var phoneWidth =  parseInt(window.screen.width); 
    var phoneScale = phoneWidth/640; 
    var ua = navigator.userAgent; 
    if (/Android (\d+\.\d+)/.test(ua) || /ios (\d+\.\d+)/.test(ua)){ 
        var version = parseFloat(RegExp.$1); 
        if(version>2.3){ 
            document.write('<meta name="viewport" content="width=640, minimum-scale = '+phoneScale+', maximum-scale = '+phoneScale+', 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">'); 
    }