Cult.fit Full Stack Developer Interview: Rounds, Questions and Preparation

What the job involves

Full Stack Developers at Indian unicorns and high-growth startups like Cult.fit build and maintain both the customer-facing web and mobile interfaces and the backend services that power them. They work across the stack, from React or React Native frontends to Node.js or Java backends, often shipping features quickly in small agile teams. Expect close collaboration with product, design, and DevOps to deliver scalable, user-friendly experiences for millions of users.

Role and Technical Knowledge

1. 1

   How would you design a REST API for a fitness class booking system like the one used by Cult.fit?

   Cover endpoints for listing classes, booking, cancelling, and checking availability. Discuss HTTP methods, status codes, authentication, and how to handle concurrent bookings.

2. 2

   What is the difference between SQL and NoSQL databases, and when would you choose one over the other for a high-growth startup?

   Explain ACID vs BASE, schema flexibility, scaling patterns, and give examples like PostgreSQL for transactions and MongoDB for flexible user profiles or activity logs.

3. 3

   Explain how React's virtual DOM works and why it improves performance for a fitness app with frequent UI updates.

   Describe the diffing algorithm, reconciliation, and how it minimises direct DOM manipulation. Relate it to real-time updates like live class schedules or workout trackers.

4. 4

   What are some common security practices you would follow when building a full stack application for a consumer app?

   Mention input validation, parameterised queries to prevent SQL injection, JWT or OAuth for auth, HTTPS, rate limiting, and secure storage of secrets.

Practical Scenarios

1. 5

   A user reports that their workout history is not loading on the app. How would you debug this across the full stack?

   Walk through checking frontend network calls, API response, server logs, database queries, and authentication tokens. Show a systematic approach from client to database.

2. 6

   You need to add a new feature to show personalised workout recommendations. How would you approach it from frontend to backend?

   Discuss gathering user data, defining an API endpoint, choosing a recommendation logic (even simple rules), and rendering it in React. Mention caching and performance.

3. 7

   How would you optimise a slow-loading page in a React app that fetches data from multiple APIs?

   Cover code splitting, lazy loading, memoisation, parallel API calls, caching, and using tools like React Query or SWR. Mention measuring with Lighthouse.

4. 8

   Describe how you would handle a deployment where a bug slipped into production. What steps would you take?

   Show a calm, methodical approach: rollback, hotfix, root cause analysis, adding tests, and improving CI/CD checks. Emphasise communication with the team.

Behavioural and HR

1. 9

   Tell me about a time you had to learn a new technology quickly for a project. How did you go about it?

   Use STAR format. Highlight self-learning, documentation, building a small prototype, and applying it successfully under a deadline.

2. 10

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

   Show enthusiasm for fast-paced environments, ownership, and impact. Connect your personal interest in fitness or consumer tech to the company's mission.

3. 11

   Describe a situation where you disagreed with a teammate on a technical approach. How did you resolve it?

   Focus on data-driven discussion, listening, and compromise. Show that you prioritise the product's success over being right.

4. 12

   Where do you see yourself in three years as a full stack developer?

   Express a desire to deepen technical expertise, take ownership of modules, mentor juniors, and contribute to architecture decisions. Align with the company's growth.

Problem Solving and Coding

1. 13

   Given an array of integers representing daily step counts, find the longest streak of days where steps were above a threshold.

   Clarify the threshold and edge cases. Use a single pass with a counter, resetting when below threshold. Discuss time and space complexity.

2. 14

   Write a function to detect if a string is a valid palindrome, ignoring non-alphanumeric characters and case.

   Use two pointers or filter and reverse. Mention Unicode considerations and test with empty strings and special characters.

3. 15

   How would you find the first non-repeating character in a string?

   Use a hash map to count frequencies, then iterate to find the first with count 1. Discuss O(n) time and O(k) space.

4. 16

   Implement a simple debounce function in JavaScript. Where would you use it in a fitness app?

   Explain closure, setTimeout, and clearTimeout. Relate it to search bars or button clicks to prevent excessive API calls.