Minesweeper

let updateBoard = function(board, click) {

    let maxRow = board.length,
        maxCol = board[0].length,
        MINE = 'M',
        BLANK = 'B',
        EMPTY = 'E',
        GAMEOVER = 'X';

    function isMine(row, col) {
        if (row < 0 || col < 0 || row >= maxRow || col >= maxCol) {
            return 0;
        }
        return board[row][col] === MINE ? 1 : 0;
    }

    function getMineCount(row, col) {
        let nearbyMines = 0;
        nearbyMines += isMine(row - 1, col - 1); // top left
        nearbyMines += isMine(row - 1, col); // top
        nearbyMines += isMine(row - 1, col + 1); // top right
        nearbyMines += isMine(row, col - 1); // left
        nearbyMines += isMine(row, col + 1); // right
        nearbyMines += isMine(row + 1, col - 1); // bottom left
        nearbyMines += isMine(row + 1, col); // bottom
        nearbyMines += isMine(row + 1, col + 1); // bottom right
        return nearbyMines;
    }

    function markAndExplore(row, col) {
        if (row < 0 || col < 0 || row >= maxRow || col >= maxCol) {
            return;
        }
        const data = board[row][col];
        if (data === EMPTY) {

            const mineCount = getMineCount(row, col);
            board[row][col] = mineCount > 0 ? `${mineCount}` : BLANK;

            if (mineCount === 0) {
                markAndExplore(row - 1, col - 1); // top left
                markAndExplore(row - 1, col); // top
                markAndExplore(row - 1, col + 1); // top right
                markAndExplore(row, col - 1); // left
                markAndExplore(row, col + 1); // right
                markAndExplore(row + 1, col - 1); // bottom left
                markAndExplore(row + 1, col); // bottom
                markAndExplore(row + 1, col + 1); // bottom right
            }
        }
    }

    let [x, y] = click;
    if (board[x][y] === MINE) {
        board[x][y] = GAMEOVER;
    }
    else {
        markAndExplore(x, y);
    }

    return board;
};