How can I handle errors in loading an iframe?

To detect whether your server is down or not, you can include an empty script file from your own domain. When the server is down, the onerror event handler will fire:

var el = document.createElement('script');
el.onerror = errorFunction;
el.src = "somebogusscript.js?" + new Date().getTime();
document.body.appendChild(el);

Note: don't forget to add a random string to the src attribute to avoid the client using a cached version (which could stop a look at the server at all).

Perhaps you could try onErrorUpdate for the event handler? I couldn't see an onError handler for iFrames. If that doesn't work, you could try onLoad and then check the source of the iframe or the title of it for a 404 message.

Such as: if (frameDoc.title == 'title the server sends for 404') {

Source:

http://bytes.com/topic/javascript/answers/166288-catch-404-when-using-iframe

iFrame Methods: http://www.java2s.com/Code/HTMLCSSReference/HTML-Tag-Reference/iframeJavaScriptMethods.htm

iFrame Properties: http://www.java2s.com/Code/HTMLCSSReference/HTML-Tag-Reference/iframeJavaScriptProperties.htm

One technique is to set a JavaScript timeout when you make the request. If your timeout fires before the iframe onload event, the content didn't load. You could then set iframe.src to about:blank, delete, or reuse the iframe.