D3 | Input box to create SVG shapes

Aim

Use input box to change visualisation

Info and sources

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="utf-8">
    <title>Input to create SVG shape</title>
    <script type="text/javascript" src="//cdnjs.cloudflare.com/ajax/libs/d3/3.4.2/d3.js"></script>

var mySVG = d3.select("body")
            .append("svg:svg")
            .attr("width", w)
            .attr("height", h);

function handleClick(event){
            console.log(document.getElementById("myRadius").value)
            draw(document.getElementById("myRadius").value)
            return false;
        }

function draw(rad){
            mySVG.append("svg:circle")
            .style("fill", "red")
            .attr("opacity", 0.4)
            .attr("r", rad)
            .attr("cx", Math.round(Math.random()*w))
            .attr("cy", Math.round(Math.random()*h))
        }

</script>

</body>
</html>

https://cdnjs.cloudflare.com/ajax/libs/d3/3.4.2/d3.js