Exemplo 03 Evento

<!DOCTYPE HTML>
<html lang="pt-br">
<head>
    <meta charset="UTF-8">
    <title>Javascript Dia 3</title>
</head>
<body>

    <h1 id="tela">3^2=?</h1>

    <button onclick="quadrado(3);">Resultado</button>

    <script type="text/javascript">

        function quadrado( x ) {
            var tela = document.getElementById("tela");
            tela.innerHTML = x * x;
        }

    </script>
</body>
</html>