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>

Author: Bhargavishnu

DIRECTORY.md

@@ -310,6 +310,7 @@
   * [ReverseString](https://github.com/TheAlgorithms/Javascript/blob/master/String/ReverseString.js)
   * [ReverseWords](https://github.com/TheAlgorithms/Javascript/blob/master/String/ReverseWords.js)
   * [ScrambleStrings](https://github.com/TheAlgorithms/Javascript/blob/master/String/ScrambleStrings.js)
+  * [ValidateCreditCard](https://github.com/TheAlgorithms/Javascript/blob/master/String/ValidateCreditCard.js)
   * [ValidateEmail](https://github.com/TheAlgorithms/Javascript/blob/master/String/ValidateEmail.js)
 
 ## Timing-Functions

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 }

String/test/ValidateCreditCard.test.js

@@ -0,0 +1,39 @@
+import { validateCreditCard } from '../ValidateCreditCard'
+
+describe('Validate credit card number', () => {
+  it('should throw error if card number is boolean', () => {
+    const invalidCC = true
+    expect(() => validateCreditCard(invalidCC)).toThrow(
+      'The given value is not a string'
+    )
+  })
+  it('returns true if the credit card number is valid', () => {
+    const validCreditCard = '4111111111111111'
+    const validationResult = validateCreditCard(validCreditCard)
+    expect(validationResult).toBe(true)
+  })
+  it('should throw an error on non-numeric character in given credit card number', () => {
+    const nonNumericCCNumbers = ['123ABCDEF', 'ABCDKDKD', 'ADS232']
+    nonNumericCCNumbers.forEach(nonNumericCC => expect(() => validateCreditCard(nonNumericCC)).toThrow(
+      `${nonNumericCC} is an invalid credit card number because ` + 'it has nonnumerical characters.'
+    ))
+  })
+  it('should throw an error on credit card with invalid length', () => {
+    const ccWithInvalidLength = ['41111', '4111111111111111111111']
+    ccWithInvalidLength.forEach(invalidCC => expect(() => validateCreditCard(invalidCC)).toThrow(
+      `${invalidCC} is an invalid credit card number because ` + 'of its length.'
+    ))
+  })
+  it('should throw an error on credit card with invalid start substring', () => {
+    const ccWithInvalidStartSubstring = ['12345678912345', '23456789123456', '789123456789123', '891234567891234', '912345678912345', '31345678912345', '32345678912345', '33345678912345', '38345678912345']
+    ccWithInvalidStartSubstring.forEach(invalidCC => expect(() => validateCreditCard(invalidCC)).toThrow(
+      `${invalidCC} is an invalid credit card number because ` + 'of its first two digits.'
+    ))
+  })
+  it('should throw an error on credit card with luhn check fail', () => {
+    const invalidCCs = ['411111111111111', '371211111111111', '49999999999999']
+    invalidCCs.forEach(invalidCC => expect(() => validateCreditCard(invalidCC)).toThrow(
+      `${invalidCC} is an invalid credit card number because ` + 'it fails the Luhn check.'
+    ))
+  })
+})