chore: Strings: Credit Card Validation (#830)

* Add credit card number validator

- Validates the credit card number based on Luhn algorithm

* Test Cases: ValidateCreditCard

* Auto-update DIRECTORY.md

* Fix: Spell check

* Add references and move const inside function

* Add comments

* Fix trailing spaces

* Add short description

* Remove trailing spaces

Co-authored-by: github-actions <${GITHUB_ACTOR}@users.noreply.github.com>

String/ValidateCreditCard.js

@@ -0,0 +1,59 @@
+/**
+ * Validate a given credit card number
+ *
+ * The core of the validation of credit card numbers is the Luhn algorithm.
+ *
+ * The validation sum should be completely divisible by 10 which is calculated as follows,
+ * every first digit is added directly to the validation sum.
+ * For every second digit in the credit card number, the digit is multiplied by 2.
+ * If the product is greater than 10 the digits of the product are added.
+ * This resultant digit is considered for the validation sum rather than the digit itself.
+ *
+ * Ref: https://www.geeksforgeeks.org/luhn-algorithm/
+ */
+
+const luhnValidation = (creditCardNumber) => {
+  let validationSum = 0
+  creditCardNumber.split('').forEach((digit, index) => {
+    let currentDigit = parseInt(digit)
+    if (index % 2 === 0) {
+      // Multiply every 2nd digit from the left by 2
+      currentDigit *= 2
+      // if product is greater than 10 add the individual digits of the product to get a single digit
+      if (currentDigit > 9) {
+        currentDigit %= 10
+        currentDigit += 1
+      }
+    }
+    validationSum += currentDigit
+  })
+
+  return validationSum % 10 === 0
+}
+
+const validateCreditCard = (creditCardString) => {
+  const validStartSubString = ['4', '5', '6', '37', '34', '35'] // Valid credit card numbers start with these numbers
+
+  if (typeof creditCardString !== 'string') {
+    throw new TypeError('The given value is not a string')
+  }
+
+  const errorMessage = `${creditCardString} is an invalid credit card number because `
+  if (isNaN(creditCardString)) {
+    throw new TypeError(errorMessage + 'it has nonnumerical characters.')
+  }
+  const creditCardStringLength = creditCardString.length
+  if (!((creditCardStringLength >= 13) && (creditCardStringLength <= 16))) {
+    throw new Error(errorMessage + 'of its length.')
+  }
+  if (!validStartSubString.some(subString => creditCardString.startsWith(subString))) {
+    throw new Error(errorMessage + 'of its first two digits.')
+  }
+  if (!luhnValidation(creditCardString)) {
+    throw new Error(errorMessage + 'it fails the Luhn check.')
+  }
+
+  return true
+}
+
+export { validateCreditCard }