var closedIsland = function(grid) {
    let m=grid.length;
    let n=grid[0].length;

    let count=0;

    function dfs(x,y,grid){
        if(x<0 || x>=grid.length || y<0 || y>=grid[0].length || grid[x][y]==1){
            return;
        }

        grid[x][y]=1;
        dfs(x+1,y,grid);
        dfs(x-1,y,grid);
        dfs(x,y+1,grid);
        dfs(x,y-1,grid);
    }

    // Exclude connected group of 0s on the corners because they are not closed island.
    for(let i=0;i<m;i++){
        for(let j=0;j<n;j++){
           if(grid[i][j]==0 &&(i==0 || i==m-1 || j==0 || j==n-1)){
               dfs(i,j,grid);
           }
        }
    }
    // Count the number of connected component of 0s on the grid.
    for(let i=1;i<m-1;i++){
        for(let j=1;j<n-1;j++){
           if(grid[i][j]==0){
               dfs(i,j,grid);
               count++;
           }
        }
    }

    return count;
};