Ajax Return Javascript Code After Submit

i want to correct my last post,

i actually want to know how to put some javascript code on my php file after the ajax form has been submitted

for example, i want to check errors, if errors are found, i want to show an alert. i want the php file for the ajax to be send back javascript code. so whe i put the javascript code php file it returns back the value of the variable. this might be called parse or parsing, im sorry if i dont explain myself well.

i put the code but its not showing the alert after i submit the form.