SOURCE

  //字符编码数值对应的存储长度：     
  //UCS-2编码(16进制) UTF-8 字节流(二进制)    
  //0000 - 007F       0xxxxxxx （1字节）     
  //0080 - 07FF       110xxxxx 10xxxxxx （2字节）     
  //0800 - FFFF       1110xxxx 10xxxxxx 10xxxxxx （3字节）    
  String.prototype.getBytesLength = function() {   
      var totalLength = 0;     
      var charCode;  
      for (var i = 0; i < this.length; i++) {  
        charCode = this.charCodeAt(i);  
         if (charCode < 0x007f)  {     
             totalLength++;     
         } else if ((0x0080 <= charCode) && (charCode <= 0x07ff))  {     
             totalLength += 2;     
         } else if ((0x0800 <= charCode) && (charCode <= 0xffff))  {     
             totalLength += 3;   
         } else{  
             totalLength += 4;   
         }          
     }  
     return totalLength;   
 }  
 var str="∀";
 console.log("字符数"+str.length+" ，字节数"+str.getBytesLength());