Sudoku.js

/**
* 
* ██████╗ ██╗   ██╗██████╗ ██╗███╗   ██╗██████╗ ███████╗██████╗ 
* ██╔══██╗██║   ██║██╔══██╗██║████╗  ██║██╔══██╗██╔════╝██╔══██╗
* ██████╔╝██║   ██║██████╔╝██║██╔██╗ ██║██║  ██║█████╗  ██████╔╝
* ██╔══██╗██║   ██║██╔══██╗██║██║╚██╗██║██║  ██║██╔══╝  ██╔══██╗
* ██║  ██║╚██████╔╝██████╔╝██║██║ ╚████║██████╔╝███████╗██║  ██║
* ╚═╝  ╚═╝ ╚═════╝ ╚═════╝ ╚═╝╚═╝  ╚═══╝╚═════╝ ╚══════╝╚═╝  ╚═╝
*                                                              
*  written by Rubinder Singh 4-Nov-2018
*  copyright ©
* 
* @param gameArr 
* returns the solved sudoku puzzle 
* note: in case there are multiple solutions this function will only return the first solution
*/
function solveSudoku(gameArr) {
    var emptySpot = nextEmtySpot(gameArr);
    var r = emptySpot[0];
    var c = emptySpot[1];

    // if the game is unsolvable don't even try to solve it
    if (!isValidSudoku(gameArr)) return gameArr;

    // if no vacant spot is left, board is solved
    if (r === -1) {
        return gameArr;
    };

    var possArr = possiblities(r, c, gameArr);

    for (var k = 0; k < possArr.length && nextEmtySpot(gameArr)[0] !== -1; k++) {
        gameArr[r][c] = possArr[k];
        solveSudoku(gameArr);
    }

    // if no possible value leads to a solution reset this value 
    if (nextEmtySpot(gameArr)[0] !== -1) gameArr[r][c] = 0;

    return gameArr;
}

function nextEmtySpot(gameArr) {
    for (var i = 0; i < 9; i++) {
        for (var j = 0; j < 9; j++) {
            if (gameArr[i][j] === 0) return [i, j];
        }
    }
    return [-1, -1];
}

function possiblities(r, c, gameArr) {
    var possArr = [];
    var row = [];
    var col = [];
    var quad = [];
    var k = 0;
    var l = 0;
    if (r <= 2) k = 0; else if (r <= 5) k = 3; else k = 6;
    if (c <= 2) l = 0; else if (c <= 5) l = 3; else l = 6;

    for (var i = 0; i < 9; i++) {
        row.push(gameArr[i][c]);
    }
    for (var j = 0; j < 9; j++) {
        col.push(gameArr[r][j]);
    }
    for (var i = k; i < k + 3; i++) {
        for (var j = l; j < l + 3; j++) {
            quad.push(gameArr[i][j]);
        }
    }

    for (var n = 1; n < 10; n++) {
        if (row.indexOf(n) === -1 && col.indexOf(n) === -1 && quad.indexOf(n) === -1) {
            possArr.push(n);
        }
    }
    return possArr;
}
function checkQuadrant(r, c, gameArr) {
    var qudarantArr = [];
    for (var i = r; i < r + 3; i++) {
        for (var j = c; j < c + 3; j++) {
            if (qudarantArr.indexOf(gameArr[i][j]) === -1 || gameArr[i][j] === 0) {
                qudarantArr.push(gameArr[i][j]);
            } else {
                return false;
            }
        }
    }
    return true;
}
function isValidSudoku(gameArr) {
    if (!checkQuadrant(0, 0, gameArr)) return false;
    if (!checkQuadrant(0, 3, gameArr)) return false;
    if (!checkQuadrant(0, 6, gameArr)) return false;

    if (!checkQuadrant(3, 0, gameArr)) return false;
    if (!checkQuadrant(3, 3, gameArr)) return false;
    if (!checkQuadrant(3, 6, gameArr)) return false;

    if (!checkQuadrant(6, 0, gameArr)) return false;
    if (!checkQuadrant(6, 3, gameArr)) return false;
    if (!checkQuadrant(6, 6, gameArr)) return false;

    for (var i = 0; i < gameArr.length; i++) {
        var rowNumbers = [];
        for (var j = 0; j < gameArr.length; j++) {
            if (rowNumbers.indexOf(gameArr[i][j]) === -1 || gameArr[i][j] === 0) {
                rowNumbers.push(gameArr[i][j]);
            } else {
                return false;
            }
        }
    }

    for (var i = 0; i < gameArr.length; i++) {
        var colNumbers = [];
        for (var j = 0; j < gameArr.length; j++) {
            if (colNumbers.indexOf(gameArr[j][i]) === -1 || gameArr[j][i] === 0) {
                colNumbers.push(gameArr[j][i]);
            } else {
                return false;
            }
        }
    }
    return true;
}

//---------------------------------------------------Test-----------------------------------------------------------------


var gameArr = [
    [2, 0, 3, 0, 0, 8, 6, 0, 7],
    [1, 4, 0, 7, 2, 6, 0, 0, 9],
    [5, 0, 7, 1, 3, 9, 4, 2, 8],
    [0, 2, 5, 0, 8, 1, 9, 0, 4],
    [4, 1, 0, 9, 0, 3, 2, 0, 5],
    [0, 7, 9, 2, 0, 5, 0, 3, 6],
    [6, 0, 2, 0, 1, 0, 0, 9, 3],
    [7, 0, 0, 5, 0, 2, 0, 0, 1],
    [0, 8, 1, 3, 6, 7, 0, 4, 0]
];

solveSudoku(gameArr);

console.log('Solved');
console.log(gameArr);
console.log(isValidSudoku(gameArr));