Антон Богдашов

Все просто, работай через events

[start page]

const childWindow = window.open('/training');
const onChangeTxtContent = (txt) => {
     if (childWindow.closed) {
         //train window closed
         return;
     }
     const event = new CustomEvent('txt-content', {detail: {txt}});
     childWindow.dispatchEvent(event);
}

[training]

window.addEventListener('txt-content', ({detail})=>{
   console.log(detail?.txt) // here is txt from parent (start) page
})

В customEvent в detail помимо txt можно пихнуть все что угодно