Document. getElementById: how to obtain the control object that is null _ basic knowledge-js tutorial

Today, I wrote a webpage. When I want to dynamically display data from other programs in the background when loading onLoad on the page, it is always null to use document. getElementById to get the control object. But check that this id does exist. See the following examples and solutions. 1. The following is a simple example. A piece of information is displayed during page loading.

The Code is as follows:

<% @ Page language = "java" import = "java. util. *" pageEncoding = "UTF-8" %>