JavaScript function to make an AJAX call

How many times have you had to go to the web, to look how to make a proper AJAX call? Many? Well.. Not anymore. Use this JavaScript function to make an AJAX call and process the response.

/**
* Purpose: to make a client-side request to the server, and obtain the response
*
* Input Params:
*  1. pURL - string - request URL
*  2. pPostVars - string - POST variables
*  3. pCallbackFunc - function reference - function that is to be called on 
*     successful response
*  4. pAsync - int - if request to be asynchronous - 1/0
*
* Output Params:   none
*
* Return Value:
*  on success - string - the response from the server
*  on failure - bool - false
*
* Notes:
*  1. By default the request will *NOT* be asynchronous.
*     This behaviour can be changed by the param pAsync.
*     1 is for asynchronous, and 0 is for synchronous.
*  2. By default the request method is GET. If however pPostVars is passed to the 
*     func, then the request method will be POST.
*     pPostVars will a set of post variables as
*     "var1=val1&var2=val2&var3=val3&varn=valn"
*  3. pCallbackFunc is a reference to a JS function. If specified, this function must 
*     mandatorily take a parameter to which the response string will be passed.
*  4. In most cases, if the request is going to be asynchronous, then it is always 
*     better to pass a callback function to do needful scripting after the response
*/
function GetServerResponse ( pURL, pPostVars, pCallbackFunc, pAsync )
{
var http_request = false;
var http_method = "GET";
var request_type = false;
var server_response;

if (pPostVars && pPostVars != "") var http_method = "POST";
if (pAsync && pAsync == 1) request_type = true;

// Browser - IE
if (window.ActiveXObject)
{
  try { http_request = new ActiveXObject ("Msxml2.XMLHTTP"); }
  catch (e)
  {
    try { http_request = new ActiveXObject ("Microsoft.XMLHTTP"); }
    catch (e) {}
  }
}

// Browser - Mozilla, ...
else if ( window.XMLHttpRequest )
{
  http_request = new XMLHttpRequest ();

    // some versions of Mozilla browsers won't work properly if
    // response from server doesn't have xml mime-type header
  if (http_request.overrideMimeType)
  {
    http_request.overrideMimeType("text/xml");
  }
}

  // unable to create xmlhttp obj
if (!http_request)
{
  alert('Error : Cannot create an XMLHTTP instance');
  return false;
}

  // return server output on successful retrieval
http_request.onreadystatechange = function() {
if ( http_request.readyState == 4 )
{
    // successfully got response
  if ( http_request.status == 200 )
  {
      // case : if callback function specified,
      // pass on the server response string to it as a parameter
    if ( pCallbackFunc && typeof(pCallbackFunc) == "function")
    {
      pCallbackFunc(http_request.responseText);
    }

    server_response = http_request.responseText;
  }

  else
  {
    var err = 'Error : Server returned a status code : ';
    err += http_request.status;
    alert ( err );
    server_response = false;
  }
}
};

  // GET method
if ( http_method == "GET" )
{
  http_request.open ( "GET", pURL, request_type );
  http_request.send ( null );
}

  // POST method
else if ( http_method == "POST" )
{
  http_request.open ( "POST", pURL, request_type );
  var post_ctype = "application/x-www-form-urlencoded";
  http_request.setRequestHeader ( "Content-Type", post_ctype );
  http_request.send ( pPostVars );
}

return server_response;
}