js实现背景图自适应窗口大小

<script type="text/javascript"> window.onresize = window.onload = function() {  var w,bgImg;  if(!!(window.attachEvent && !window.opera)) {   w = document.documentElement.clientWidth;  } else {   w = window.innerWidth;  }if(w>1650){   document.getElementById ('msg').value ='窗口大小：' + 'width:' + w;   bgImg = document.getElementById('bg').getElementsByTagName('img')[0];   bgImg.width = (w - 5);  }else{  document.getElementById ('msg').value ='窗口大小：' + 'width:' + w;  bgImg = document.getElementById('bg').getElementsByTagName('img')[0];  bgImg.width = 1650; } }</script>