The userscript causes interference with itself when it is opened on multiple tabs simultaneously

I'm facing an issue with Greasemonkey 3.17 on FF56 and Tampermonkey on Vivaldi that doesn't seem to have any solution on Google.

Here's an example script:

// ==UserScript==
// @name        stackoverflow.com
// @namespace   siod87gbnwf87rnsfdkn
// @include     https://stackoverflow.com/*
// @version     1
// ==/UserScript==

alert(1);
alert(2);

It's a simple script that should display one message box followed by another after clicking OK. The issue arises when performing the following steps:

  • Visit stackoverflow.com

  • Open a question in a new tab using the middle mouse button twice rapidly, so that the second tab opens as the first one is still loading

Both tabs will display "1" in the popup box, but you'll notice that upon clicking OK on the first tab, the script pauses until you click OK on the second tab as well. Only after dealing with both popups from the second tab will the alert on the first tab trigger.

Why aren't the scripts running independently of each other? This behavior isn't specific to alert, as I've encountered the same issue with more complex scripts. However, for demonstration purposes, the alert function illustrates the problem effectively.

javascriptgreasemonkeyuserscriptstampermonkey

Answer №1

What is the reason scripts cannot run independently from one another?

Scripts are considered to run separately as they do not share the global scope. However, certain blocking actions like the alert function are controlled by the browser itself, and the sequence of execution is not defined in the ECMAScript standard.

To avoid this issue, it is recommended to refrain from using blocking functions such as alert. Instead, opt for non-blocking methods like console.log or console.info for displaying output.

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