<script>
  function one_row() {
    var s1=" <img src='counters/0.png' class='cell' id='";
    var s2="' cn='0' onClick='cell_clicked(this);'>";
    for (var i=0; i<5; i++) {
      document.write(s1 + i + s2);
    }
    document.write('<div>0</div>');
  }
</script>

You probably didn't think to put it in a function but remember that we have to do 4 more rows!
So how do we use this function?
Before we go there, let me show you a better way of storing JavaScript functions.
Rather than cluttering up our index.html file, we store them in separate files.
So create a file called one_row.js and store the above code in it but without the script tags.

To make this function available to our html, we add this line before we use the function.

<script src='one_row.js'></script>

Then the body of our html can be simply:

<div class='row'>
  <script>one_row();</script>
</div>