JS制作手机端自适应缩放显示

<script>
var _width = parseInt(window.screen.width);
var scale = _width/640;
var ua = navigator.userAgent.toLowerCase();
var result = /android (\d+\.\d+)/.exec(ua);
if (result){
var version = parseFloat(result[1]);
if(version>2.3){
document.write('<meta name="viewport" content="width=640, minimum-scale = '+scale+', maximum-scale = '+scale+', target-densitydpi=device-dpi">');
}else{
document.write('<meta name="viewport" content="width=640, target-densitydpi=device-dpi">');
}
} else {
document.write('<meta name="viewport" content="width=640, user-scalable=no, target-densitydpi=device-dpi">');
}
</script>

<script type="text/javascript" src="http://code.jquery.com/jquery-1.7.2.min.js"></script>
<script type="text/javascript">
$(function(){
 if (navigator.userAgent.match(/iPhone/i) || navigator.userAgent.match(/iPad/i)) {
  var viewportmeta = document.querySelector('meta[name="viewport"]');
   if (viewportmeta) {
    viewportmeta.content = 'width=device-width, minimum-scale=1.0, maximum-scale=1.0, initial-scale=1.0';
  document.addEventListener('touchstart', function () {
   viewportmeta.content = 'viewportmeta.content = width=device-width, minimum-scale=0.25, maximum-scale=1.6';
  }, false);
   document.addEventListener('orientationchange', function () {
   viewportmeta.content = 'viewportmeta.content = width=device-width, minimum-scale=1.0, maximum-scale=1.0, initial-scale=1.0';
  }, false);
   }
  }
});
</script>

<meta charset="UTF-8">
<meta content="width=device-width, initial-scale=1.0, maximum-scale=1.0, user-scalable=0" name="viewport">
<meta content="yes" name="apple-mobile-web-app-capable">
<meta content="black" name="apple-mobile-web-app-status-bar-style">
<meta content="telephone=no" name="format-detection">