smart-truncate.js

function truncate(string, words) {
  var max_chars = 6 * parseInt(words);
  var words_array = string.split(" ");
  var index = parseInt(words) - 1;
  var truncated = words_array.slice(0, index).join(" ");

  if ( truncated.length > max_chars ) {
    truncated = string.slice(0, max_chars) + "...";
  } else if ( truncated === string ) {
    truncated = string;
  } else {
    truncated = truncated + "...";
  }

  return truncated;
}

truncate("I enjoy playing counter strike source.", 3); 
// => I enjoy playing...

truncate("I enjoy playing counter strike source.", 6);
// => I enjoy playing counter strike sourc...

truncate("I enjoy playing counter strike source.", 100);
// => I enjoy playing counter strike source.