en.javascript.info/1-js/4-data-structures/3-string/3-truncate/solution.md

The maximal length must be maxlength, so we need to cut it a little shorter, to give space for the ellipsis.

Note that there is actually a single unicode character for an ellipsis. That's not three dots.

function truncate(str, maxlength) {
  return (str.length > maxlength) ? 
    str.slice(0, maxlength - 1) + '…' : str;
}