why this show undefined in javaScript :   
    <body>
    <html>
<body>
    <p id="demo">Click the button to change the text in this paragraph.</p>
    
    <button onclick="myFunction()">Try it</button>
    <p id="para"></p>
    
    <script>
    var len ;
    function myFunction() {
    	 len = document.getElementById("demo").value() ;
    
        document.getElementById("demo").innerHTML = "Hello World";
    
        document.getElementById("para").innerHTML = len ;
    
    }
    </script>
    
    </body>
</html>

I don't understand why this is saying that the len is undefined. Please help. 