<h1>Arealet av en sirkel</h1>
  


  <h3>Input</h3>

  Radius: <input type="number" id="radius">
  
  
  
  <button id="btn">Beregn areal</button>



  <h3>Resultat</h3>
  
  <div id="Areal"></div>

  document.getElementById("radius").value

  let r = document.getElementById("radius").value

  const area_circle = function () {
    let r = document.getElementById("radius").value;
    return Math.PI * r ** 2;
  }
  

 onclick="document.getElementById('Areal').innerHTML = area_circle()"
    

  <script>
    const area_circle = function () {
      let r = document.getElementById("radius").value;
      return Math.PI * r ** 2;
    }
  </script>

<button id="btn" onclick="document.getElementById('Areal').innerHTML 
                          = area_circle()">Beregn areal</button>