if a class exists jquery

if ($(".mydivclass")[0]){
    // Do something if class exists
} else {
    // Do something if class does not exist
}

if ($("#about").hasClass("opened")) {
  $("#about").animate({right: "-700px"}, 2000);
}

/* Answer to: "jquery check if element has class starting with" */

if(!$(this).is('[class*="answerbox"]')) {
    //Finds element with no answerbox class
} else {
    //The element has already an answerbox class
}