How do I resist the urge to smack this girl?

She's the friend that sometimes she's okay, but then she starts to be totally rude and I just want to slap her. She gets mad over the dumbest things. It's like I have to tiptoe around her and watch what I say around her because she will just get angry over nothing. Like once in gym we were playing volley ball and she goes saying, "Gosh Joanna... you need to quit missing the ball." And I say, "You act as if you never miss the ball either." And I swear, she'll suddenly get so freakin angry and she'll be like "JOANNA SHUT UP. GOSH WHY DO YOU ALWAYS HAVE TO SAY SOMETHING?" She's like this with all of her friends. And if she doesn't get what she wants, she pouts and complains until she gets it. And yesterday on Facebook she was going on and on about how her favorite team was going to win the football game. Well, I was for the opposing team and when my team won, I had said, "Oh really? Because it my team won. So haha. :P" And then she goes typing in annoying all caps and saying stupid stuff that didn't make sense and really taking the situation seriously, as if I truly cared. So then I responded, once again not being serious and being totally obvious that I wasn't serious, and then she goes deleting my post. She can't stand it when someone says something she doesn't like. A few times I've come close to smacking her and she always has to keep going on and on even if I ignore her. And eventually I can't take it anymore and just switch to ***** mode. But I know she's not worth it.