Sudoku box region safe condition

sudoku-box-condition.js

// puzzleArray is the game board being solved. A 9x9 matrix
// emptyCell = {rowIndex: INT , colIndex: INT } INT = coordinates of currently empty cell
// num = integer value 1-9 being tested

const boxSafe = (puzzleArray, emptyCell, num) => {
  // Define top left corner of box region for empty cell
  boxStartRow = emptyCell.rowIndex - (emptyCell.rowIndex % 3) 
  boxStartCol = emptyCell.colIndex - (emptyCell.colIndex % 3)
  let safe = true

  for ( boxRow of [0,1,2] ) {  // Each box region has 3 rows
    for ( boxCol of [0,1,2] ) { // Each box region has 3 columns
      // Is num is present in box region?
      if ( puzzleArray[boxStartRow + boxRow][boxStartCol + boxCol] == num ) { 
        safe = false // If number is found, it is not safe to place
      }
    }
  }
  return safe
}