<script>
do
    {
        var size = prompt('What chessboard size do you want?');
        if(size == NaN || size<=0)
        {
        alert('Try to enter an integer.');
        size=null;
        }
        else
        break;
    }
    while (size==null)
    var secSize=size;

var step1 = '';
var step2 = '';
var summ = "";

for (var i=0;i<size;i++)
    {
        step1+='# ';
        step2+=' #';
    }
    alert(step1);
    alert(step2);
for (var i=0;i<=size;i++)
{
    if(size%2==1)
    summ+=step1+'\n';

    else
    summ+=step2+'\n';
}
console.log(summ);
alert('Done.');
</script>