Dispatching an XMLHttpRequest

Dispatching an XMLHttpRequest
I'll start at the beginning of the Ajax sequence: creating and dispatching an XMLHttpRequest from the browser. Unfortunately, the method to create an XMLHttpRequest differs from browser to browser. The JavaScript function in Listing 2 smoothes out these browser-dependent wrinkles, detecting the correct approach for the current browser and returning an XMLHttpRequest ready to use. It's best to think of this as boilerplate code: simply copy it into your JavaScript library and use it when you need an XMLHttpRequest.

Code:

/*
 * Returns a new XMLHttpRequest object, or false if this browser
 * doesn't support it
 */
function newXMLHttpRequest() {

  var xmlreq = false;

  if (window.XMLHttpRequest) {

    // Create XMLHttpRequest object in non-Microsoft browsers
    xmlreq = new XMLHttpRequest();

  } else if (window.ActiveXObject) {

    // Create XMLHttpRequest via MS ActiveX
    try {
      // Try to create XMLHttpRequest in later versions
      // of Internet Explorer

      xmlreq = new ActiveXObject("Msxml2.XMLHTTP");

    } catch (e1) {

      // Failed to create required ActiveXObject

      try {
        // Try version supported by older versions
        // of Internet Explorer

        xmlreq = new ActiveXObject("Microsoft.XMLHTTP");

      } catch (e2) {

        // Unable to create an XMLHttpRequest with ActiveX
      }
    }
  }

  return xmlreq;
}