Summary of solutions to the "sys undefined" error in Ajax

Error: The "sys undefined" error occurs when customizing scripts in Ajax applications.
Solution 1: Obtain the pagerequestmanager instance statement (for example, var prm = SYS. webforms. pagerequestmanager. getinstance ();
) In the function.
Solution 2: place the JS Code after the scriptmanager control is defined.
Cause analysis: The scriptmanager control is placed in the body.
JavaScript code is placed before the scriptmanager control.
Because HTML is loaded row by row.
Because var PRM = SYS. pagerequestmanager. getinstance ();
It was called before loading the JS Code of the scriptmanager control,
So there is a situation where sys is undefined.
Solution 3: (I have tried it successfully !)
In the configuration file Add under Node

Solution 4:
If "system. Web. Extensions. dll" is referenced, it will be OK.
The Web. config configuration is as follows: