After the HR introduction, you follow these steps:

1. (Round 1) Home assignment: You have 1 hour and 30 minutes to complete 2 coding challenges. The challenges are of increasing difficulty (and fun).

2. (Round 1) Skills Interview: You receive a project a few hours before the interview and have to demonstrate that you can navigate it and add a small feature. Before you get to the coding part, you are asked a few system design questions (45 minutes).

3. (Round 1) Behavioural Interview: An informal 1-hour conversation where you are asked about leadership and how you handled past situations in a team. It's better if you have a look at the Klarna leadership principles.

4. (Round 2) Team Fit Interview: You have a chat with your potential colleagues, who tell you more about the team you are going to be joining. At this stage, you can decide whether you like the team or not. In case you don't, you can have other interviews with other teams.

5. (Round 2) Home Logical Test: You get a 15-minute Kattis test to be done online. Make sure you have tried a couple of similar tests before.

6. (Round 2) Skills Interview: A 1 hour and 15-minute chat with an Engineering Manager, usually around system design and architecture. It could include coding, but it's really up to the interviewer.

7. Offer and negotiation: You get offered salary + RSU. It's easier to negotiate on the bonuses rather than the actual base salary.

Given a small Node.js project: does the function have concurrency issues?