1. beforeunload事件用于捕获选项卡关闭和后退按钮按下。
2. pageshow事件用于区分重新加载和实际页退出。
3. waspreaded标志防止函数在重新加载时触发。事件.preventDefault（）和事件。
4. beforeunload中的returnValue是可选的，用于显示确认对话框。

let wasReloaded = false;
    
    window.addEventListener('beforeunload', function (event) {
        if (wasReloaded) {
            return; // Skip execution if the page was reloaded
        }
    
        // Indicate that the page is about to unload
        // (can be used for confirmation dialogs or other actions)
        event.preventDefault();
        event.returnValue = 'Are you sure you want to leave?';
    
        // Trigger the function on actual page exit (not reload)
        window.addEventListener('unload', function () {
            if (!isSubmit) {
                sendDownloadEmail();
            }
        });
    });
    
    window.addEventListener('pageshow', function (event) {
        if (event.persisted) {
            wasReloaded = true;
        }
    });

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The best answer is here，也有一个不稳定的答案，你可以测试。

let before_unload_time
window.addEventListener('beforeunload', () => {
  before_unload_time = new Date().getTime()
})
window.addEventListener('unload', () => {
  if (new Date().getTime() - before_unload_time >= 4) {
    localStorage.test = 'refresh:' + before_unload_time
  } else {
    localStorage.test = 'close:' + before_unload_time
  }
})

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