It's an approach that truncate a string given the maximum size, but following some conditions

truncateString.js

function truncateString(str, num) {
  var points = '...';
  return num < str.length ?
    ((num < points.length) ?
       str.substr(0, num).concat(points) :
       str.substr(0, num - points.length).concat(points)) :
    str;
}