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luhn-algorithm/script.js

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+window.activated_drawer_item_link = "/luhn-algorithm/";
+
+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() {
+    // 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");
+}

luhn-algorithm/style.css

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+:root {
+    --mdc-theme-primary: #00897b;
+    --mdc-theme-secondary: #d84315;
+}
+.mdc-text-field-helper-text {
+    margin-bottom: 10px;
+}