How to load local script files as fallback in case

Question

This question already has an answer here:

• Best way to use Google's hosted jQuery, but fall back to my hosted library on Google fail 22 answers

I’m using a CDN for the following javascript:

• https://ajax.googleapis.com/ajax/libs/jquery/1.5.1/jquery.min.js
• https://ajax.googleapis.com/ajax/libs/jqueryui/1.8.10/jquery-ui.min.js
• http://ajax.aspnetcdn.com/ajax/mvc/3.0/jquery.validate.unobtrusive.min.js
• http://ajax.aspnetcdn.com/ajax/jquery.validate/1.7/jquery.validate.min.js

For each one, how can I revert to using local copy in the instance where it may be blocked/unavailable?

Answer 1

You need to know, how you can make sure that a lib was loaded successfully. For instance:

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

So this trys to load jQuery (1.5.1) from the google CDN. Since <script> tags do block the overall render & execution process (if not explicitly told different), we can check right after that if the jQuery object is found within window. If not, just fallback by writing another <script> tag into the document, referencing a local copy.

Answer 2

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'
        ],   .......

Answer 3

Best to do all this script loading with your own Javascript code.

First try to load the CDN file by inserting a new SCRIPT element into the DOM. Then check that it has loaded by looking for an object that it defines. If the object does not appear, then insert another SCRIPT element to load the local copy. Probably best to clean up the DOM and remove SCRIPTs which failed to load as well.

Don't forget to account for timing issues, i.e. load is not instant.

Answer 4

I answered a similar questions at jquery ui - how to use google CDN

You can make the call using

<link rel="stylesheet" href="http://ajax.googleapis.com/ajax/libs/jqueryui/1.8.9/themes/base/jquery-ui.css" type="text/css" media="all" /> 
<script src="http://ajax.aspnetcdn.com/ajax/jQuery/jquery-1.5.min.js" type="text/javascript"></script>
<script src="http://ajax.googleapis.com/ajax/libs/jqueryui/1.8.9/jquery-ui.min.js" type="text/javascript"></script>

You can also link to other Ui themes by changes the name of the theme. In This case change the name base to any other theme name /base/jquery-ui.css to any other theme.

<link rel="stylesheet" href="http://ajax.googleapis.com/ajax/libs/jqueryui/1.8.9/themes/base/jquery-ui.css" type="text/css" media="all" />

Check out the jQuery UI Blog for a link of all CDN links http://blog.jqueryui.com/

If you want to revert back to your host in case Google failed, you can do

<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.5.1/jquery.min.js"></script>
<script type="text/javascript">
if (typeof jQuery == 'undefined')
{
    document.write(unescape("%3Cscript src='/jquery.js' type='text/javascript'%3E%3C/script%3E"));
}
</script>

Answer 5

I don't think any of the solutions could be painless if the related CDN is really filtered out. (like for example iptables/drop, misconfigured routers.) If you don't trust them use everything locally... And you spare yourself a few surprises/calls from users on exotic network.