Cannot the problem of the Set property ' onclick ' of NULL

Today saw a W3school JS Click event Test Case, Details page: Http://www.w3school.com.cn/tiy/t.asp?f=js_dom_event_onclick4, its code is:

<! DOCTYPE html>
 
Click the effect is normal, and I usually test code when the general habit of JS code in the head tag, the above code if the JS code moved to the head tag, the browser will error, hint: uncaught typeerror:cannot Set Property ' OnClick ' of NULL.
 
analysis of the code,w3school the way the browser first loaded the button node to execute the JS, so I changed the code to:  
 
 
 
<! DOCTYPE html>  <button id= "MYBTN" > click here </button><p id= "Demo" ></p></body></ Html>
 
The test passes, stating that the node needs to be loaded before the OnClick event can be executed.
 
 

 

Cannot the problem of the Set property ' onclick ' of NULL