Sequentially add class to child

addclass.js

 // add class to next child
  $.fn.add_class = function(options) {
    var opts = $.extend({}, $.fn.add_class.defaults, options);
    return this.each(function() {
      var children = opts.child;
      var index = 0;
      function addClassToNextChild() {
        if (index == children.length) return;
        children.eq(index++).addClass(opts.usedclass);
        window.setTimeout(addClassToNextChild, opts.delay);
      }
      addClassToNextChild();
    });
  }
  
  $.fn.add_class.defaults = {
    child: $('article'),
    usedclass: 'is-active',
    delay: 200
  };