How to stop closing a page with unsaved changes.

function beforeUnloadListener(event) {
event.preventDefault();
return event.returnValue = ‘Are you sure you want to exit?’;
};

// A function that invokes a callback when the page has unsaved changes.
onPageHasUnsavedChanges(() => {
window.addEventListener(‘beforeunload’, beforeUnloadListener);
});

// A function that invokes a callback when the page’s unsaved changes are resolved.
onAllChangesSaved(() => {
window.removeEventListener(‘beforeunload’, beforeUnloadListener);
});