Tic Tac Toe using JavaScript

<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
    <style type="text/css">
        .btn {
            width: 55px;
            height: 55px;
            font-size: 25px;
        }
    </style>
    <title></title>
</head>
<body>


        <div>
            <table>
                <tr>
                    <td>
                        <input id="2Player" type="button" value="TwoPlayer" onclick="SelectGame(this.id)" name="2Player" />
                    </td>
                    <td>
                        <input id="Computer" type="button" value="Computer" onclick="SelectGame(this.id)" name="Computer" />
                    </td>
                </tr>
            </table>
            <table>
                <tr>
                    <td>
                        <input id="A1" type="button" onclick="checkUser(this.id);" class="btn" name="A1" />
                    </td>
                    <td>
                        <input id="A2" type="button" onclick="checkUser(this.id);" class="btn" name="A2" />
                    </td>
                    <td>
                        <input id="A3" type="button" onclick="checkUser(this.id);" class="btn" name="A3" />
                    </td>
                </tr>
                <tr>
                    <td>
                        <input id="B1" type="button" onclick="checkUser(this.id);" class="btn" name="B1" />

                    </td>
                    <td>

                        <input id="B2" type="button" onclick="checkUser(this.id);" class="btn" name="B2" />
                    </td>
                    <td>
                        <input id="B3" type="button" onclick="checkUser(this.id);" class="btn" name="B3" />
                    </td>
                </tr>
                <tr>
                    <td>
                        <input id="C1" type="button" onclick="checkUser(this.id);" class="btn" name="C1" />

                    </td>
                    <td>
                        <input id="C2" type="button" onclick="checkUser(this.id);" class="btn" name="C2" />

                    </td>
                    <td>
                        <input id="C3" type="button" onclick="checkUser(this.id);" class="btn" name="C3" />
                    </td>
                </tr>

            </table>
        </div>

    <script type="text/javascript">
        var result = "";
        var CheckForWinningOpton = "";
        var player = true;
        var totalCount = 0;
        var gameChosen = 0;

        var SelectGame = function (clickedId) {
            if (document.getElementById(clickedId).value == "TwoPlayer") {
                gameChosen = 0;
                document.getElementById("Computer").disabled = true;
            }
            else {
                gameChosen = 1;
                document.getElementById("2Player").disabled = true;
            }
        }
        function checkUser(clickedId) {
            if (gameChosen == 0) {
                totalCount = totalCount + 1;
                if (player == true) {
                    document.getElementById(clickedId).value = 'X';
                    document.getElementById(clickedId).disabled = true;
                    clear(false);
                }
                else {
                    document.getElementById(clickedId).value = 'O';
                    document.getElementById(clickedId).disabled = true;
                    clear(true);
                }
                checkForWinner();
            }
            else {
                //Check Whether Computer has any option to win this game
                //Block if Human is going to win
                //Pocket the Corner Place if first 2 Condition Fails
                //If corner places are full then go for Empty places
                    document.getElementById(clickedId).value = 'X';
                    document.getElementById(clickedId).disabled = true;
                    clear(false);

               result= CheckForWinningOpton("O");
                if(result==null)
                {
                    result = CheckForWinningOpton("X")
                    if(result==null)
                    {
                        result = LookforCorner();
                        if (result == null)
                            {
                            result = LookforEmpty();
                        }
                    }
                }

                document.getElementById(result).value = 'O';
                document.getElementById(result).disabled = true;
                clear(true);

                checkForWinner();
            }

        }


        CheckForWinningOpton = function (Value) {
            var A1 = document.getElementById("A1").value;
            var A2 = document.getElementById("A2").value;
            var A3 = document.getElementById("A3").value;

            var B1 = document.getElementById("B1").value;
            var B2 = document.getElementById("B2").value;
            var B3 = document.getElementById("B3").value;

            var C1 = document.getElementById("C1").value;
            var C2 = document.getElementById("C2").value;
            var C3 = document.getElementById("C3").value;



            if (A1 == Value && A2 == Value && A3 == "")
                return "A3";
            if (A1 == Value && A3 == Value && A2 == "")
                return "A2";
            if (A2 == Value && A3 == Value && A1 == "")
                return "A1";

            if (B1 == Value && B2 == Value && B3 == "")
                return "B3";
            if (B1 == Value && B3 == Value && B2 == "")
                return "B2";
            if (B3 == Value && B2 == Value && B1 == "")
                return "B1";

            if (C1 == Value && C2 == Value && C3 == "")
                return "C3";
            if (C1 == Value && C3 == Value && C2 == "")
                return "C2";
            if (C3 == Value && C2 == Value && C1 == "")
                return "C1";
            //Check Vertical


