This is a basic example of how to grab a user email from a form submission and send them an email

mailApp.js

function myFunction(e){
//pass inital event parameter to access form responses 
//use e.values to get whatever form values you want 
var userEmail = e.values[1]; 

//use MailApp.sendEmail to send email 
var subject = "Email Sent from Google Form"; 
var body = "Email sent from google forms body"; 

MailApp.sendEmail(userEmail, subject, body); 

//Learn more at this URL: https://developers.google.com/apps-script/reference/mail/mail-app#sendemailrecipient-subject-body

  
}