sudoku.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) {

  // Validate rows

  for (var y=0; y<BOARD_SIZE; y++) {
    if (!validateGroup(board[y])) {
      return false
    }
  }

  // Validate columns

  for (var x=0; x<BOARD_SIZE; x++) {
    var group = [];

    for (var y=0; y<BOARD_SIZE; y++) {
      group.push(board[y][x]);
    }

    if (!validateGroup(group)) {
      return false
    }
  }

  // Validate groups

  for (var x=0; x<BOARD_SIZE; x+=SECTION_SIZE) {
    for (var y=0; y<BOARD_SIZE; y+=SECTION_SIZE) {

      var group = [];

      for (var sx=x; sx<(x+SECTION_SIZE); sx++) {
        for (var sy=y; sy<(y+SECTION_SIZE); sy++) {
          group.push(board[sy][sx]);
        }
      }

      if (!validateGroup(group)) {
        return false
      }
    }
  }

  return true;
}

function validateGroup(group) {

  if (group.length !== BOARD_SIZE) {
    return false;
  }

  for (var i=1; i<=BOARD_SIZE; i++) {
    if (group.indexOf(i) === -1) {
      return false;
    }
  }

  return true;
}