win.webContents.on('will-prevent-unload', event => {
    let choice = dialog.showMessageBoxSync({
      title:'do you want to close',
      buttons:['cancel','yes']
    });
    if(choice === 1)  event.preventDefault();
    //...
  })

  win.webContents.on('will-prevent-unload', async event => {
    let choice = await dialog.showMessageBox({
      title:'do you want to close',
      buttons:['cancel','yes']
    });
    if(choice === 1) event.preventDefault();  //this code is useless
    //...
  })