Fixed it to make sure it's not called twice. But still might have some issues I overlooked...

This one's interesting. I like the fault tolerance it adds.

// Provide a tick for any queued jsdom operations to wrap up without error            
setTimeout(() => dom.window.close(), 0)

Maybe we should make it an opt-in/opt-out option.

Alternatively we could also catch all errors after dom.window.close() and print a small note a la: "path/to/xxx.svelte" produced an error after the window was closed. This is most likely caused by the window closing before all async code has been resolved. Please verify the content of "/path/to/xxx.html". This message can be disabled by setting "tossr.strict" to false.

So the reason I was getting this error is basically that I was calling $ready() as soon as remote data was loaded. This appears to have caused a race condition between:

• The document being serialized
• Svelte operations to update the DOM

As you observed, the error seems to have occurred because the page was serialized and dom.window.close() called before Svelte had a chance to finish "doing its thing". So, 'twas user error, but still not a reason to fail this way, as valid HTML was returned.

On your suggestion: I'm not sure how you would be able to catch errors after dom.window.close() given how they are intercepted. This would work in a one-shot production serverless function, but in dev mode how would you know which rendering operation caused the error, given that you are just looking for any unhandled async rejection?

Also note that without the guard to call resolveHtml() only once, it was actually getting called twice during this race, as evidenced by that console.log message getting printed out twice. Perhaps the closing of the window caused $ready() to fire a second time.

Anyway, I'm using this code on Netlify and it seems to have gotten rid of all the silent failures, so I'll let you decide what to do with this. (Netlify doesn't like an error being thrown after the HTML is generated: if it hasn't sent the response back yet, it'll do something weird as in roxiness/routify-starter#96)

I was calling $ready() as soon as remote data was loaded

This should be perfectly fine. Any chance there's an async call running after the data is loaded?

I'm puzzled by resolveHtml being called twice. The three calls should be mutually exclusive. That is except for the event where $ready() is called after tossr has timed out.

removeEventListener should fix that.

if (dom.window._document) {
  console.log(`[tossr] ${url} Waited for the event "${eventName}", but timed out after ${timeout} ms.`);
  resolveHtml()
  // guard against a second call
  dom.window.removeEventListener(eventName, resolveHtml) //added
}

I will probably have to bite the bullet and write some more extensive tests for tossr soon.

Any chance there's an async call running after the data is loaded?

I'm puzzled by resolveHtml being called twice. The three calls should be mutually exclusive. That is except for the event where $ready() is called after tossr has timed out.

This might be the same problem. Some async code runs after load, fires the app-loaded event again, and resolveHtml() is called a second time before dom.window.close().

Debugging this further seems above my pay grade. Though I'm using these two commits in my deployment so far, and I haven't see any runtime errors or double-calls. I'll check back in here if I notice anything else.

Much appreciated! @mizzao