How to only have one instance of a window open while feeding info into it.

Multi instance comms.js

let isMainWindow = false; // keep track of main instance.
// Of course the main instance should be kept track of in a better way than this, perhaps find a way of uniquely identifying the tabs
// and storing the ID of the active tab in localStorage.

window.addEventListener('load', e => {

  if (!window.localStorage.mainWindowVacant) window.localStorage.mainWindowVacant = "true"; // initialsise window.localStorage
  if (!window.localStorage.openInstances) window.localStorage.mainWindowVacant = "true"; // initialsise window.localStorage

  if (window.localStorage.mainWindowVacant !== "false") {
    // Take Over Main Instance
    isMainWindow = true;
    window.localStorage.mainWindowVacant = "false";
    watchInstance();
  } else { // can't take over main window
    window.localStorage.openInstances += (document.querySelector('.name').value || "untitled") + ", ";
    //exit
    window.closeHandler();
    alert('closing');
  }
})

window.addEventListener('unload', closeHandler); // listen for a close event and vacate main position

function closeHandler() {
  if (isMainWindow) {
    window.localStorage.mainWindowVacant = "true"; // this "vacates" the main window position allowing another tab to take the main window's place. ie the main window has been closed.
  }
}

async function watchInstance() {
  let prevAmnt = window.localStorage.openInstances;
  setInterval(e => {
    if (window.localStorage.openInstances !== prevAmnt) {
        // do some logic once the openInstances updates (ie a new tab has been opened and is ready to be migrated to main tab)
      showOpenInstances();
    }
  }, 1)
}

async function showOpenInstances() { 
    // just a function that provides feedback to the user once a new tab has been opened.
    // use this function to update the tabs list and load other stuff.
    // Put some kind of logic here that will focus the window. If that's even possible.
  const instances = window.localStorage.openInstances.split(', ');
  document.querySelector('.body').innerHTML = instances.reduce((a, i) => a + i + "<br />");
}

// @RahulJain