How to check if a 9x9 grid is a valid sudoku board in JavaScript

JavaScript, algorithms, matrices

Here is one way to implement an algorithm to check if a 9x9 grid is a valid sudoku board:

validSudokuTry in REPL
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function isValid(grid) {
  return grid.every(checkRow) &&
         transpose(grid).every(checkRow) &&
         expandSubGrids(grid).every(checkRow);
}
function checkRow(row) {
  const tracker = new Set();
  for (const cell of row) {
    if (cell !== '.' && tracker.has(cell)) {
      return false;
    }
    tracker.add(cell);
  }
  return true;
}
function expandSubGrids(grid) {
  const three = Math.sqrt(grid.length);
  return grid.map(
    (_,i) => grid.map(
      (_, j) => grid[(i/three|0)*three+j/three|0][i%three*three+j%three]  
    )
  )
}
function transpose(grid) {
  return grid[0].map(
    (_, col) => grid.map(row => row[col]) 
  );
}

Tests can be found at REPL.it