socket.io/lib/xmlhttprequest.js

// browser shim for xmlhttprequest module
var hasCORS = require('has-cors');

module.exports = function(opts) {
  var xdomain = opts.xdomain;
  if (xdomain && 'undefined' != typeof XDomainRequest && !hasCORS) {
    return new XDomainRequest();
  }

  // XMLHttpRequest can be disabled on IE
  try {
    if ('undefined' != typeof XMLHttpRequest && (!xdomain || hasCORS)) {
      return new XMLHttpRequest();
    }
  } catch (e) { }

  if (!xdomain) {
    try {
      return new ActiveXObject('Microsoft.XMLHTTP');
    } catch(e) { }
  }
}