Question

I am using Ruby on Rails v3.0.9 and jQuery 1.6. I am trying to retrieve and display the favicon image from some web sites.

For instance, in order to do that for a single web site I am using the following code:

$jQ.ajax({
  type:    'GET',
  url:     'http://www.facebook.com/favicon.ico',
  data:    '',
  error:   function(jqXHR, textStatus, errorThrown) {
    $jQ('#facebook_favicon').replaceWith('ERROR');
  },
  success: function(data, textStatus, jqXHR) {
    $jQ('#facebook_favicon').replaceWith("<img width='16px' height='16px' src='http://www.facebook.com/favicon.ico' />");
  }
});

I use the above code that performs an HTTP request in order to check if the favicon was found: if so (on success), it will replace the #facebook_favicon related to an HTML tag with the just found favicon.ico image otherwise it will replace that with an ERROR text.

However, when I load the page where the above JavaScript is placed and I inspect the Firebug Console I get the following:

It means that the request was successfully performed (HTTP status codes are all 200, except for Gmail) but I can not display the icon image on success. Why? What is the problem?

I noted that in Firebug Responses are all blank, like this:

What I can do to accomplish what I would like to make (I am referring mostly to check if a web site favicon is found)?

---

If I use code like the following it will work but it doesn’t check the favicon presence:

$jQ('#facebook_favicon').replaceWith("<img width='16px' height='16px' src='http://www.facebook.com/favicon.ico' />");

Answer 1

You can’t do XmlHttpRequests on URLs that are not on the same domain/port/protocol than the current page.

You can, however, check that an image exists by using an Image object and the onload and onerror events:

var img = new Image;
var src = 'http://www.facebook.com/favicon.ico';
img.onload = function() {
    // the image exists, display it
};
img.src = src;

You can wrap this in a function:

function imageExists(src, callback) {
    var img = new Image;
    img.onload = function() {
        callback(src, true);
    };
    img.onerror = function() {
        callback(src, false);
    };
    img.src=src;
}

And use it like this:

imageExists('http://www.facebook.com/favicon.ico', function(src, exists) {
    if (exists) {
        $('body').append('<p>The image loaded correctly!</p>');
        $('<img>').attr('src', src).appendTo('body');
    } else {
        $('body').append('<p>oops, error</p>');
    }
});

Test it here: http://jsfiddle.net/YpBsJ/2/