Adds the given class to 1 element from the selection and return it:

setClassRand.js

// extend jQuery:
$.fn.setClassRand = function(cssClass) {
	return this.eq( Math.floor( Math.random() * this.length ) ).addClass(cssClass);
};

//usage:
//add the class "teaser__video--active" and play it:
var video = $(".teaser__video").setClassRand("teaser__video--active");
if (video.length) {
  video[0].play();
}