How to load local script files as fallback in cases where CDN are blocked/unavailable? [duplicate]

Question

To confirm that cdn script loaded you can check for existence any variable/function this script defines, if it is undefined - then cdn failed and you need to load local script copy.

On this principle are based solutions like that:

<script src="//ajax.googleapis.com/ajax/libs/jquery/1.5.1/jquery.min.js"></script>
<script>window.jQuery || document.write('<script src="js/libs/jquery-1.5.1.min.js">\x3C/script>')</script>

(if there is no window.jQuery property defined cdn script didn't loaded).

You may build your own solutions using this method. For instance, jquery tooltip plugin creates $.tooltip() function so we can check it with code like this:

<script>
    if (typeof $.tooltip === 'undefined') {
        document.write('<script src="js/libs/jquery.tooltip.min.js">\x3C/script>');
    }
</script>

I would have looked into a plugin like yepnopejs

yepnope([{
  load: 'http:/­/ajax.googleapis.com/ajax/libs/jquery/1.5.1/jquery.min.js',
  complete: function () {
    if (!window.jQuery) {
      yepnope('local/jquery.min.js');
    }
  }
}]);

Takes an array of object to check for, check the documentation at the site

<script src="//ajax.googleapis.com/ajax/libs/jquery/1.4.2/jquery.js"></script>
<script>!window.jQuery && document.write(unescape('%3Cscript src="js/libs/jquery-1.4.2.js"%3E%3C/script%3E'))</script>

Taken from HTML5 Boilerplate.

I use http://fallback.io/

  fallback.load({
        // Include your stylesheets, this can be an array of stylesheets or a string!
        page_css: 'index.css',

        // JavaScript library. THE KEY MUST BE THE LIBARIES WINDOW VARIABLE!
        JSON: '//cdnjs.cloudflare.com/ajax/libs/json2/20121008/json2.min.js',

        // Here goes a failover example. The first will fail, therefore Fallback JS will load the second!
        jQuery: [
            '//ajax.googleapis.com/ajax/libs/jquery/1.9.0/jquery.FAIL_ON_PURPOSE.min.js',
            '//ajax.googleapis.com/ajax/libs/jquery/1.9.0/jquery.min.js',
            '//cdnjs.cloudflare.com/ajax/libs/jquery/1.9.0/jquery.min.js'
        ],   .......

first thing - shouldn't you include them in different order?

something like this should work:

<script src="http://ajax.aspnetcdn.com/ajax/jquery.validate/1.7/jquery.validate.min.js"></script>
<script src="http://ajax.aspnetcdn.com/ajax/mvc/3.0/jquery.validate.unobtrusive.min.js"></script>
<script>jQuery.fn.validate || document.write('<script src="js/jquery.validate.min.js">\x3C/script><script src="js/jquery.validate.unobtrusive.min.js">\x3C/script>'</script>

what I'm doing here is simply checking if the first plugin (jQ validate) has been loaded. by checking for a static validate function on jQuery.fn object. I can't check the second script same way, because it's not adding anything anywhere, just proxying existing methods, so it's easier to assume that if the first one works, the second one will work too - after all, they are provided by the same CDN.