createXHR(): 创建兼容浏览器的XMLHttpRequest

function createXHR()
{
    if (typeof XMLHttpRequest != "undefined")
    {
        return new XMLHttpRequest();
    } // end if
    else if (window.ActiveXObject)
    {
        var aVersions = ["MSXML2.XMLHttp.6.0", "MSXML2.XMLHttp.3.0"];
        for (var i = 0; i < aVersions.length; ++i)
        {
            try
            {
                var oXHR = new ActiveXObject(aVersions[i]);
                return oXHR;
            } // end try
            catch (oError)
            {
                // do nothing
            } // end catch
            
        } // end for
        
    } // end else if
    
    throw new Error("XMLHttp object could not be created.")
    
} // end createXHR();

来源: professional ajax 2nd edition, page39

function createXHR()
{
  try { return new XMLHttpRequest(); } catch (e) {}
  try { return new ActiveXObject("Msxml2.XMLHTTP.6.0"); } catch (e) {}
  try { return new ActiveXObject("Msxml2.XMLHTTP.3.0"); } catch (e) {}
  try { return new ActiveXObject("Msxml2.XMLHTTP"); } catch (e) {}
  try { return new ActiveXObject("Microsoft.XMLHTTP"); } catch (e) {}
  return null;
} // end createXHR()

来源: ajax完全手册中文版 62页