Graph DFS

    const [ ROWS, COLS ] = [ grid.length, grid[0].length ];

    const neighbors = [ [1,0], [-1, 0], [0,1], [0, -1] ];
    const isValid = (r,c) => !(r < 0 || r >= ROWS || c < 0 || c >= COLS);
    const seen = new Set();

    const dfs = (r, c) => {
        let result = 0;
        for (const [ dR, dC ] of neighbors) {
            const newR = r + dR;
            const newC = c + dC;

            if (!isValid(newR, newC)) continue;
            if (seen.has([newR, neweC])) continue;

            seen.add([newR, newC]);
            result += dfs(newR, newC);
        }
        return result;
    }

    return dfs(START_NODE);