Calculate the volume of cylinder with JavaScript using prototype

I'm a JavaScript noob and beginner so don't get too hard on me.

I need to calculate the volume of a cylinder using a constructor function and prototype.

I've got a form with two inputs which I'll be getting the values from to do the calculation with. I have to create a new instance when the button is clicked and the calculation has to be outputted in the outcome input.

I seem to be stuck at the part to get the values out of the inputs as my console always says that 'hoogte' and 'diameter' is undefined when I click the button.

I've been looking at this for almost 24h now, but I'm not getting any progress..

This is my code:

<form action="">
    <label>
        Diameter: <input type="text" id="diameter"><br><br>
        Hoogte: <input type="text" id="hoogte"> <br><br>
        <input type="button" value="bereken" id="berekenBtn"><br><br>
        <input type="text" id="uitkomst">
    </label>
</form>

<script>
    window.addEventListener("DOMContentLoaded", function () {
        document.getElementById("berekenBtn").addEventListener("click", bereken);
    });

    function Cylinder(hoogte, diameter) {
        this.hoogte = hoogte;
        this.diameter = diameter;
    }

    Cylinder.prototype.volume = function () {
        var radius = document.getElementById('diameter') = this.diameter / 2;
        var hoogte = document.getElementById('hoogte') = this.hoogte;

        var berekening = Math.PI * radius * radius * hoogte;
        //berekening.toFixed(4);
    }


    function bereken() {
        var myCylinder = new Cylinder(
            document.getElementById("uitkomst").value = Cylinder()
        )
        console.log(myCylinder);
    }
</script>

 <form action="">
        <label>
            Diameter: <input type="number" id="diameter"><br><br>
            Hoogte: <input type="number" id="hoogte"> <br><br>
            <input type="button" value="bereken" id="berekenBtn"><br><br>
            <input type="text" id="uitkomst">
        </label>
    </form>


<script>
         window.addEventListener("DOMContentLoaded", function () {
        document.getElementById("berekenBtn").addEventListener("click", bereken);
    });

    function Cylinder(hoogte, diameter) {
        this.hoogte = hoogte;
        this.diameter = diameter;
    }

    Cylinder.prototype.volume = function () {
        var radius =  this.diameter / 2;
        var hoogte = this.hoogte;

        var berekening = Math.PI * radius * radius * hoogte;
        return berekening;
    }


    function bereken() {
        var diameter = document.getElementById("diameter").value;
        var hoogte = document.getElementById("hoogte").value;
        var myCylinder = new Cylinder(diameter, hoogte);
        var result = myCylinder.volume();
        document.getElementById("uitkomst").value = result;
    }

    </script>

I have modified some code, try to understand this, hope it will helps you !