JavaScript的Onclick事件

Question

请帮助我。我有一个按钮，我想创建一个操作，这样，每当单击该按钮时，就会将100添加到特定div中的值。另外，我不希望刷新页面时div中的值恢复到以前的值。

<h4>Activity Earnings:<h4><br>
<h4 id="demo"> 0 </h4>

 <button onclick="myFunction(whatever number 
   that 
  is met in the demo's value, 100)">earn</button>

  <script>
   function myFunction(the number met in the  
   demo for instance: 0 is currently there,  100) {
   x1 = demo's value + 100;
   document.getElementById("demo").innerHTML  
   =  x1;
   }
</script>