<!DOCTYPE html>
<html>
  <head>
    <meta charset="UTF-8" />
    <title>Document</title>
  </head>
  <body>
    <button id="btn1">按钮1</button>
    <button id="btn2">按钮2</button>
    <button onclick="demo()">按钮3</button>

    <script type="text/javascript" >
      const btn1 = document.getElementById('btn1')
      btn1.addEventListener('click',()=>{
        alert('按钮1被点击了')
      })

      const btn2 = document.getElementById('btn2')
      btn2.onclick = ()=>{
        alert('按钮2被点击了')
      }

      function demo(){
        alert('按钮3被点击了')
      }

    </script>
  </body>
</html>

reactjs基础知识：原生事件绑定_javascript