8.8 Smarter computer playing Tic-Tac-Toe

var board = new TRUGameBoard(SIZE, SIZE, "borderLine", 400, 400, "LightGrey");
board.draw('game-board');  // 'game-board' is the id of an <div> in which the game board is displayed.
        
// Event handler registration for mouse 'click'
board.cellsEventListener('click', function(e) {
    var pos = board.position(e);  // pos: {row:r, col:c}
            
    // Person
    // If there is not 'X' or 'O' mark, i.e., not occupied
    if (board.cellContent(pos.row, pos.col) != 'X' && board.cellContent(pos.row, pos.col) != 'O' ) {
        ttt_mark(board, pos.row, pos.col, 'X', X_O_SIZE);  // Mark the cell with 'X'
                
        // Now computer's turn
        ttt_computer_turn(board, X_O_SIZE);
    }
            
});
        
// For coloring on the mouse movement
board.cellsEventListener('mouseenter', function(e) {
    var pos = board.position(e);
    var current_color = board.cellBackgroundColor(pos.row, pos.col);
    board.cellMark2(pos.row, pos.col, current_color);
    board.cellBackgroundColor(pos.row, pos.col, 'LightSteelBlue');
});
board.cellsEventListener('mouseleave', function(e) {
    var pos = board.position(e);
    var previous_color = board.cellMark2(pos.row, pos.col);
    board.cellBackgroundColor(pos.row, pos.col, previous_color);
});

Check if all marked; if so, return;  // It is implemented.
Defense first:  // You need to complete some parts.
    For each empty cell,
        Check if the user can win with the cell;
        If so, use the cell and return;
Offense:  // It is implemented.
    Find the best position and use it;