The problem Shopify- Ecommerce website solves

Frontend is built with React js which helps in creating components which could be used multiple times with just few lines of code. Also react doesn't go to server to fetch different pages hence it works faster too.
I have used Redux for managing, updating states and in order to keep track of all actions going on and debug easily.
Add to cart feature is there, even when user is not logged in the products he has added to the cart will still be visible on next visit.
Signup/ login/logout features.
Update profile feature.
Sends Email to user on signing up.
Functional Search Box with autoCompletion using Trie Data structure.
Filters are also present.
FAQ section is also present where users can ask their queries.

Challenges I ran into

Implementing cart function using redux. Learning Redux itself is difficult and then using it for implemeting was something little tricky I found as I had to use ternary operator for that.
Which algo to use for my search box was also challenging. I used Trie data structure for autocompletion then.