Textarea do not change with Jquery when I add a text

Textarea do not change with Jquery when I add a text

I need to reset a textarea to its first value with Jquery and Ajax. My script does'nt work when I change the valu in the textarea.
Please see my JsFiddle and its explanation :
https://jsfiddle.net/peter45/9aq8b18t/
Thank you for your help.