Koo Full Stack Developer Interview: Rounds, Questions and Preparation

What the job involves

A Full Stack Developer at high-growth Indian startups builds and maintains both client-side and server-side components of web applications, often working across the entire stack from React or Vue frontends to Node.js or Python backends and databases. Day to day involves writing APIs, integrating third-party services, optimising performance for scale, and collaborating with product and design teams in fast-paced sprint cycles. In consumer-facing unicorns like Koo, you may also work on features that impact millions of users, requiring attention to scalability, security, and quick iteration.

Role and Technical Knowledge

1. 1

   Explain the difference between REST and GraphQL. When would you choose one over the other for a social media app like Koo?

   A strong answer covers REST's multiple endpoints versus GraphQL's single endpoint and flexible queries, and discusses trade-offs like over-fetching, caching, and real-time updates. Mention that GraphQL can be better for complex, evolving frontends but adds complexity.

2. 2

   How does the event loop work in Node.js, and why is it important for handling many concurrent requests?

   Explain the single-threaded event loop, non-blocking I/O, and how callbacks and promises are queued. Relate it to scalability in a startup handling high traffic.

3. 3

   What are the key differences between SQL and NoSQL databases? Which would you use for storing user posts and why?

   Compare schema flexibility, scalability, and consistency. For posts, a NoSQL database like MongoDB might be chosen for flexible schema and horizontal scaling, but mention that SQL can be used if relationships are complex.

4. 4

   Explain how you would implement authentication and authorization in a full stack app. What are JWT and OAuth?

   Cover session-based vs token-based auth, JWT structure, and OAuth for third-party login. Emphasise security best practices like HTTPS, token expiry, and refresh tokens.

Practical Scenarios

1. 5

   You are building a feature to show a user's feed. How would you design the API and database schema to ensure fast loading and pagination?

   Discuss REST endpoints with query parameters for pagination (e.g., limit and offset), indexing in the database, and caching strategies. Mention considering cursor-based pagination for large datasets.

2. 6

   A page on your app is loading slowly. How would you debug and optimise it?

   Outline steps: use browser dev tools to identify bottlenecks, check network requests, optimise images, lazy loading, code splitting, and backend query optimisation. Mention measuring before and after.

3. 7

   How would you handle a situation where two users try to update the same post simultaneously?

   Explain optimistic vs pessimistic locking, versioning, or using transactions. Discuss how to prevent race conditions and ensure data integrity.

4. 8

   You need to integrate a third-party payment gateway. What steps would you take from frontend to backend?

   Cover secure API key handling, server-side validation, webhook integration, and error handling. Emphasise never exposing secrets on the client and testing in sandbox mode.

Behavioural and Teamwork

1. 9

   Describe a time you worked in a team to build a project. What was your role and how did you handle disagreements?

   Use STAR method: Situation, Task, Action, Result. Highlight collaboration, communication, and willingness to compromise for the team's goal.

2. 10

   Tell me about a challenging technical problem you faced and how you solved it.

   Focus on the problem-solving process, resources used (documentation, peers), and the outcome. Show persistence and learning.

3. 11

   How do you prioritise tasks when you have multiple deadlines?

   Discuss using urgency and importance matrix, communicating with stakeholders, and breaking tasks into smaller chunks. Mention flexibility in startup environments.

4. 12

   Give an example of how you learned a new technology quickly for a project.

   Show self-driven learning through online courses, documentation, or side projects. Emphasise applying it effectively under time constraints.

HR and Culture Fit

1. 13

   Why do you want to work at a high-growth startup like Koo rather than a large MNC?

   Express enthusiasm for fast-paced environments, ownership, and impact. Align with the company's mission and mention adaptability.

2. 14

   Where do you see yourself in the next two years?

   Show ambition to grow technically and take on more responsibility. Avoid saying you want to leave or start your own company immediately.

3. 15

   How do you handle feedback and criticism?

   Demonstrate openness to learning, give an example of incorporating feedback, and emphasise a growth mindset.

4. 16

   What are your salary expectations?

   Research typical fresher salaries in Indian startups, provide a range, and express flexibility based on overall compensation and learning opportunities.