sudokuOn.js

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

var INVALID_BOARD_1 = [
  [ 8,9,5,7,6,2,1,3,6 ],
  [ 2,7,1,9,6,3,4,8,5 ],
  [ 4,6,3,5,8,1,7,9,2 ],
  [ 9,3,4,6,1,7,2,5,8 ],
  [ 5,1,7,2,3,8,9,6,4 ],
  [ 6,8,2,4,5,9,3,7,1 ],
  [ 1,5,9,8,7,4,6,2,3 ],
  [ 7,4,6,3,2,5,8,1,9 ],
  [ 3,2,8,1,9,6,5,4,7 ]
];

var INVALID_BOARD_2 = [
  [ 8,9,5,7,6,2,1,3,6 ],
  [ 2,7,1,9,6,3,4,8,5 ],
  [ 4,6,3,5,8,1,7,9,2 ],
  [ 9,3,4,6,1,7,2,5,8 ],
  [ 5,1,7,2,3,8,9,6,4 ],
  [ 6,8,2,4,5,9,3,7,1 ],
  [ 1,5,9,8,7,4,7,2,3 ],
  [ 7,4,6,3,2,5,8,1,9 ],
  [ 3,2,8,1,9,6,5,4,7 ]
];

function assert(condition, error) {
  if (!condition) {
    throw new Error(error);
  }
}

var BOARD_SIZE = 9;
var SECTION_SIZE = 3;

assert(validateSudoku(VALID_BOARD) === true, 'Expected VALID_BOARD to be valid');
assert(validateSudoku(INVALID_BOARD_1) === false, 'Expected INVALID_BOARD_1 to be invalid');
assert(validateSudoku(INVALID_BOARD_2) === false, 'Expected INVALID_BOARD_2 to be invalid');

console.log('All tests passing');

function validateSudoku(board) {

  var validator = groupValidator();

  // Validate rows

  var x = 0, y = 0;

  while (true) {

    if (!validator.check(board[y][x])) {
      return false
    }

    x += 1;

    if (x === BOARD_SIZE) {
      x = 0;
      y += 1;
      validator.reset();

      if (y === BOARD_SIZE) {
        break;
      }
    }
  }

  // Validate columns

  var x = 0, y = 0;

  while (true) {

    if (!validator.check(board[y][x])) {
      return false
    }

    y += 1;

    if (y === BOARD_SIZE) {
      y = 0;
      x += 1;
      validator.reset();

      if (x === BOARD_SIZE) {
        break;
      }
    }
  }

  // Validate sections

  var x = 0, y = 0;

  while (true) {

    if (!validator.check(board[y][x])) {
      return false
    }

    x += 1;

    if (x % SECTION_SIZE === 0) {
      x -= SECTION_SIZE;
      y += 1;

      if (y % SECTION_SIZE === 0) {
        x += SECTION_SIZE;
        y -= SECTION_SIZE;
        validator.reset();
      }
    }

    if (x === BOARD_SIZE) {
      x = 0;
      y += SECTION_SIZE;
    }

    if (y === BOARD_SIZE) {
      break
    }
  }

  return true;
}

function groupValidator() {

  var numbers = {};

  return {
    check: function(number) {

      if (number < 1 || number > BOARD_SIZE) {
        return false;
      }

      // hash-map access is O(1), woot
      if (numbers[number]) {
        return false;
      }

      numbers[number] = true;
      return true;
    },

    reset: function() {
      numbers = {};
    }
  }
}