeout(doMyStuff, 0);

    function doMyStuff() {
        // Your stuff here. All images in the original markup are guaranteed
        // to have been loaded (or failed) by the `load` event, and you know
        // that other handlers for the `load` event have now been fired since
        // we yielded back from our `load` handler

That doesn't mean that other code won't have scheduled itself to run later (via setTimeout, for instance, just like we did above but with a longer timeout), though.

So there are some things you can do to try to be last, but I don't believe there's any way to actually guarantee it without having full control of the page and the scripts running on it (I take it from the question that you don't).

(* There are some edge cases where the thread can be suspended in one place and then allow other code to run in another place [for instance, when an ajax call completes while an alert message is being shown, some browsers fire the ajax handler even though another function is waiting on the alert to be dismissed], but they're edge cases and there's still only one thing actively being done at a time.)

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