Ellipses on a string - truncate and ... in javascript

function truncate(str, num) {
  if(num >= str.length){
    return str;
  }
  num = num - 3 >= 0 ? num-3 : num;

  return str.slice(0, num)+'...';
}

truncate("A-tisket a-tasket A green and yellow basket", 11);

truncate("Absolutely Longer", 2);

truncate("A-tisket a-tasket A green and yellow basket", 11) ;