my countries.html
<script src='countries.js'></script>
<script>
  for (var i=0;i<countries.length;i++) {
    var c0=countries[i]; // eg 'LAOS'
    var letters=c0.split(''); // eg ['L','A','O','S']
    var arr=[]; // for storing the set
    for (var j=0;j<letters.length;j++) {
      var l=letters[j]; // eg 'L'
      for (k=0;k<countries.length;k++) {
        var c=countries[k]; // eg 'ALGERIA'
        var first_letter=c.split('')[0]; // eg 'A'
        if (l==first_letter) { // success
          arr.push(c);
          break; // move on to the next letter
        }
      }
    }
    // arr = (eg) ['LEBANON','ALGERIA','OMAN','SUDAN']
    // prepare for display on the screen
    var s='[';
    var t=',';
    for (var j=0;j<arr.length;j++) {
      if (j==(arr.length-1)) t='],<br>'; // last one
      s=s+"'"+arr[j]+"'"+t;
    }
    document.write(s);
  }
</script>