Formulaire réactif de vanille javascript

<html lang="en">
    <head>
        <meta charset="UTF-8">
        <meta http-equiv="X-UA-Compatible" cont`ent="IE=edge">
        <meta name="viewport" content="width=device-width, initial-scale=1.0">
        <title>Name Echoer</title>

        <script type="text/javascript">
            function evaluateUserInput(){
                let userInputFirstName = document.getElementById("first_name").value;
                let userInputLastName = document.getElementById("last_name").value;
                let outputDisplay = "Your name is: " + userInputFirstName + " " + userInputLastName;
                document.getElementById("output").innerHTML = outputDisplay;
                return false;
            }
        </script>
    </head>
    
    <body>
        <form id="myform">
          First name:<br><input id="first_name" type="text"><br>
          Last name:<br><input id="last_name" type="text"><br>
          <button id="submit_button" onClick="return evaluateUserInput();">Submit</button>
        </form> 
        <p id="output">placeholder text until the `submit_button` is clicked</p>
    </body>
</html>
Nervous Nightingale