按钮事件侦听器始终返回相同的值

Question

preload.js

const ipcRenderer = require('electron').ipcRenderer;
window.addEventListener('DOMContentLoaded', () => {
  a = ""
  function add_chr(val){a=a+val;console.log(a,val)}
  for(var i of ["0","1","2","3","4","5","6","7","8","9","+","-","*","/","."]){
    document.getElementById(i).addEventListener("click",()=>{add_chr(i)})
  } 
})

示例按钮：

<td><button type="button" id="9">9</button></td>
<td><button type="button" id="8">8</button></td>
<td><button type="button" id="7">7</button></td>

在for循环中设置的事件侦听器始终传递“”。无论我单击哪个按钮，都可以使用add_char函数。为什么只传递数组中存在的最后一个字符？