js 事件防止冒泡 event.stopPropagation()

<div id="app">
  <button id="btn">click</button>
</div>

const btn = document.getElementById('btn');

btn.addEventListener('click', function(event){
  event.stopPropagation();//阻止冒泡
}, false);

btn.onclick = function(event) {
  event.stopPropagation();
}