Othello board in JavaScript

This is a JavaScript Othello board. It doesn't play, it just keeps score, highlights the available moves, and provides a playing surface.

function replace_text (parent, text)
{
    clear (parent);
    append_text (parent, text);
}
function clear(element)
{
    if (!element)
        return;
    while (element.lastChild)
        element.removeChild (element.lastChild);
}

// Clear a box

function clearbox (boxid)
{
//    alert (boxid);
    boxid.value = '';
}

function create_node(type,parent)
{
    if (!type) {
        alert ("No type given to create_node");
        exit;
    }   
    if (!parent) {
        alert ("No parent given to create_node");
        exit;
    }   
    var new_node = document.createElement (type);
    if (!new_node) {
        alert ("Creation of node of type '"+type+"' failed.");
        exit;
    }
    parent.appendChild (new_node);
    return new_node;
}

// Create a node of type "type", attach it to "parent", then attach
// some text to that node.

function create_text_node (type,parent,text)
{
    var new_node = create_node (type,parent);
    var text_node = document.createTextNode (text);
    if (!text_node) {
        alert ("Creation of text node with text '"+text+"' failed.");
        exit;
    }
    new_node.appendChild(text_node);
    return new_node;
}

// Add some text to a node

function append_text (parent,text)
{
    var text_node = document.createTextNode (text);
    if (!text_node) {
        alert ("Creation of text node with text '"+text+"' failed.");
        exit;
    }
    parent.appendChild(text_node);
}

function append_text_by_id (parent_id, text)
{
    var parent = getbyid (parent_id);
    clear (parent);
    append_text (parent, text);
}

// Clear a box

function clearbox (boxid)
{
//    alert (boxid);
    boxid.value = '';
}

function clearboxbyname (boxname)
{
//    alert (boxid);
    var boxid = getbyid (boxname);
    boxid.value = '';
}

function getbyid (id)
{
    var element = document.getElementById (id);
    if (! element) {
        alert ("Could not find element with id "+id);
        exit;
    }
    return element;
}

// Get the value of something.


function getvalue (id)
{
    var element = getbyid (id);
    var value = element.value;
    if (! value)
        return;
/*
BKB 2009-09-20 22:47:43:
This is commented out in order to make "canvas3.js" work.

        alert ("Element with id "+id+" has no value.")
        exit;
    }
*/
    return value;
}

var board=new Array();
for (var i=0;i<=8;i++)
    board[i]=new Array();

function init()
{
    for (var x=1;x<=8;x++)
        for (var y=1;y<=8;y++)
            if (x>3&&y>3&&x<6&&y<6)
                board[x][y]=(x+y)%2?"b":"w";
            else
                board[x][y]=0;
}

function piece(x,y,colour)
{
    var square = getbyid ("square"+x+y);
    append_text (square, String.fromCharCode (parseInt ("25CF",16)));
    square.className = square.className.replace (/empty/, colour);
}

function change_piece(x,y,colour)
{
    var square = getbyid ("square"+x+y);
    square.className = square.className.replace (/b/, "funky");
    square.className = square.className.replace (/w/, "b");
    square.className = square.className.replace (/funky/, "w");
}

var next="b";

/* change_ok if it is OK to change the squares. */

function check_valid (x,y,colour,change_ok)
{
    var min_x = x > 1 ? x - 1 : 1;
    var min_y = y > 1 ? y - 1 : 1;
    var max_x = x < 8 ? x + 1 : 8;
    var max_y = y < 8 ? y + 1 : 8;
    var found_opposite = false;
    if (board[x][y])
        return false;
    for (test_x = min_x; test_x <= max_x; test_x++) {
        for (test_y = min_y; test_y <= max_y; test_y++) {
            if (test_x == x && test_y == y)
                continue;
            var neighbour_colour = board[test_x][test_y];
            if (neighbour_colour != 0 && neighbour_colour != colour) {
                var x_offset = test_x - x;
                var y_offset = test_y - y;
                    //alert (x_offset +" "+ y_offset);
                for (t=1;;t++) {
                    var probe_x = x+t*x_offset;
                    var probe_y = y+t*y_offset;
                    //alert ("probing "+t+" "+probe_x +" "+ probe_y);
                    if (probe_x < 1 || probe_y < 1 ||
                        probe_x > 8 || probe_y > 8)
                        break;
                    if (board[probe_x][probe_y] == 0)
                        break;
                    if (board[probe_x][probe_y] == colour) {
                        found_opposite = true;
                        var s;
                        if (change_ok) {
                        for (s=1; s < t; s++) {
                            var change_x = x+s*x_offset;
                            var change_y = y+s*y_offset;
                            board[change_x][change_y] = colour;
                            change_piece (change_x, change_y, colour);
                        }
                        }
                        break;
                    }
                }
            }
        }
    }
    return found_opposite;
}

