отреагировать перед загрузкой
window.addEventListener('beforeunload', (event) => {
event.returnValue = `Are you sure you want to leave?`;
});
TP
window.addEventListener('beforeunload', (event) => {
event.returnValue = `Are you sure you want to leave?`;
});
let formChanged = false;
myForm.addEventListener('change', () => formChanged = true);
window.addEventListener('beforeunload', (event) => {
if (formChanged) {
event.returnValue = 'You have unfinished changes!';
}
});