code: function randomString(len = 32) {
    let code = '';
    const chars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789';
    const maxPos = chars.length;
    for (let i = 0; i < len; i += 1) {
      code += chars.charAt(Math.floor(Math.random() * (maxPos + 1)));
    }
    return code;
  }
docs: 生成指定长度的随机字符串