How can I modify the XMLHttpRequest responsetext received by another function?

Question

I am trying to modify the responseText received by a function that I cannot modify. This function creates a XMLHttpRequest that I can attach to, but I have been unable to \"

Answer 1

I needed to intercept and modify a request response so I came up with a little bit of code. I also found that some websites like to use response as well as the responseText which is why my code modifies both.

The Code

var open_prototype = XMLHttpRequest.prototype.open,
intercept_response = function(urlpattern, callback) {
   XMLHttpRequest.prototype.open = function() {
      arguments['1'].match(urlpattern) && this.addEventListener('readystatechange', function(event) {
         if ( this.readyState === 4 ) {
            var response = callback(event.target.responseText);
            Object.defineProperty(this, 'response',     {writable: true});
            Object.defineProperty(this, 'responseText', {writable: true});
            this.response = this.responseText = response;
         }
      });
      return open_prototype.apply(this, arguments);
   };
};

the first param of the intercept_response function is a regular expression to match the request url and the second param is the function to be used on the response to modify it.

Example Of Usage

intercept_response(/fruit\.json/i, function(response) {
   var new_response = response.replace('banana', 'apple');
   return new_response;
});