Javascript function does not work on onclick of checkbox

I try to call a Javascript function through an onclick attribute of a checkbox.

The js should show another checkbox if the first one is checked etc.

Now the function isnt working. I tried to use just an alert() for testing without any result...

  Fitness:<input type="checkbox" id="fitnessCheck" onclick="checkFunction()">
    Beauty:<input type="checkbox" id="beautyCheck">
    Streetwear:<input type="checkbox" id="streetwearCheck">
    Luxus:<input type="checkbox" id="luxusCheck" onClick="checkFunction()">

Datenschutz: <input style="display:none" type="checkbox"  id="datenschutzCheck">
<script>
checkFunction(){
//get Checkboxes
var fitnessCheckbox = document.getElementById("fitnessCheck");
var beautyCheckbox = document.getElementById("beautyCheck");
var streetwearCheckbox = document.getElementById("streetwearCheck");
var luxusCheckbox = document.getElementById("luxusCheck");
var datenschutzCheckbox = document.getElementById("datenschutzCheck");

if(fitnessCheckbox.checked == true || beautyCheckbox.checked == true || streetwearCheckbox.checked == true || luxusCheckbox.checked == true){
    datenschutzCheckbox.style.display ="block"; 
}
}</script>