判断IE版本的代码

var ie = ( function() {
  var undef = undefined,
      v = 3,
      div = document.createElement( 'div' ),
      all = div.getElementsByTagName( 'i' )

  while (
    div.innerHTML = '<!--[if gt IE ' + (++v) + ']><i></i><![endif]-->',
    all[ 0 ]
  );

  return v > 4 ? v : undef
})()