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There are several ways to communicate between browser windows

  • Server conversation (async, network delays)
  • localstorage (sync)
  • BroadcastChannel (most modern, does not appear to be supported in Safari)
  • IndexedDB (async)

It looks like localstorage will do for the current application’s purposes of launching another window to do some work and then report back a limited amount of information and closing.


The remote client window uses localstorage.setItem(key,value) to talk back to main window. Both key and value must be strings. JSON.stringify(object) may help.

Main window uses window.addEventListener("storage", storageEventHandler, false) to register the function storageEventHandler(evt) to be notified when the storage is modified. The main window can then examine evt.key to get the key of the item in local storage that changed and evt.newValue to get the new value of the changed item. Alternately, if only one key is in involved, the main window can use localStorage.getItem(key).

Demo Code

The demo must run in a server context not file:// and the browser windows should not be in “private” mode.

  <h1>Web localStorage (read)</h1>
  <p><a href="client.html" target="_blank">Load Client</a></p>
  <h1><span id='theString'></span></h1>

<script src="scriptRead.js"></script>

Body of HTML page

"use strict";

window.addEventListener("storage", storageEventHandler, false);
const answerFieldSpan = document.getElementById("theString");

function storageEventHandler(evt) {
  // let theString = localStorage.getItem("theString");
  // answerFieldSpan.innerText = theString;

  answerFieldSpan.innerText = `${evt.key} : ${evt.newValue}`;


Simulation of client window (launched by main window or manually)

	<h1>Graphic Page Writing Back via Local Storage</h1>
	<form action=""></form>
	<label for="the-string">String to store:</label>
	<input type="text" name="saveValue" id="the-string">
	<button id='save'>Save</button>

<script src="scriptSet.js"></script>

Body of HTML page

"use strict";

let theString = "stop";
let check;

const saveButton = document.getElementById("save");
const theStringField = document.getElementById("the-string");

saveButton.onclick = (ev) => {
  theString = theStringField.value;
  localStorage.setItem("theString", theString);
  check = localStorage.getItem("theString");




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