Truncate a String

//My solution for this
function truncateString(str, num) {
  // Clear out that junk in your trunk
  //var newstr = '';
  if(num <= 3){
    str = str.slice(0,num) + '...';
  }
  else if(str.length > num){
    str = str.slice(0,num-3) + '...';
  }

  return str;
}

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

Its quite simple but was getting stuck. Finally