Edit in JSFiddle

var trainers = ['ash', 'misty', 'brock'];
var index = 0;

function change_text(){
  index++;  
  return trainers[index - 1]; 
}

$('ul:first li').each(function(index, el){
  var txt = change_text;
  $(this).text(txt);
});

<ul>
    <li>abc</li>
    <li>def</li>
    <li>ghi</li>
</ul>