Calculate or verify a Luhn (Mod10) checksum

luhn-checksum.js

/**
 * MIT No Attribution
 *
 * Copyright 2015 Giovanni Thenstead
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 * SOFTWARE.
 *
 *******************************************************************************
 *
 * Calculate a Luhn (Mod10) checksum
 *   -or-
 * Verify the Luhn checksum of a credit card or IMEI number
 *
 * This implementation was adapted from the following sources:
 *   - https://en.wikipedia.org/w/index.php?title=Luhn_algorithm&oldid=688221366
 *   - https://gist.github.com/ShirtlessKirk/2134376
 */

var luhn = {
  /* Calculates the Luhn checksum */
  calculate: function(digits) {
    var sum = this.sum(digits, false);
    return (sum * 9) % 10;
  },

  /* Verifies if a number is a valid Luhn checksum */
  verify: function(digits) {
    var sum = this.sum(digits, true);
    return sum > 0 && sum % 10 === 0;
  },

  /* Sum each digit from right to left, and double
     every second digit. If the double exceeds 9,
     then sum its digits (i.e., 654321 -> 358341
     -> 24) */
  sum: function(digits, even) {
    var sum = 0,
        digit = 0,
        i = digits.length;

    while (i--) {
      digit = Number(digits[i]);
      sum += (even = !even) ? this.computed[digit] : digit;
    }

    return sum;
  },
  
  /* Create a precomputed list based on doubling
     each digit, as described in sum(). */
  computed: [0, 2, 4, 6, 8, 1, 3, 5, 7, 9]
};


var helpers = {
  /* Appends a Luhn checksum to the end of a number */
  createLuhnId: function(number) {
    var digits = String(number);
    return digits + luhn.calculate(digits);
  },

  /* Checks if a credit card or IMEI number is valid */
  isLuhnId: function(number) {
    return luhn.verify(String(number));
  }
};

Hi,
Do you have a license for your code?
Thanks

@hrieke Thanks for mentioning this. An open license (MIT-0) is now included with this code.