[JavaScript] Check Website Availability and Then Redirect by JSONP

Implement JSONP service at the website of a.com

I wrote three posts on how to implement JSONP service on Google App Engine, please refer to following posts:

• JSONP on Google App Engine Python
• JSONP with Anonymous Callback Function
• Use Object Instance Function as JSONP Callback Function

Insert JavaScript availability checking and redirection code at the website of b.com

Here comes the tricky part of the solution. We will use JSONP with Anonymous Callback Function to check if the website of a.com is available for the visitor, and if available, the visitor will be redirect to a.com. To achieve this, insert the following JavaScript to the webpage of the website of b.com (Assume the JSONP URL of a.com is /jsonp).

1
2
3
4
5
6
7
8
9

if (window.location.host == 'b.com') {
  var callback = function() {
    window.location = 'http://a.com' + window.location.pathname + window.location.search;
  };
  var qry = '?callback=' + encodeURIComponent('('+callback.toString()+')');
  var ext = document.createElement('script');
  ext.setAttribute('src', 'http://a.com/jsonp' + qry);
  document.getElementsByTagName("head")[0].appendChild(ext);
}

In line 2~4 is the JSONP callback function, which does only one thing: redirect the user to the website of a.com. If the website of a.com is down or not available, the callback function will not be executed and hence the visitor will still stay at the website of b.com. If the website of a.com is available, the callback function will be executed and hence the visitor will be redirected to a.com. This is exactly the solution we need.