how to make the browser save the user data entered the first time without having him to enter it again?

Question

I know I should use localstorage but my code made the user enter the data every time and if he didn't it will be saved as null
do you have any codes for that?

Answer 1

1. Local Storage is unsafe for sensitive information: I can go in your computer and copy everything you have in Local Storage, if I paste that in my computer I will be logged in as you.
2. Cookies are a bit safer, but I think you can still do whatever you want with them.
3. You should use JWT authentication (You'll have to do some research)