I faced a similar problem, trying to test if jQuery is already present on a page, and if not force it's load, and then execute a function. I tried with @David Hellsing workaround, but with no chance for my needs. In fact, the onload instruction was immediately evaluated, and then the $ usage inside this function was not yet possible (yes, the huggly "$ is not a function." ^^).
So, I referred to this article : https://developer.mozilla.org/fr/docs/Web/Events/load and attached a event listener to my script object.
var script = document.createElement('script');
script.type = "text/javascript";
script.addEventListener("load", function(event) {
console.log("script loaded :)");
onjqloaded();
});
script.src = "https://ajax.googleapis.com/ajax/libs/jquery/3.3.1/jquery.min.js";
document.getElementsByTagName('head')[0].appendChild(script);
For my needs, it works fine now. Hope this can help others :)