Count number of columns in a table row

Why not use reduce so that we can take colspan into account? :)

function getColumns(table) {
    var cellsArray = [];
    var cells = table.rows[0].cells;

    // Cast the cells to an array
    // (there are *cooler* ways of doing this, but this is the fastest by far)
    // Taken from https://stackoverflow.com/a/15144269/6424295
    for(var i=-1, l=cells.length; ++i!==l; cellsArray[i]=cells[i]);

    return cellsArray.reduce(
        (cols, cell) =>
            // Check if the cell is visible and add it / ignore it
            (cell.offsetParent !== null) ? cols += cell.colSpan : cols,
        0
    );
}

document.getElementById('table1').rows[0].cells.length

cells is not a property of a table, rows are. Cells is a property of a row though

You could do

alert(document.getElementById('table1').rows[0].cells.length)

fiddle here http://jsfiddle.net/TEZ73/