Great question! Browsers intentionally restrict direct detection of registered URL schemes for security and privacy reasons, but there are reliable indirect methods to handle this scenario. Here's how you can approach it:

The Timer + Visibility State Trick #

This is the most widely used method, leveraging the fact that when a registered app opens, the browser tab will lose focus (and its visibility state will switch to hidden). If the app isn't registered, the browser won't switch away, so we can detect that with a timer.

Here's a working code example tailored to your use case:

<a href="customapp:12345" id="launchAppBtn">Open My App</a>

<script>
document.getElementById('launchAppBtn').addEventListener('click', (e) => {
  // Capture the exact time of the click
  const clickTimestamp = Date.now();

  // Set a reasonable timeout (adjust based on typical app launch times)
  setTimeout(() => {
    // Check if the page is still visible after the timeout
    if (document.visibilityState === 'visible') {
      // If visible, the app likely wasn't registered/launched
      alert('It looks like our app isn\'t installed yet! Please download it to continue.');
      // You could replace the alert with a styled modal or redirect to your app's download page
    }
  }, 2000); // 2 seconds is a safe default; tweak if needed
});
</script>

Key Notes on This Method: #

• Browser Behavior Variations: Some browsers (like Chrome) will show a system prompt asking if the user wants to open the app even if it's registered. If the user clicks "Cancel", the page remains visible, so the timer will still trigger the install prompt—this is expected behavior.
• Timeout Adjustment: If your app takes longer to launch (e.g., a large desktop app), increase the timeout to 3-4 seconds to avoid false positives.
• Edge Cases: If the user switches back to the browser tab within the timeout window, you might get a false positive. This is rare, but you can mitigate it by checking the time difference between the click and the timeout (e.g., only trigger if more than 1.5 seconds have passed and the page is still visible).

Why Direct Detection Isn't Possible #

Browsers block direct access to scheme registration status to prevent malicious sites from fingerprinting users (e.g., detecting which apps are installed on their system). So all solutions rely on indirect behavioral cues like the visibility state trick above.

Bonus: Enhance the User Experience #

Instead of a basic alert, you can:

• Show a styled modal with a download button pointing to your Windows app's installer.
• Add a fallback link to your web-based alternative if the user can't or won't install the native app.

内容的提问来源于stack exchange，提问作者Michael Chourdakis