Run function only when tab/window is actually closed

In the end I did something really convoluted. But it works for me.

On the React front end, I make the call to delete as soon as the event is triggered. Since I can not detect if the user clicks to stay, I do setTimeout and give it 1s on the front end.

useEffect(() => window.addEventListener('beforeunload', async (e) => {
  e.preventDefault()
  e.returnValue = ''
  await axios.post('/api/delete', { stay: false })
  setTimeout(async () => {
   await axios.post('/api/delete', { stay: true })
  }, 1000)
 }), [])

And on the Express back end, I wait 2s for a cancellation to occur, if not then go ahead and delete.

let timer

app.post('/api/delete', (req, res) => {
  if (req.body.stay) {
    clearTimeout(timer)
  } else {
    clearTimeout(timer)
    timer = setTimeout(() => console.log('delete a file here'), 2000)
  }
  res.end()
})

If anyone can come up with something better I will accept that answer.