damm.js

var table = [
  [0, 3, 1, 7, 5, 9, 8, 6, 4, 2],
  [7, 0, 9, 2, 1, 5, 4, 8, 6, 3],
  [4, 2, 0, 6, 8, 7, 1, 3, 5, 9],
  [1, 7, 5, 0, 9, 8, 3, 4, 2, 6],
  [6, 1, 2, 3, 0, 4, 5, 9, 7, 8],
  [3, 6, 7, 4, 2, 0, 9, 5, 8, 1],
  [5, 8, 6, 9, 7, 2, 0, 1, 3, 4],
  [8, 9, 4, 5, 3, 6, 2, 0, 1, 7],
  [9, 4, 3, 8, 6, 1, 7, 2, 0, 5],
  [2, 5, 8, 1, 4, 3, 6, 7, 9, 0]
];

function generate(input) {
  if (typeof input !== 'string') {
    throw new Error('Input must be a string.');
  }
  if (!input.match(/^\d+$/)) {
    throw new Error('Input must only contain digits.');
  }

  var row = 0;
  for(var i = 0; i < input.length; i++) {
    var col = input.charAt(i);
    row = table[row][col];
  }
  return row.toString();
}

function append(input) {
  return input + generate(input);
}

function verify(input) {
  return generate(input) === '0';
}

exports.generate = generate;
exports.append = append;
exports.verify = verify;