生成隨機字符串

randomString(len) {
      len = len || 6
      const chars = 'ABCDEFGHJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz'
      const charLen = chars.length
      let str = ''
      for (let i = 0; i < len; i++) {
        str += chars.charAt(Math.floor(Math.random() * charLen))
      }
      return str
    }

生成隨機字母字符串