Sudoku recursive solver function & shuffler

sudoku-fill-puzzle.js

// startingBoard is a 9x9 matrix of zeros

const numArray = [1, 2, 3, 4, 5, 6, 7, 8, 9]

const shuffle = array => {
  let newArray = [...array]
  for ( let i = newArray.length - 1; i > 0; i-- ) {
      const j = Math.floor( Math.random() * ( i + 1 ) );
      [ newArray[ i ], newArray[ j ] ] = [ newArray[ j ], newArray[ i ] ];
  }
  return newArray;
}

const fillPuzzle = startingBoard => {
  const emptyCell = nextEmptyCell(startingBoard)
  // If there are no more zeros, the board is finished, return it
  return startingBoard if (!emptyCell) 

  for (num of shuffle(numArray) ) {   
    // counter is a global variable tracking the number of iterations performed in generating a puzzle
    // Most puzzles generate in < 500ms, but occassionally random generation could run in to
    // heavy backtracking and result in a long wait. Best to abort this attempt and restart.
    // See initializer function for more
    counter++
    if ( counter > 20_000_000 ) throw new Error ("Recursion Timeout")

    if ( safeToPlace( startingBoard, emptyCell, num) ) {
      // If safe to place number, place it
      startingBoard[ emptyCell.rowIndex ][ emptyCell.colIndex ] = num 
      
      // Recursively call the fill function to place num in next empty cell
      return startingBoard if ( fillPuzzle(startingBoard) )
        
      // If we were unable to place the future num, that num was wrong. 
      // Reset it and try next
      startingBoard[ emptyCell.rowIndex ][ emptyCell.colIndex ] = 0 
    }
  }
  // If unable to place any number, return false, 
  // causing previous round to go to next num
  return false 
}