获取准确字符串字节长度

function byte_length(s){
var sum = 0;
for(var i=0;i<s.length;i++){
if(s.charCodeAt(i)<=0x7F){
sum++;
}else if(s.charCodeAt(i)<=0x7FF){
sum += 2;
}else if(s.charCodeAt(i)<=0xFFFF){
sum += 3;
}else if(s.charCodeAt(i)<=0x1FFFFF){
sum += 4;
}else if(s.charCodeAt(i)<=0x3FFFFFF){
sum += 5;
}
}
return sum;
}