SOURCE

function trigger(elem,event){
  if(document.all) {
  	elem.event();
  } else {
    var evt = document.createEvent("Events"); //还有onchange则是HtmlEvents
    evt.initEvent(event,true,true);
    elem.dispatchEvent(evt);
  };
}

var box = document.querySelectorAll('.box');
box[0].onclick=()=>{
  alert('我被调用了');
};
trigger(box[0],'click')

<div class="box"></div>

.box{
  width:100%;
  height: 300px;
  background: #888;
}