Update App.js

[sunnyjangra98]

Currently there is an issue, i.e. as follow:-

1. Add new user (user id will be 4)
2. Delete any user (let say user id 2)
3. Now add another user (user id will be 4 again), as user.length will be 3 (due to deletion at step 2)
4. Now delete user id created at step 1 or step 3, it'll delete both users because of same user id, hence updating it to different value each time using Date.now()