Check input isn't null before evaluating the number

luhn-algorithm/script.js

@@ -12,6 +12,10 @@ document.addEventListener("keyup", function(event) {
 
 // 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) {
@@ -26,4 +30,5 @@ function checkNumber() {
 
         // displays evaluation message
         $("#validation_message").html(((sum % 10) == 0) ? "Number is valid" : "Number is invalid");
+    }
 }