            if (A1 == Value && B1 == Value && C1 == "")
                return "C1";
            if (B1 == Value && C1 == Value && A1 == "")
                return "A1";
            if (A1 == Value && C1 == Value && B1 == "")
                return "B1";

            if (A2 == Value && B2 == Value && C2 == "")
                return "C2";
            if (B2 == Value && C2 == Value && A2 == "")
                return "A2";
            if (A2 == Value && C2 == Value && B2 == "")
                return "B2";

            if (A3 == Value && B3 == Value && C3 == "")
                return "C3";
            if (B3 == Value && C3 == Value && A3 == "")
                return "A3";
            if (A3 == Value && C3 == Value && B3 == "")
                return "B3";

            //Check Diagonal

            if (A1 == Value && B2 == Value && C3 == "")
                return "C3";
            if (B2 == Value && C3 == Value && A1 == "")
                return "A1";
            if (A1 == Value && C3 == Value && B2 == "")
                return "B2";


            if (C1 == Value && B2 == Value && A3 == "")
                return "A3";
            if (B2 == Value && A3 == Value && C1 == "")
                return "C1";
            if (C1 == Value && A3 == Value && B2 == "")
                return "B2";

            else
            return null;

        }

        function LookforCorner()
        {
            var A1 = document.getElementById("A1").value;
            var A2 = document.getElementById("A2").value;
            var A3 = document.getElementById("A3").value;

            var B1 = document.getElementById("B1").value;
            var B2 = document.getElementById("B2").value;
            var B3 = document.getElementById("B3").value;

            var C1 = document.getElementById("C1").value;
            var C2 = document.getElementById("C2").value;
            var C3 = document.getElementById("C3").value;



            if (A1 != "" && A3== "")
                return "A3";

            if (A3 != "" && A1 == "")
                return "A1";

            if (C1 != "" && C3 == "")
                return "C3";

            if (C3 != "" && C1 == "")
                return "C3";
            else
            return null;
        }
        var LookforEmpty=function()
        {
            var A1 = document.getElementById("A1").value;
            var A2 = document.getElementById("A2").value;
            var A3 = document.getElementById("A3").value;

            var B1 = document.getElementById("B1").value;
            var B2 = document.getElementById("B2").value;
            var B3 = document.getElementById("B3").value;

            var C1 = document.getElementById("C1").value;
            var C2 = document.getElementById("C2").value;
            var C3 = document.getElementById("C3").value;



            if (A1 == "")
                return "A1";
            if (A2 == "")
                return "A2";
            if (A3 == "")
                return "A3";


            if (B1 == "")
                return "B1";
            if (B2 == "")
                return "B2";
            if (B3 == "")
                return "B3";


            if (C1 == "")
                return "C1";
            if (C2 == "")
                return "C2";
            if (C3 == "")
                return "C3";
            else
            return null;


        }
        var checkForWinner = function () {
            var A1 = document.getElementById("A1").value;
            var A2 = document.getElementById("A2").value;
            var A3 = document.getElementById("A3").value;

            var B1 = document.getElementById("B1").value;
            var B2 = document.getElementById("B2").value;
            var B3 = document.getElementById("B3").value;

            var C1 = document.getElementById("C1").value;
            var C2 = document.getElementById("C2").value;
            var C3 = document.getElementById("C3").value;


            var checkUserWins = false;

            if (A1 == A2 && A2 == A3 && A1 != "") {
                checkUserWins = true;
                alert(A1 + " Wins");
            }
            if (B1 == B2 && B2 == B3 && B1 != "") {
                checkUserWins = true;
                alert(B1 + " Wins");
            }
            if (C1 == C2 && C2 == C3 && C1 != "") {
                checkUserWins = true;
                alert(C1 + " Wins");
            }

            if (A1 == B1 && B1 == C1 && A1 != "") {
                checkUserWins = true;
                alert(A1 + " Wins");
            }
            if (A2 == B2 && B2 == C2 && A2 != "") {
                checkUserWins = true;
                alert(A2 + " Wins");
            }
            if (A3 == B3 && B3 == C3 && A3 != "") {
                checkUserWins = true;
                alert(A3 + " Wins");
            }

            if (A1 == B2 && B2 == C3 && A1 != "") {
                checkUserWins = true;
                alert(A1 + " Wins");
            }
            if (A3 == B2 && B2 == C1 && A3 != "") {
                checkUserWins = true;
                alert(A3 + " Wins");
            }
            else if (totalCount == 9 && checkUserWins==false) {
                alert("Game Draw");
            }

        }
        var clear = function (value) {
            player = value;
        }
    </script>
</body>
</html>