window.onbeforeunload中的多个XHR调用

window.onbeforeunload = function() {
  // 创建一个Promise数组，用于存储所有的XHR调用
  var xhrPromises = [];

  // 发送第一个XHR调用
  var xhr1 = new XMLHttpRequest();
  xhr1.open('GET', 'https://example.com/api/data1', true);
  xhr1.send();
  xhrPromises.push(new Promise(function(resolve, reject) {
    xhr1.onload = function() {
      resolve(xhr1.responseText);
    };
    xhr1.onerror = function() {
      reject(xhr1.statusText);
    };
  }));

  // 发送第二个XHR调用
  var xhr2 = new XMLHttpRequest();
  xhr2.open('GET', 'https://example.com/api/data2', true);
  xhr2.send();
  xhrPromises.push(new Promise(function(resolve, reject) {
    xhr2.onload = function() {
      resolve(xhr2.responseText);
    };
    xhr2.onerror = function() {
      reject(xhr2.statusText);
    };
  }));

  // 等待所有的XHR调用完成
  Promise.all(xhrPromises)
    .then(function(results) {
      // 所有的XHR调用都已完成，可以执行其他操作
      console.log(results);
    })
    .catch(function(error) {
      // 处理错误
      console.error(error);
    });

  // 返回一个字符串，用于显示给用户
  return '您确定要离开此页面吗？';
};