code: function rand16Num(len) {
      let result = [];
      for (let i = 0; i < len; i++) {
          result.push('0123456789abcdef'.charAt(
              Math.floor(Math.random() * 16))
          );
      }
      return result.join('');
  }
docs: Generates a random GUID legal string of the given length.
  
  @param {number} len 要生成串的长度
  
  @return {string} 指定长度的16进制数随机串