Limit the input character length to 1 character in English and 2 characters in Chinese

js Although it provides a function to calculate the number of bytes occupied by a string , However, the number of bytes occupied by Chinese characters cannot be calculated correctly , Such as document.form1.username.value=“ Hello, Shanghai ”,document.form1.username.value.length The return is 5 instead of 10, So it brings some trouble to programming , Such as restricting the input of fixed length characters , Intercepting fixed length strings can not achieve the desired effect because of this problem , The following is a simple function written by myself , Used to intercept a fixed length string , Both Chinese and English are applicable .

// js Intercepting string , Both Chinese and English can be used 
//  If the given string is greater than the specified length , Intercepts the specified length and returns , If not, return the source string .
    cutstr(str,len) {
        var str_length = 0;
        var str_len = 0;
        var str_cut = new String();
        var str_len = str.length;
           for(var i = 0;i<str_len;i++)
          {
             var a = str.charAt(i);
             str_length++;
             if(escape(a).length > 4)
             {
              // The length of Chinese characters after encoding is greater than 4
              str_length++;
              }
              str_cut = str_cut.concat(a);
              if(str_length>=len)
              {
              str_cut = str_cut.concat("...");
              return str_cut;
              }
         }
         // If the given string is less than the specified length , Then return the source string ;
         if(str_length<len){
          return  str;
         }
    },

Such as cutstr(" Hello, Shanghai ",6); What will be returned will be : How do you do ...