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otherWindow.postMessage(message, targetOrigin, [transfer]);

<div>
    <input id="text" type="text" value="tutorialpro" />
    <button id="sendMessage">Send Message</button>
</div>
<iframe loading="lazy" id="receiver" src="https://c.tutorialpro.org/tutorialprotest/postMessage_receiver.html" width="300" height="360">
    <p>Your browser does not support iframes.</p>
</iframe>
<script>
window.onload = function() {
    var receiver = document.getElementById('receiver').contentWindow;
    var btn = document.getElementById('sendMessage');
    btn.addEventListener('click', function (e) {
        e.preventDefault();
        var val = document.getElementById('text').value;
        receiver.postMessage("Hello " + val + "!", "https://c.tutorialpro.org");
    });
}
</script>

<div id="recMessage">
Hello World!
</div>
<script>
window.onload = function() {
    var messageEle = document.getElementById('recMessage');
    window.addEventListener('message', function (e) {  // Listen for message event
        alert(e.origin);
        if (e.origin !== "https://www.tutorialpro.org") {  // Verify message origin
            return;
        }
        messageEle.innerHTML = "Message received from " + e.origin + ": " + e.data;
    });
}
</script>