leetcode-36.js

var isValidSudoku = function(board) {
  for (let i=0; i < board.length; i++) {
    if (!isValidColumn(board, i) || !isValidRow(board, i) || !isValidSubBox(board, i)) {
      return false
    }
  }  
  return true
};

function isValidColumn(board, col) {
  const nums = new Set()
  for (let i=0; i < board.length; i++) {
    const num = board[i][col]
    if (nums.has(num)) return false
    if (num !== ".") nums.add(num)
  }
  return true
}

function isValidRow(board, row) {
  const nums = new Set()
  for (let i=0; i < board.length; i++) {
    const num = board[row][i]
    if (nums.has(num)) return false
    if (num !== ".") nums.add(num)
  }
  return true
}

function isValidSubBox(board, index) {
  const row = Math.floor(index/3) * 3
  const col = (index % 3) * 3
  
  const nums = new Set()
  for (let i=row; i < row + 3; i++) {
    for (let j=col; j < col + 3; j++) {
      const num = board[i][j]
      if (nums.has(num)) return false
      if (num !== ".") nums.add(num)
    }
  }
  return true
}