Just sharing my experience, won't share specifics of any questions but sharing the concepts covered and some material I used. I worked really hard and enjoyed the prep process. After the interview I felt I had nothing to do:P. On the day of interivew I felt I could have performed much better. Alas! I still made it.

Online Coding:
Questions similar to:
https://leetcode.com/problems/number-of-islands/
https://leetcode.com/discuss/interview-question/411357/

Phone Screen:
Technical: Creating a reusable component, make it accessible, style it, optimize the code
This round did not have any behaviour question however the interviewer did ask about my favorite feature in a framework and comparison between different front end frameworks, my project experience, most challenging project why etc.

Onsite:
Each interviewer started the round with small talk 2-3min, then went on to behaviour questions 15-20min, followed by techincal questions 20-25min and ended with questions for the interviewer.

Round 1: (Bar Raiser)
LP: Insist on High Standards, Deliver Result
Technical: Tree quetion, used DFS to solve it and the complexity of question was incrementally increased.

Round 2: (System Design)
LP: Disagree and Commit, Ownership
Design: I almost bombed this round, It was around a system i had not practiced nor enjoyed, but managed to cover basic concepts like load balancing, scaling, cache, protocols. Used an online app to draw the design. I started with getting the requirement the interviewer mostly drived this round and went into details wherever he liked.

Round 3: (Front End)
LP: Customer Obsession
Technical - A basic tabable ui which is responsive, fires events, accessible, optimized, localized etc.

Round 4: (Algorithm/Front End)
LP: Earn Trust of others
Technical - a short algorithm for an online game. again did not go that well and my solution was not really optimized but towards the end talked about ways to make the code more maintainable and optimized

Mostly LP questions were similar to the one on this site:
https://interviewgenie.com/blog-1/2018/1/10/ownership. - There are questions for each LP.

Result:
I was not confident with my performance in system design round and final round however, the recruiter called in a week with an offer. :)

Preparation Material:
There was a month gap between phone screen and onsite so I studied the following:

Technical:
Leetcode:
40-50 medium/easy questions.
Topics covered - Sorting, DFS, BFS, Trie, Object Design, Linked List (single/doubly), etc..
https://www.youtube.com/channel/UCIPzukkCO01wPnMo1DZXIcQ/videos - search for linked list implementation by this guy it's really good for a basic js implementation

System Design:
*** System Design, Youtube videos by Gaurav Sen, TechDummies, Tushar Roy.
Design: Uber, Twitter, Cache, Netflix, etc
Concepts: Distributed System, CDN, Caching, Load Balancer, Heartbeats, Message Queues, Database SQL vs noSQL, Sharding, Storages, Microservices, Hashing, API Gateway, HTTP Protocols etc

Front End:
Frameworks, JS Concepts, OOPs, Design Patterns, Basic Regex Patterns,
David Sharrif Blogs, Tutorials by Mosh
Some good videos:
https://www.youtube.com/user/Fidde12345 - check the front end engineer interview, it sheds light on some good practices.
Practiced creating basic ui eg: carousel, accordion, star pattern, progress bar in both vanilla js and a framework

Behaviour:
Kept 3 Examples handy for Customer Obsession, Ownership, Insist on High Standards, Deliver Result, Disagree and Commit and 1 for others. Basically i had around 25 short stories and whenever I got some free time I would think of my experience for a principle.

Interview Questions to ask: https://github.com/viraptor/reverse-interview - this really helped I kept 2-3 interesting questions for each interviewer based on their role to make sure that the round ends on a good note:P

Overall experience was really good, interviewers were really nice and provided hints in between. The key was to clarify, think out loud and optimize before jumping to solution and explain the interviewer thes decisions made while coding. Also a week before the interview there was a prep call and recruiter gave great pointers