Number of Islands

let numIslands = function(matrix) {

    if (!matrix.length) return 0;

    let land = 1, visited = 'X',
        counter = 0,
        maxRow = matrix.length - 1,
        maxCol = matrix[0].length - 1;

    function exploreAndMark(row, col) {
        if (row < 0 || col < 0 || row > maxRow || col > maxCol) {
            return;
        }

        if (matrix[row][col] === land) {
            matrix[row][col] = visited;

            // explore up
            exploreAndMark(row - 1, col);

            // explore down
            exploreAndMark(row + 1, col);

            // explore left
            exploreAndMark(row, col - 1);

            // explore right
            exploreAndMark(row, col + 1);
        }
    }

    for (let row = 0; row <= maxRow; row++) {
        for (let col = 0; col <= maxCol; col++) {
            if (matrix[row][col] === land) {
                counter++;
                exploreAndMark(row, col);
            }
        }
    }

    return counter;
}