testPostMessage.html

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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
    <title>First window</title>
</head>
<body>

    <script>
        window.addEventListener("message", function(e) {
            console.log("Received message: " + e.data);
            e.source.postMessage("Message received " + e.data, e.origin);
        }, false);
        var w = window.open("https://dragon-alien.codio.io/postMessageTest.html");
       setTimeout(function () {
            console.log("send message on timeout");
            w.postMessage("Initial data", "*");
        }, 3000);
    </script>
    
</body>
</html>