<script type="text/javascript">

function steamIdToLink(steamId)
{
  var
    parts  = steamId.split(':'),
  
    server = +parts[1],
    authId = +parts[2],

    profile = [7656119, 7960265728 + authId * 2 + server].join('');
  
  return '<a href="http://steamcommunity.com/profiles/' + profile + '">'
          + steamId +
         '</a>';
};

document.writeln( steamIdToLink('STEAM_0:1:12345678') );

</script>