Implement Jekyll again

Done again as navigation drawer wasn't closing when it was supposed to

_site/luhn-algorithm/script.js

@@ -0,0 +1,34 @@
+document.addEventListener("keyup", function(event) {
+    // stop any code that may normally run when enter key pressed
+    event.preventDefault();
+    // if enter key is pressed, run Luhn algorithm
+    if (event.keyCode === 13) {
+        checkNumber();
+    } else {
+        // if number in input field is being edited, remove previous number evaluation message
+        $("#validation_message").html(null);
+    }
+});
+
+// function for validating number entered with Luhn algorithm
+function checkNumber() {
+    // define input string and remove non-numerical characters
+    var input = document.getElementsByTagName("input")[0].value.replace(/\D/g,'');;
+    // check string isn't empty
+    if (input) {
+        // retrieve digits & reverse
+        var digits = document.getElementsByTagName("input")[0].value.split("").reverse(), sum = 0;
+        digits.forEach(function(currentDigit, index) {
+            newDigit = Number(currentDigit);
+            if (index % 2 == 1) {
+                if ((newDigit *= 2) > 9) {
+                    newDigit -= 9;
+                }
+            }
+            sum += newDigit;
+        });
+
+        // displays evaluation message
+        $("#validation_message").html(((sum % 10) == 0) ? "Number is valid" : "Number is invalid");
+    }
+}