What are the common causes?
The most obvious cause of this error are that you forgot to include jQuery before you used it, but there are also more subtle causes that you’ll not see until you deploy your site to production:
1. Your CDN-hosted jQuery might be blocked
If you are using a CDN-hosted version of jQuery such as Google’s Hosted Libraries, these CDNs might be blocked by a filter or proxy service on your customers' connection. We typically see this issue with requests originating from Chinese or Indonesian IP addresses.
2. Your CDN-hosted jQuery is down or timing out
Another common example of this bug is when the CDN hosting your jQuery script unreliable or slow to load. Browsers typically have a timeout of around 20-30 seconds for each script tag.
How can I fix this?
Luckily there is a simple solution to most of these issues. You can easily provide a locally-hosted fallback version of jQuery as follows:
// First try loading jQuery from Google's CDN <script src="//ajax.googleapis.com/ajax/libs/jquery/1.11.0/jquery.min.js"></script> // Fall back to a local copy of jQuery if the CDN fails <script> window.jQuery || document.write('<script src="http://mysite.com/jquery.min.js"><\/script>')) </script>
If you have a rails app, you can alternatively use the jquery-rails gem, which automates this fallback process for you.
This technique is used on many popular sites, including jquery.com, and solves most
jQuery is not defined errors. If the CDN jQuery fails load, it will almost certainly load fine from your own domain, but you'll also see the benefits of using a CDN-hosted jQuery for most of your users.
Is this is a problem on my site?