Why can't set `document.getElementById` to be global variable?

Question

For the code structure here

var mydo=sessionStorage.getItem("action");
function to_delete(){
    var _table=document.getElementById("showTable");
    //omit
    }
window.onload=function(){
    to_delete();
    }

I get desired result. Now rewrite the code structure as below:

var mydo=sessionStorage.getItem("action");
var _table=document.getElementById("showTable");
function to_delete(){
    //omit
    }
window.onload=function(){
    to_delete();
    }

An error occur, TypeError: _table is null.
Why can't set document.getElementById to be global variable?

Answer 1

In your rewrite var _table=document.getElementById("showTable"); runs before the document loads so the element does not exist.

Instead declare the global var _table; outside the function and assign to it in the load event.