sudoku.js

const myPuzzle = [

  1,    null, 9,        null, null, null,   null, 6,    null,
  5,    3,    null,     null, 7,    null,   null, null, null,
  null, null, 4,        null, 2,    6,      null, null, null,

  null, 5,    null,     null, null, null,   null, null, 4,
  null, 7,    null,     6,    3,    4,      null, 2,    null,
  6,    null, null,     null, null, null,   null, 7,    null,
  
  null, null, null,     7,    4,    null,   9,    null, null,
  null, null, null,     null, 1,    null,   null, 5,    2,
  null, 9,    null,     null, null, null,   4,    null, 3

];

const puzzleRowByRowIdx = (puzzle, i) => puzzle.slice(i*9, i*9+9);


const puzzleColByColIdx = (puzzle, i) =>
  [ puzzle[i], puzzle[i+9], puzzle[i+18], puzzle[i+27], puzzle[i+36], puzzle[i+45], puzzle[i+54], puzzle[i+63], puzzle[i+72] ];

const puzzleSquareBySquareIdx = (puzzle, i) => {
  const cell0 = [0,3,6,27,30,33,54,57,60][i];
  
  return [
    puzzle[cell0], puzzle[cell0+1], puzzle[cell0+2],
    puzzle[cell0+9], puzzle[cell0+10], puzzle[cell0+11],
    puzzle[cell0+18], puzzle[cell0+19], puzzle[cell0+20]
  ];
}

const validateGroup = (group=[]) => 
  group
    .reduce(
      (acc, num, idx, nums) => acc && (num===null||nums.indexOf(num)===idx),
      true
    );


const validatePuzzle = puzzle => {
  for(let i = 0; i < 9; i++) {

    const rowIsValid = validateGroup(puzzleRowByRowIdx(puzzle, i));
    if(!rowIsValid) {
      return false;
    }
    
    const colIsValid = validateGroup(puzzleColByColIdx(puzzle, i));
    if(!colIsValid) {
      return false;
    }
    
    const squareIsValid = validateGroup(puzzleSquareBySquareIdx(puzzle, i));
    if(!squareIsValid) {
      return false;
    }
    
  }
  return true;
}



const checkPuzzleCompletion = puzzle =>
  puzzle.filter(x=>x).length===81;



const solve = (puzzle, cell = 0, tryWith = 1) => {
  // set recursion bounds  
  if(cell > 80) {
    return false;
  }

  if( checkPuzzleCompletion(puzzle) ) {
    return validatePuzzle(puzzle)
      ? puzzle
      : false;
  }

  const value = puzzle[cell];
  
  // move to next empty cell if already a number in current cell...
  if(value!==null) {
    return solve(puzzle, cell+1);
  }
  
  
  let attempt = [].concat(puzzle);

  attempt[cell] = tryWith;
  const isValid = validatePuzzle(attempt);

  if(isValid) {

    const solved = solve(attempt, cell+1);
    if(solved) {
      return solved;
    }
  }
  
  // not valid with current attempt, or unsolveable branch...
  if(tryWith < 9) {
    return solve(puzzle, cell, tryWith+1);
  }

  return false;
  
}


console.log( solve(myPuzzle) );