function check_any_valid (colour)
{
    var any_valid = false;
    for (x=1;x<=8;x++) {
        for (y=1;y<=8;y++) {
            var valid = check_valid (x,y,colour, false);
            if (valid) {
                any_valid = true;
                var square = getbyid ("square"+x+y);
                square.className += " valid";
            }
//          alert (x+" "+y+ " "+valid+" "+any_valid);
        }
    }
    return any_valid;
}

function declare_victory (white, black)
{
    if (white > black)
        alert ("White wins");
    else if (white < black)
        alert ("Black wins");
    else
        alert ("Tie");
}

function update_game ()
{
    var white = 0;
    var black = 0;
    var remaining = 0;
    for (var x = 1; x <= 8; x++) {
        for (var y = 1; y <= 8; y++) {
            if (board[x][y]=="w")
                white++;
            else if (board[x][y]=="b")
                black++;
            else
                remaining++;
        }
    }
    if (white+black+remaining != 64) {
        alert ("Inconsistency");
    }
    var wscore    = getbyid ("wscore");
    var bscore    = getbyid ("bscore");
    var remaining_td = getbyid ("remaining");
    var next_td = getbyid ("next");
    replace_text (wscore, white);
    replace_text (bscore, black);
    replace_text (remaining_td, remaining);
    if (remaining == 0) {
        declare_victory (white, black);
    } else {
        next_td.className = "square "+ next;
        if (!check_any_valid (next)) {
            alert (next+" has no moves");
            next=(next=="b"?"w":"b");
            next_td.className = "square "+ next;
            if (!check_any_valid (next)) {
                alert ("Neither does "+next+".");
                declare_victory (white, black);
            }
        }
    }
}

function reset_valid ()
{
    for (var x = 1; x <= 8; x++) {
        for (var y = 1; y <= 8; y++) {
            var square = getbyid ("square"+x+y);
            if (square.className.match(/valid/))
                square.className = "empty square";
        }
    }    
}

function play(x,y)
{
    if (board[x][y])
        return;
    if (!check_valid (x, y, next, true)) {
//      alert ("No go joe");
        return;
    }
    // Valid
    reset_valid ();
    board[x][y] = next;
    piece (x,y,next);
//    alert ("play "+x+","+y+" colour: "+colour);
    next=(next=="b"?"w":"b");
    update_game ();
}

function create_board ()
{
//    alert ("Creating board");
    init ();
    var board_table = getbyid ("board");
    var top_tr = create_node ("tr", board_table);
    for (x=0;x<9;x++) {
        var th;
        if (x)
            th=create_text_node ("th", top_tr, x);
        else 
            th=create_node ("th", top_tr);
        th.className = "square";
    }
    for (y=1;y<=8;y++) {
        var tr = create_node ("tr", board_table);
        for (x=0;x<9;x++) {
            if (x) {
                var td = create_node ("td", tr);
                td.className="empty square";
                td.id="square"+x+y;
                (function (x,y) {
                    td.onclick = function () {play (x,y);}
                }(x,y));
                if (board[x][y] != 0) {
                    piece (x, y, board[x][y]);
                }
            } else {
                var th = create_text_node ("th", tr, y);
                th.className="square";
            }
            
        }
    }
    update_game ();
}

<table id="board">

</table>
<table>
<tr>
<td class="w">White:</td><td id="wscore">0</td>
</tr>
<tr>
<td class="b">Black:</td><td id="bscore">0</td>
</tr>
<tr>
<td>Remaining:</td><td id="remaining">64</td>
</tr>
<tr>
<td>Next:</td><td class="square b" id="next">●</td>
</tr>
</table>

.square {width:50;height:50;font-size:40}
.w {background-color:green; color:white;}
.b {background-color:green; color:black;}
.empty {background-color:green}
.valid {background-color:fuchsia}