Detect Browser Compatibility with the Request Object

Here’s an example of an entire compatibility check:

  /* Wrapper function for constructing a request object.
   Parameters:
    reqType: The HTTP request type, such as GET or POST.
    url: The URL of the server program.
    asynch: Whether to send the request asynchronously or not. */

  function httpRequest(reqType,url,asynch){
      //Mozilla-based browsers
      if(window.XMLHttpRequest){
          request = new XMLHttpRequest();
      } else if (window.ActiveXObject){
          request=new ActiveXObject("Msxml2.XMLHTTP");
          if (! request){
              request=new ActiveXObject("Microsoft.XMLHTTP");
          }
      }
      //the request could still be null if neither ActiveXObject
      //initialization succeeded
      if(request){
          initReq(reqType,url,asynch);
      } else {
          alert("Your browser does not permit the use of all "+
                "of this application's features!");
      }
  }
 /* Initialize a request object that is already constructed */
  function initReq(reqType,url,bool){
      /* Specify the function that will handle the HTTP response */ 
  request.onreadystatechange=handleResponse;
      request.open(reqType,url,bool);
      request.send(null); 
  }

“Use the Request Object to POST Data to the Server” [Hack #2] shows how to implement a POST request withXMLHttpRequest.