Card Number Validation API

A REST API endpoint that validates credit/debit card numbers using the Luhn algorithm (mod 10).

Features

• Card Number Validation: Validates card numbers using the Luhn algorithm
• Card Type Detection: Identifies card types (Visa, Mastercard, American Express, Discover)
• Input Sanitization: Handles card numbers with spaces and dashes
• Input Validation: Proper error handling for invalid inputs

Technology Stack

• Node.js with TypeScript
• Express.js
• Jest for testing

Project Structure

card-number-validation-api/
├── src/
│   ├── index.ts           # Express app and API endpoint
│   ├── cardValidator.ts   # Card validation logic (Luhn algorithm)
│   ├── cardValidator.test.ts # Unit tests
│   └── types.ts           # TypeScript interfaces
├── package.json
├── tsconfig.json
├── jest.config.js
└── README.md

Installation

1. Install dependencies:

npm install

Running the Project

Development Mode

npm run dev

The server will start on http://localhost:3000

Production Mode

npm run build
npm start

Running Tests

npm test

API Endpoint

POST /api/validate-card

Validates a card number and returns the validation result.

Request

{
  "cardNumber": "4111111111111111"
}

Success Response (200)

{
  "valid": true,
  "cardNumber": "4111111111111111",
  "message": "Valid card number",
  "cardType": "Visa"
}

Invalid Card Response (422)

{
  "valid": false,
  "cardNumber": "4111111111111112",
  "message": "Invalid card number (failed Luhn check)",
  "cardType": "Visa"
}

Bad Request Response (400)

{
  "error": "Bad Request",
  "message": "Card number is required"
}

Design Decisions

Validation Algorithm

The API uses the Luhn algorithm (also known as mod 10) to validate card numbers. This is the industry-standard algorithm used by most payment processors to verify card numbers before processing.

Input Handling

• Card numbers can be submitted with or without spaces (e.g., 4111 1111 1111 1111)
• Card numbers can include dashes (e.g., 4111-1111-1111-1111)
• The API automatically sanitizes input before validation

HTTP Status Codes

• 200 OK: Card number is valid
• 400 Bad Request: Missing or invalid input (e.g., missing card number field)
• 422 Unprocessable Entity: Card number is present but invalid (fails validation)
• 500 Internal Server Error: Unexpected server error

Card Type Detection

The API can identify the following card types based on the card number prefix:

• Visa: Starts with 4
• Mastercard: Starts with 51-55 or 2221-2720
• American Express: Starts with 34 or 37
• Discover: Starts with 6011, 644-649, or 65

Length Validation

Card numbers must be between 13 and 19 digits (standard for most card types).

Testing

The project includes unit tests for the CardValidator class covering:

• Valid card numbers (Visa, Mastercard, American Express)
• Invalid card numbers (failed Luhn check)
• Non-digit characters
• Invalid lengths (too short or too long)
• Card type detection

Running the Project

Install dependencies

npm install

Build

npm run build

Run tests

npm test

Start development server

npm run dev