Medicare Number Validation - Typescript

medicare-number-validation.ts

/**
 * Australian Medicare Number Validation
 * 
 * Validates an 11-digit Medicare card number using the official checksum algorithm.
 * 
 * Structure (11 digits total):
 * - Identifier: 8 digits (first digit must be 2-6, indicates state/territory)
 * - Checksum: 1 digit (weighted sum mod 10)
 * - Issue Number: 1 digit (card reissue count)
 * - IRN: 1 digit (Individual Reference Number)
 * 
 * Checksum algorithm: Multiply first 8 digits by weights [1,3,7,9,1,3,7,9], sum, mod 10
 * 
 * @param medicare - 11-digit Medicare number (spaces/dashes allowed)
 * @returns { isValid, error?, components? }
 * 
 * @example
 * validateMedicare("2951 8731 2 5 1") // { isValid: true, components: { ... } }
 * validateMedicare("21234567891")     // { isValid: false, error: "Invalid checksum" }
 * 
 * @see https://doccy.com.au/medicare-number-validation
 * @author Louis J Sisk, Jordi Hermoso
 */
const validateMedicare = (medicare: string) => {
  const weights = [1, 3, 7, 9, 1, 3, 7, 9] as const;
  const digits = medicare.replace(/\D/g, "");

  if (digits.length !== 11) {
    return { isValid: false, error: `Expected 11 digits, got ${digits.length}` };
  }

  const [identifier, checksum, issueNumber, irn] = [
    digits.slice(0, 8),
    digits[8],
    digits[9],
    digits[10],
  ];

  const firstDigit = parseInt(identifier[0]!, 10);
  if (firstDigit < 2 || firstDigit > 6) {
    return { isValid: false, error: `First digit must be 2-6, got ${firstDigit}` };
  }

  const expectedChecksum = identifier
    .split("")
    .reduce((sum, d, i) => sum + parseInt(d, 10) * weights[i]!, 0) % 10;

  if (expectedChecksum !== parseInt(checksum!, 10)) {
    return { isValid: false, error: `Invalid checksum: expected ${expectedChecksum}, got ${checksum}` };
  }

  return {
    isValid: true,
    components: { identifier, checksum, issueNumber, irn },
  };
};