Create card validator with all 4 rules and tests for valid and invalid card numbers

Author: KhotKeys

Sprint-3/4-stretch/card-validator.js

@@ -0,0 +1,26 @@
+// Validates a credit card number string against the following rules:
+// - Must be exactly 16 digits (no letters or other characters)
+// - Must contain at least two different digits
+// - The final digit must be even
+// - The sum of all digits must be greater than 16
+
+function isValidCardNumber(cardNumber) {
+  // Rule 1: must be exactly 16 digit characters
+  if (!/^\d{16}$/.test(cardNumber)) return false;
+
+  const digits = cardNumber.split("").map(Number);
+
+  // Rule 2: at least two different digits must be present
+  if (new Set(digits).size < 2) return false;
+
+  // Rule 3: final digit must be even
+  if (digits[15] % 2 !== 0) return false;
+
+  // Rule 4: sum of all digits must be greater than 16
+  const sum = digits.reduce((acc, d) => acc + d, 0);
+  if (sum <= 16) return false;
+
+  return true;
+}
+
+module.exports = isValidCardNumber;

Sprint-3/4-stretch/card-validator.test.js

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+const isValidCardNumber = require("./card-validator");
+
+// Valid cards
+test("should return true for a valid card number", () => {
+  expect(isValidCardNumber("9999777788880000")).toEqual(true);
+  expect(isValidCardNumber("6666666666661666")).toEqual(true);
+});
+
+// Rule 1: must be exactly 16 digits
+test("should return false when card contains non-digit characters", () => {
+  expect(isValidCardNumber("a92332119c011112")).toEqual(false);
+});
+
+test("should return false when card has fewer than 16 digits", () => {
+  expect(isValidCardNumber("123456789012345")).toEqual(false);
+});
+
+test("should return false when card has more than 16 digits", () => {
+  expect(isValidCardNumber("12345678901234567")).toEqual(false);
+});
+
+// Rule 2: at least two different digits
+test("should return false when all digits are the same", () => {
+  expect(isValidCardNumber("4444444444444444")).toEqual(false);
+});
+
+// Rule 3: final digit must be even
+test("should return false when final digit is odd", () => {
+  expect(isValidCardNumber("6666666666666661")).toEqual(false);
+});
+
+// Rule 4: sum of all digits must be greater than 16
+test("should return false when sum of digits is not greater than 16", () => {
+  expect(isValidCardNumber("1111111111111110")).toEqual(false);
+});