Tic Tac Toe: Check diagonal win

Question

I created a Tic Tac Toe game in JavaScript and HTML. In the game the user can choose how many columes and rows he wants to play with . I was abble to check win in a row and a colum but I don't have any idea for a diagonal. i will be glad if someon can give me a code example that can fit my program.

Here is the JavaScript code:

var rows;
var cols;
var Array;
var tor = 0;
function XO() {
    rows = +prompt("Enter rows : ");
    cols = +prompt("Enter cols : ");

    intBoard = new Array(rows);
    var r, c;
    for (r = 0; r < intBoard.length; r++) {
        intBoard[r] = new Array(cols);
        for (c = 0; c < cols; c++) {
            intBoard[r][c] = 0;
        }
    }

    var idNum = 0;
    var strToShow = "<table border='1'>";
    for (var r = 0; r < rows; r++) {
        strToShow = strToShow + "<tr>";
        for (var c = 0; c < cols; c++) {
            strToShow = strToShow + "<td id='" + idNum.toString() +
                " ' onclick='TdClicked(this)' width='80px' height='80px' >";
            //strToShow = strToShow + idNum.toString();
            strToShow = strToShow + "<img src='background.jpg' width='80px' height='80px' />";
            strToShow = strToShow + "</td>";
            idNum++;
        }
        strToShow = strToShow + "</tr>";
    }
    strToShow = strToShow + "</table>";
    document.write(strToShow);
}


var showfirst = 0;
function TdClicked(tdClickedThis) {
    var idClicked = tdClickedThis.id;
    var rowClicked = Math.floor(idClicked / cols);
    var colClicked = idClicked % cols;
    var rightcol = colClicked + 1;
    var leftcol = colClicked - 1;
    var upRow = rowClicked - 1;
    var downRow = rowClicked + 1;
    var idRightDown = downRow * cols + rightcol;
    var idLeftDown = downRow * cols + leftcol;
    var idRightUp = upRow * cols + rightcol;
    var idLeftUp = upRow * cols + leftcol;


    if (showfirst == 0) {
        document.getElementById(idClicked).innerHTML = "<img width='80px' height='80px' src='wolf.jpg'/>";
        showfirst++;
        intBoard[rowClicked][colClicked] = 1;

        for (c = 0; c < cols; c++) {           // col win
            if (intBoard[rowClicked][c] != 1)
                c = cols + 111;   //break;
        }
        if (c == cols)
            alert("PLAYER X WIN!!")


        for (r = 0; r < rows; r++) {           //row win
            if (intBoard[r][colClicked] != 1)
                r = rows + 111;   //break;
        }
        if (r == rows)
            alert("PLAYER X WIN!!")


        
    }

    else {
        document.getElementById(idClicked).innerHTML = "<img width='80px' height='80px' src='fox.png'/>";
        showfirst--;
        intBoard[rowClicked][colClicked] = 2;

        for (c = 0; c < cols; c++) {            //col win 
            if (intBoard[rowClicked][c] != 2)
                c = cols + 111;   //break;
        }
        if (c == cols)
            alert("PLAYER O WIN!!")


        for (r = 0; r < rows; r++) {            // row win
            if (intBoard[r][colClicked] != 2)
                r = rows + 111;   //break;
        }
        if (r == rows)
            alert("PLAYER O WIN!!")

    }

}