JavaScript String Byte Length of UTF-8

function byteLengthOf(s){
    //assuming the String is UCS-2(aka UTF-16) encoded
    var n=0;
    for(var i=0,l=s.length; i<l; i++){
        var hi=s.charCodeAt(i);
        if(hi<0x0080){ //[0x0000, 0x007F]
            n+=1;
        }else if(hi<0x0800){ //[0x0080, 0x07FF]
            n+=2;
        }else if(hi<0xD800){ //[0x0800, 0xD7FF]
            n+=3;
        }else if(hi<0xDC00){ //[0xD800, 0xDBFF]
            var lo=s.charCodeAt(++i);
            if(i<l&&lo>=0xDC00&&lo<=0xDFFF){ //followed by [0xDC00, 0xDFFF]
                n+=4;
            }else{
                throw new Error("UCS-2 String malformed");
            }
        }else if(hi<0xE000){ //[0xDC00, 0xDFFF]
            throw new Error("UCS-2 String malformed");
        }else{ //[0xE000, 0xFFFF]
            n+=3;
        }
    }
    return n;
}