sudoku.js

class SudokuValidator {
  constructor(board) {
    if (board.length !== board[0].length) {
      throw new Error('Board must be square');
    }
    
    if (Math.sqrt(board.length) !== Math.floor(Math.sqrt(board.length))) {
      throw new Error('Board size must be a square number')
    }
    
    this.board = board;
    this.size = board.length;
    this.gridSize = Math.sqrt(board.length);
    this.axes = ['row', 'col'];
  }

  _buildTracker = () => new Array(this.size)
  
  _validateAxis = (axis) => {
    if (!this.axes.find(i => i === axis)) throw new Error('Axis must be either col or row');
    let tracker;
    for (let row = 0; row < this.size; row++) {
      for (let col = 0; col < this.size; col++) {
        tracker = this._buildTracker();
        const cell = this.board[axis === 'row' ? col : row][axis === 'row' ? row : col];
        if (tracker[cell]) return false;
        tracker[cell] = true;
      }
    }
    return true;
  }

  _validateSquares = () => {
    let tracker;
    for (let bigRow = 0; bigRow < this.size; bigRow += this.gridSize) {
      for (let bigCol = 0; bigCol < this.size; bigCol += this.gridSize) {
        tracker = this._buildTracker();
        for (let littleRow = 0; littleRow < this.gridSize; littleRow++) {
          for (let littleCol = 0; littleCol < this.gridSize; littleCol++) {
            const cell = this.board[bigRow + littleRow][bigCol + littleCol];
            if (tracker[cell]) return false;
            tracker[cell] = true;
          }
        }
      }
    }
    return true;
  }

  validate = () => {
    return this._validateAxis('row') &&
      this._validateAxis('col') &&
      this._validateSquares();
  }
}

this implementation assumes cell values are 0-9