let input = document.getElementById("input");
input.addEventListener("change", function( event ) {
updateMessage(event);
}, true);
function updateMessage(event) {
let message = document.getElementById("message");
message.innerHTML = 'Message: ' + event.target.value;
}
<html>
<body>
<input type="text" id="input">
<p id="message">
Message:
</p>
</body>
</html>