$.fn.on(eventType, selector, selector, handler) みたいな物を。挙動はちょっと異なります。

delegate.js

var _delegate = function(elms, type, selector, callback){
  elms = (_type(elms) === "string") ? document.querySelectorAll(elms)
    : (! elms.length) ? [elms]
    : elms;
  Array.from(elms).forEach(function(elm){
    elm.addEventListener(type, function(e){
      var valid = !! Array.from(this.querySelectorAll(selector))
      .filter(function(el){
        return el === e.target;
      }).length;
      if(valid){
        callback.call(this, e);
      }
    }, false);
  });
};

howtouse.js

_delegate("#container", "click", ".childItem", function(e){
  console.log(e.target);
});