Javascript错误：未捕获TypeError：无法读取null的属性'value'[duplicate](Javascript error: Uncaught TypeError: Cannot read property 'value' of null [duplicate])

 
 
 这个问题在这里已有答案： 
 
 
 
  
 为什么jQuery或像getElementById这样的DOM方法找不到元素？ 6个答案 
 
 
 
 
 我有两个问题。 
 1）我写了这个html标签，并在单独的文件中创建了javascript。 
 
<input type="text" id="callno" value="3333">
 
 在js中，我有 
 
var PhoneNumber = document.getElementById("callno").value;
 
 但是，当我在浏览器（谷歌浏览器）中查看控制台时，我发现有一个错误未捕获的TypeError：无法读取null的属性'value' 。 
 
 2）每当我更新html标签的值时，如何自动更新PhoneNumber的值，我怎么能这样做。 

 
 
This question already has an answer here:
 
 
 
  
 Why does jQuery or a DOM method such as getElementById not find the element?  8 answers  
 
 
 
 
I have two questions.
 1) I wrote this html tag and created javascript in separate file.
 
<input type="text" id="callno" value="3333">
 
In the js, I have 
 
var PhoneNumber = document.getElementById("callno").value;
 
However, when I look at the console in the browser (Google Chrome) I saw that there is an error Uncaught TypeError: Cannot read property 'value' of null. 
 
2) How can I make that each time when I update value of html tag in above the value of PhoneNumber updated automatically.