if (!isActive) {
      $parent.toggleClass('open');
  } // best

Damian Conway of Perl fame wrote an excellent style guide for C in the early/mid 90s which I'm pretty sure explained why (I can't find my copy):

Programmer A:

  if (!isActive) $parent.toggleClass('open');

Programmer B:

  if (!isActive) 
      $parent.toggleClass('open');
      doSomethingElse();

Oh dear.

Yep, that's why I always demand that if you're not going to use curlies, you make it a one-liner.

    // okay
    if (!isActive) $parent.toggleClass('open');

    // not okay
    if (!isActive)
      $parent.toggleClass('open');

That way you know when you change it to two lines, you have to add curlies.

I believe HTML5 boilerplate implements the smelly pattern.

https://github.com/h5bp/html5-boilerplate/blob/master/index....