License key generator and validator in Javascript

license.js

/*
  This generator does not generate valid keys for any software, its merely a demonstration of 
  how to create a simple license key registration and validation system. You could store the 
  generated license keys on the server, and add online-validation when the user wants to use
  the entered key to register your application.
  
  ** DO NOT ** use these as is in your production code (DUHH!)
*/

// Master key is the sum of the license key characters. This sum is the same for every key.
const masterKey = 399;
// These are the characters that are valid for the license key, change these as you like.
const charset = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789';
// This is the prefix that is used to start your license key. You can change this for different products.
const prefix = 'SAMP';

/**
 * Generate license key. 
 * This function generates multiple license keys.
 *
 * ** Disclaimer **
 * This is just a very simple license mechanism that can be integrated into
 * your own software. You can change the charset, length and code to generate
 * your own license keys - but update the validation function accordingly.
 */
function generateRegCode() {
  // Add the prefix to the licensekey.
  let result = prefix;
  let j = 0;

  // Keep trying to find a valid license key.
  while (j !== masterKey) {
    result = prefix;
    j = 0;

    // We need 15 characters for our license key + the prefix of 4 characters + 3 dashes = 22 characters total.
    for (let i = 1; i <= 15; i++) {
      const k = Math.floor(Math.random() * charset.length);
      j += k;
      result += charset[k];
    }
  }

  // Add the dashes to make the license key easier to read/write.
  result = result.slice(0, 4) + '-' + result.slice(4, 8) + '-' + result.slice(8, 12) + '-' + result.slice(12);

  // Return the generated license key.
  return result;
}

/**
 * Validate license key.
 * This function validates generated license keys and can be used
 * in your website to check if the user entered a valid license key.
 * ** DO NOT ** put this code on the client-side!!
 */
function validateRegCode(regCode) {
  // Total length should be 22 characters long.
  if (regCode.length !== 22) {
    return false;
  }

  // If there are no dashes on these positions, the formatting is wrong.
  if (
    regCode[4] !== '-' ||
    regCode[9] !== '-' ||
    regCode[14] !== '-'
  ) {
    return false;
  }

  // Calculate the sum of the license key.
  let j = 0;
  for (let i = 5; i < regCode.length; i++) {
    if (regCode[i] !== '-') {
      const k = charset.indexOf(regCode[i]);
      if (k === -1) {
        return false;
      }
      j += k;
    }
  }

  // Validate the sum with the masterkey.
  return j === masterKey;
}

/**
  * Generate a list of License keys.
  */
const licenseKeys = Array.from({ length: 10 }).map(() => (generateRegCode()));

/**
  * Validate the generated list of license keys.
  */
console.log('Valid generated license keys:');
licenseKeys.forEach((licenseKey) => {
    if (validateRegCode(licenseKey)) {
        console.log(`${licenseKey} = VALID`);
    } else {
        console.log(`${licenseKey} = INVALID!`);
    }
});

/**
  * List of BAD (random) license keys.
  */
const badLicenseKeys = [
    'SAMP-111S-3069-GT7862Q',
    'SAMP-661S-1234-GT7862A',
    'SAMP-6254-4A4A-GHS62Q6',
    'SAMP-4R2U-23X9-84E367X',
    'SAMP-W9A1-X88O-ZYV8P42'
];

/**
  * Validate list of BAD license keys.
  */
console.log('\nBad random license keys:')
badLicenseKeys.forEach((licenseKey) => {
    if (validateRegCode(licenseKey)) {
        console.log(`${licenseKey} = VALID`);
    } else {
        console.log(`${licenseKey} = INVALID!`);
    }
});