Zepto Instant Full Stack Developer Interview: Rounds, Questions and Preparation

What the job involves

A Full Stack Developer at an Indian unicorn like Zepto Instant builds and maintains both the customer-facing web apps and the backend services that power them. Day to day, you might work on a React dashboard for order tracking, write Node.js APIs for inventory updates, or optimize database queries for real-time delivery slots. You collaborate with product, design, and DevOps teams to ship features fast in a high-growth, high-traffic environment.

Role and Technical Knowledge

1. 1

   Explain the difference between client-side rendering and server-side rendering. When would you choose one over the other for a high-traffic e-commerce app?

   A strong answer covers performance, SEO, and initial load time. Mention that SSR is better for SEO-critical pages like product listings, while CSR suits dynamic dashboards.

2. 2

   How would you design a REST API for a quick-commerce app that needs to fetch nearby stores and their inventory?

   Cover endpoints, HTTP methods, query parameters for location and filters, and response structure. Discuss pagination, caching, and rate limiting for high traffic.

3. 3

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

   Explain ACID vs BASE, schema flexibility, and scaling. For orders, a relational database often makes sense due to transactions and consistency, but NoSQL can work for product catalogs.

4. 4

   How do you ensure security in a full-stack application? Give examples for both frontend and backend.

   Mention input validation, authentication (JWT, OAuth), HTTPS, CORS, and protection against XSS and SQL injection. On the frontend, talk about secure storage of tokens and avoiding eval.

Practical Scenarios

1. 5

   You notice that the checkout page is slow for users in a particular city. How would you debug and fix this?

   A strong answer includes checking network latency, CDN usage, database query performance, and server response times. Suggest optimizations like caching, code splitting, or regional servers.

2. 6

   A feature you built is causing frequent server errors in production. How do you handle the situation?

   Show a structured approach: check logs, reproduce the issue, roll back if needed, and communicate with the team. Emphasize monitoring tools and post-mortem learning.

3. 7

   You need to add real-time order tracking to the app. What technologies and architecture would you use?

   Discuss WebSockets, Socket.io, or server-sent events. Mention backend pub/sub with Redis or Kafka, and how to scale connections. Also consider fallback to polling.

4. 8

   How would you optimize a slow database query that fetches order history for a user?

   Cover indexing, query analysis with EXPLAIN, avoiding N+1 queries, and caching frequent queries. Also mention pagination and limiting data returned.

Behavioural and HR

1. 9

   Why do you want to work at a fast-growing startup like Zepto Instant instead of a large established company?

   Show enthusiasm for impact, learning, and ownership. Mention adaptability and desire to solve real problems at scale. Avoid saying you just want a job.

2. 10

   Describe a time you had to learn a new technology quickly for a project. How did you approach it?

   Use a specific example from college projects or internships. Highlight your learning process, resourcefulness, and how you applied it successfully.

3. 11

   Tell me about a time you worked in a team to build something. What was your role and how did you handle disagreements?

   Focus on collaboration, communication, and conflict resolution. Show that you listen to others and can compromise for the team's goal.

4. 12

   How do you prioritize tasks when you have multiple deadlines? Give an example.

   Discuss using urgency and importance, breaking down tasks, and communicating with stakeholders. Provide a concrete instance from academics or projects.

Problem Solving and Coding

1. 13

   Write a function to find the first non-repeating character in a string. Explain your approach and its time complexity.

   A strong answer uses a hash map to count frequencies in one pass, then a second pass to find the first with count 1. Time complexity O(n), space O(n).

2. 14

   Given an array of order amounts, find two orders that sum to a target delivery fee. How would you optimize for large inputs?

   Use a hash set to store complements. Explain O(n) time and O(n) space. Mention edge cases like duplicate values and no solution.

3. 15

   Design a simple URL shortener like the ones used for promotional links. What are the key components?

   Cover encoding (base62), database schema, redirection service, and caching. Discuss scalability, collision handling, and analytics.

4. 16

   How would you implement a rate limiter for an API endpoint to prevent abuse?

   Describe token bucket or sliding window algorithms. Mention using Redis for distributed rate limiting and returning 429 status codes with retry headers.