How do i write a CDN fallback expression when loading a script asynchronously?

Question

I'm setting up my jQuery bundle as follows:

var bundle = new ScriptBundle("~/bundles/jquery", "//ajax.googleapis.com/ajax/libs/jquery/1.11.3/jquery.min.js");

// Path to the version of the file on your server (in case the CDN fails)
bundle.Include("~/scripts/jquery-{version}.js");

// JS expression to run, to test if CDN delivered the file or not
bundle.CdnFallbackExpression = "window.$";

bundles.Add(bundle);

In order to increase my Google PageSpeed Insights score I chose to load the script asynchronously:

@Scripts.RenderFormat(@"<script src=""{0}"" async></script>", "~/bundles/jquery")

But now, it seems to always fail the CDN fallback expression and ends up loading the script twice; once from my fallback bundle and once from the CDN when the async call finishes. I assume because when it runs the fallback test, the async call to the CDN hasn't finished.

<script src="//ajax.googleapis.com/ajax/libs/jquery/1.11.3/jquery.min.js" async=""></script>
<script>(window.$)||document.write('<script src="/bundles/jquery"><\/script>');</script>
<script src="/bundles/jquery"></script>

Is there a more intelligent way I can write my fallback expression?

Is there a better solution entirely?

I'd load jQuery closer to the bottom but then it'd break any inline calls that depend on it.

Answer 1

<script id="srequire" src="//ajax.googleapis.com/ajax/libs/jquery/1.11.3/jquery.min.jsWRONG" async="async"></script>
<script type="text/javascript">
    function sOnerror() {
        console.log("sOnerror");
        //if (typeof window.jQuery === 'undefined') {
            var scr = document.createElement("script");
            scr.src = "/bundles/jquery";
            scr.async = true;
            document.body.appendChild(scr);
       //}
    }
    var element = document.getElementById("srequire");
    console.log("element", element);
    if (element.addEventListener) {
        //element.addEventListener("load", onsuccess, false);
        element.addEventListener("error", sOnerror, false);
    } else if (element.attachEvent) {
        //element.attachEvent("onload", onsuccess);
        element.attachEvent("onerror", sOnerror);
    } else {
        //element.onload = onsuccess;
        element.onerror = sOnerror;
    }
</script>

for u to test it I put WRONG in the js name to check that the script works I checked it on ff chrome ie opra and edge you can use requirejs but that also fail sometimes from cdn that's why I use this script happy to share hope it helps

"If the load resulted in an error (for example a DNS error, or an HTTP 404 error) Executing the script block must just consist of firing a simple event named error at the element."

source w3.org