simplifed a bit

pixel_calculator/pixel_calculator.html

@@ -95,10 +95,6 @@
         let result = document.getElementById("result");
         let calcButton = document.getElementById("calculate");
 
-        const getChannels = () => parseInt(channels.selectedOptions[0].value);
-
-        const getChannel = () => parseInt(channel.selectedOptions[0].value);
-
         channels.addEventListener('change', function () {
 //            let maxOffset = getChannels()-1;
             doCalc();
@@ -119,7 +115,10 @@
             let x = parseInt(document.getElementById("x").value);
             let y = parseInt(document.getElementById("y").value);
 
-            let calculation = (y * w + x) * getChannels() + getChannel();
+            // calculate index in pixels array:
+            let calculation = (y * w + x) *  
+                parseInt(channels.selectedOptions[0].value) +  
+                parseInt(channel.selectedOptions[0].value);
             result.innerHTML = calculation;
         }
     </script>