js truncate method for string without cutting word

trunc.js

String.prototype.trunc = function(n) {
  if (this.length <= n) {
    return this;
  }
  var truncated = this.substr(0, n);
  if (this.charAt(n) === ' ') {
    return truncated;
  }
  return truncated.substr(0, truncated.lastIndexOf(' '));
};