Swapping values between cells in html Javascript tables

Question

I've written a code that has a 4x4 table with numbers 1-15 in each cell plus "empty" in one cell.

When I click any adjacent cell (above, left, right or below) to "empty", it swaps the two value. It seems to work well in the middle 2 rows but not the top and bottom row. Can anyone point out the problem?

<script type="text/javascript">

    var test = 0;
    var startNumber = 0;
    var arrayNumbers = new Array()

    for (i = 0; i < 16; ++i) {
        if (i == 15) { arrayNumbers[i] = "empty"; }
        else {
            ++startNumber;
            arrayNumbers[i] = startNumber
        }
    }

    shuffle(arrayNumbers);

    document.write("<table id = \"table1\" border = \"1\">")

    for (k = 0; k < 4; ++k) {
        document.write("<tr>")
        for (i = 0; i < 4; ++i) {
            document.write("<td>" + arrayNumbers[test] + "</td>");
            ++test
        }
        document.write("</tr>")
    }

    document.write("</table>")
    // Table Made

    function shuffle(array) {
        var currentIndex = array.length, temporaryValue, randomIndex;

        // While there remain elements to shuffle...
        while (0 !== currentIndex) {

            // Pick a remaining element...
            randomIndex = Math.floor(Math.random() * currentIndex);
            currentIndex -= 1;

            // And swap it with the current element.
            temporaryValue = array[currentIndex];
            array[currentIndex] = array[randomIndex];
            array[randomIndex] = temporaryValue;
        }

        return array;
    }

    var table = document.getElementById('table1'),
cells = table.getElementsByTagName('td');

    var i = 0, len = cells.length

    for (var i = 0, len = cells.length; i < len; i++) {
        cells[i].onclick = function (index) {

            return function () {
                if (cells[index - 1].innerHTML == "empty" && index !== 4 && index !== 8 && index !== 12) {
                    cells[index - 1].innerHTML = cells[index].innerHTML;
                    cells[index].innerHTML = "empty";
                }
                else if (cells[index + 1].innerHTML == "empty" && index !== 3 && index !== 7 && index !== 11) {
                    cells[index + 1].innerHTML = cells[index].innerHTML;
                    cells[index].innerHTML = "empty";
                }
                if (cells[index - 4].innerHTML == "empty") {
                    cells[index - 4].innerHTML = cells[index].innerHTML;
                    cells[index].innerHTML = "empty";
                }
                if (cells[index + 4].innerHTML == "empty") {
                    cells[index + 4].innerHTML = cells[index].innerHTML;
                    cells[index].innerHTML = "empty";
                }
            }

        }
        (i)
    }

</script>

Answer 1

Looks to me like the issue is checking elements in the array which don't exist throwing an error. i.e. you will call using your method index-4 = cells[-1] when someone clicks on box 3. I would suspect all your edges would be susceptible to errors.

In each loop you could simply check the bounding condition as well:

return function () {
    if ((index-1 >= 0) 
           && cells[index - 1].innerHTML == "empty" 
           && index !== 4 
           && index !== 8 
           && index !== 12) {
        cells[index - 1].innerHTML = cells[index].innerHTML;
        cells[index].innerHTML = "empty";
    }
    else if ((index+1<=15) 
           && cells[index + 1].innerHTML == "empty" 
           && index !== 3 
           && index !== 7 
           && index !== 11) {
        cells[index + 1].innerHTML = cells[index].innerHTML;
        cells[index].innerHTML = "empty";
    }
    ...
}

Doing the same for your +/- 4 as well. That should at least get you closer. Also I would recommend installing something you can use to debug your javascript (Firebug is awesome), as this error would have been apparent quickly.