Make my userscript wait for other scripts to load

[Edit: I'm replacing the original, confusing question with a simplified example demonstrating the problem.] Background I'm trying to write a userscript which will run in Chrome. This script needs to call a JavaScript function `AlertMe()` that is outside of the userscript -- this function is part of the page and contains variables that are generated dynamically on the server-side, so it isn't possible to re-write this function in my userscript. Code Script on the page (visit the page): ``` <script type="text/javascript"> function AlertMe() { alert("Function AlertMe was called!"); // then do stuff with strings that were dynamically generated // on the server so that I can't easily rewrite this into the userscript } </script> ``` My userscript (install it in Chrome): ``` function tryAlert() { if (typeof AlertMe == "undefined") { console.log('AlertMe is undefined.'); window.setTimeout(tryAlert, 100); } else { AlertMe(); } } tryAlert(); ``` The Problem When I tried to simply call the function, Chrome's console let me know that `AlertMe is not defined`. Thinking that this was because my userscript was running before all other scripts had been loaded, I used `setTimeout` to wait for the `AlertMe` function to become defined. Unfortunately, if you install the script then visit the page, you'll see that this just outputs `AlertMe is undefined.` forever and never calls the function. If you type `typeof AlertMe` into Chrome's console, it will correctly respond with `"function"`, so why is it that my userscript always thinks that `AlertMe` is undefined?