<script>


function numOfNeighbour( mat,  i,  j,rows,cols)
{
    var count = 0;

    // UP
    if (i > 0  && mat[i - 1][j]==='X')
        count++;

    // LEFT
    if (j > 0  && mat[i][j - 1]==='X')
        count++;

    // DOWN
    if (i < rows-1  && mat[i + 1][j]==='X')
        count++;

    // RIGHT
    if (j < cols-1 && mat[i][j + 1]==='X')
        count++;

    return count;
}

function findPerimeter( mat)
{
    var perimeter = 0;
    var rows=mat.length;
    var cols=mat[0].length;

    // Traversing the matrix and finding ones to
    // calculate their contribution.
    for (var i = 0; i < rows; i++)
        for (var j = 0; j < cols; j++)
            if (mat[i][j] && mat[i][j]==='X')
                perimeter += (4 - numOfNeighbour(mat, i ,j,rows,cols));

    return perimeter;
}

console.log("perimeter of ['XOOXO', 'XOOXO', 'OOOXO', 'XXOXO', 'OXOOO'] is ",findPerimeter(['XOOXO', 'XOOXO', 'OOOXO', 'XXOXO', 'OXOOO'])); //24
console.log("perimeter of ['XOOO', 'XOXO', 'XOXO', 'OOXX', 'OOOO'] is "     ,findPerimeter(['XOOO', 'XOXO', 'XOXO', 'OOXX', 'OOOO'])); //18
</script>