function encodeBase64(str) { return bto...

text
function encodeBase64(str) {
  return btoa(unescape(encodeURIComponent(str)));
}

document.getElementById('submit-btn').onclick = async function() {
  const codeInput = document.getElementById('code');
  const outputElem = document.getElementById('output');
  const stageIndicator = document.getElementById('stage-indicator');
  const firstChallenge = document.getElementById('first-challenge');
  const secondChallenge = document.getElementById('second-challenge');
  const code = codeInput.value;

  if (!code.trim()) {
    alert('代码不能为空！');
    return;
  }

  this.disabled = true;
  document.getElementById('output-area').style.display = "none";
  
  const payload = { code: encodeBase64(code) };
  let outputText = '', isErr = false, isSuccess = false;

  try {
    const resp = await fetch('/run', {
      method: 'POST',
      headers: {'Content-Type': 'application/json'},
      body: JSON.stringify(payload)
    });
    const data = await resp.json();

    if (data.output !== undefined) {
      outputText = data.output;
    } else if (data.error) {
      outputText = data.error;
      isErr = true;
    } else if (data.message) { 
      outputText = data.message;
      isSuccess = true;
      stageIndicator.textContent = '当前阶段：第二关 (golf)';
      codeInput.value = '';
      codeInput.placeholder = '第二关：编写代码完成序数词挑战，要求长度小于 37 字符。';
      
      // 隐藏第一关示例并显示第二关示例
      firstChallenge.style.display = "none";  // 隐藏第一关示例
      secondChallenge.style.display = "block"; // 显示第二关示例
    } else if (data.flag) { 
      outputText = '恭喜你！成功获得 Flag：\n\n' + data.flag;
      isSuccess = true;
      stageIndicator.textContent = '挑战完成！🎉';
    } else {
      outputText = JSON.stringify(data, null, 2);
    }
  } catch (e) {
    outputText = '网络或服务器错误：' + e;
    isErr = true;
  }

  outputElem.textContent = outputText;
  outputElem.className = isErr ? "err" : (isSuccess ? "success" : "");
  document.getElementById('output-area').style.display = "block";
  this.disabled = false;
};

帮我分析一下这道js代码，获得flag