```js
  const setDirty = _ => { window.dirty = true; document.body.classList.add("dirty") }
  const clearDirty = _ => { window.dirty = false; document.body.classList.remove("dirty") }
  window.clearDirty()

  window.addEventListener("beforeunload", function(event) {                                                              
    if( window.dirty ) {
      event.returnValue = "There are unsaved changes.";
      return
    }
    event.preventDefault()
  })
```