Winternship 2025 Case Studies – Solutions

Welcome to my repository of solved lessons from the Winternship 2025 Case Studies.
This repo showcases my progress in mastering modern web development concepts through practical, hands-on challenges across TypeScript, React (Vite), MongoDB, and now Express.

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📘 Overview

The Winternship 2025 program provides structured case studies across:

• TypeScript → mastering language fundamentals, advanced types, generics, and design patterns
• React (Vite) → building interactive UIs with hooks, state management, and component-driven architecture
• MongoDB → designing aggregation pipelines for analytics and data transformation, CRUD operations, and ACID-compliant transactions
• Express → backend routing, middleware, and REST APIs

Each case study bridges theory and practice with real-world coding exercises.

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🚀 What’s Inside

• CaseStudy1 (TypeScript)

  • Functions (optional, default, rest parameters, callbacks, recursion, overloads)
  • Advanced Types (union, utility, mapped types)
  • Generics (generic classes and functions)
  • Design Patterns (Observer, Strategy)
  • Classes & Access Modifiers (inheritance, private/protected members)

• react/

  • Vite-based React app (my-react-app)

    • Component-driven solutions with hooks
    • State management patterns
    • Code-splitting and optimization
    • 🌐 Live Demo

  • Vite-based React app (my-task-app)

    • Standalone mini-app built with React + TypeScript
    • Interactive Task Manager (add, toggle, delete tasks with useReducer)
    • 🌐 Live Demo

• mongodb/

  • Case Study 1: MovieFlix Analytics (solution.js)

    • Aggregation pipelines using $match, $group, $project, $sort, $limit
    • Example queries: views, ratings, genres
    • Runnable via mongosh or Node.js driver

  • Case Study 2: FastBite CRUD Challenge (menu-crud.js)

    • Demonstrates insertOne, find, updateOne, deleteOne
    • Idempotent script using upsert and $addToSet for repeatable runs
    • Runnable via mongosh or Node.js driver

  • Case Study 3: FinTrust Transactions & Refunds (refund-transaction.js, seed-refund.js)

    • Implements multi-document ACID transactions in MongoDB
    • Secure money transfers with replica set enabled
    • Refund workflow: balance adjustments, transaction status updates, and audit trail logging
    • Prints balances before and after refund for verification
    • Demonstrates rollback prevention and double-refund safeguards

• express/

  • Lesson 1: Getting Started (app.js)
    • Basic Express server setup with npm init and npm install express
    • Routes: / (homepage), /events (list of activities), /contact (JSON with email + phone)
    • Demonstrates res.send() vs res.json() responses
    • Runnable via node app.js
  • Lesson 2: Modular routes (app.js, routes/)
    • Split routes into routes/events.js, routes/classes.js, routes/contact.js
    • Mounted routers in app.js with app.use('/path', router)
    • Organized project for scalability and team collaboration
    • Runnable via node app.js
  • Lesson 3: PATCH endpoint + validation (app.js, routes/products.js)
    • Implements PATCH /products/:id/inStock to update only the inStock field
    • Validates input: returns 400 Bad Request if inStock is missing or not a boolean
    • Returns 404 Not Found if product ID does not exist
    • Demonstrates partial updates, clear status codes, and modular routing
    • Runnable via node app.js
  • Lesson 4: POST /transfer endpoint (app.js, routes/transfer.js)
    • Implements POST /transfer to move points between accounts
    • Uses fixed valid UUIDs for Alice and Bob to simplify testing
    • Validates: both IDs must be valid UUIDs, points must be a positive integer
    • Returns 404 Not Found if sender/receiver not found, 400 Bad Request if invalid UUIDs or insufficient points
    • Responds with 200 OK and updated balances on success
    • Demonstrates transaction-like workflow, input validation, and structured error handling
    • Runnable via node app.js
  • Lesson 5: BakingController (app.js, routes/baking.js)
    • Implements POST /baking/start to begin baking an order

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🛠️ How to Run

Clone the repository and run any case study as described below.

TypeScript (General Case Studies)

Most lessons can be run directly by compiling the TypeScript file and executing the generated JavaScript:

cd Internship_caseStudies/CaseStudy1
tsc lesson-name.ts
node lesson-name.js

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TypeScript (Lesson 19)

Lesson 19 is a standalone TypeScript project and uses external dependencies.

cd Internship_caseStudies/CaseStudy1/nineteen
npm install
tsc
node dist/index.js

This will install required dependencies (typedi, reflect-metadata), compile the project using the local tsconfig.json, and run the output from the dist directory.

React (Vite)

cd Internship_caseStudies/react/my-react-app
npm install
npm run dev

This will start the Vite dev server and open the app in your browser.

Task Manager (Vite)

cd Internship_caseStudies/react/my-task-app
npm install
npm run dev

This will start the Vite dev server and open the app in your browser.
You can then interact with the Task Manager mini-app: add tasks, toggle completion, and delete tasks.

MongoDB

Case Study 1: MovieFlix Analytics

cd Internship_caseStudies/mongodb
npm install
node solution.js

Case Study 2: FastBite CRUD Challenge

cd Internship_caseStudies/mongodb
npm install
node menu-crud.js

Case Study 3: FinTrust Transactions & Refunds

cd Internship_caseStudies/mongodb
npm install
node seed-refund.js   # seed sample users + transaction
node refund-transaction.js   # run refund workflow

Express

Lesson 1: Getting Started

cd Internship_caseStudies/express/lesson1
npm install
node app.js

Then visit:

• http://localhost:3000/ → homepage
• http://localhost:3000/events → JSON list of events
• http://localhost:3000/contact → JSON with email + phone

Lesson 2: Modular routes

cd Internship_caseStudies/express/lesson2
npm install
node app.js

Then visit:

• http://localhost:3000/events → JSON list of events
• http://localhost:3000/classes → JSON list of classes
• http://localhost:3000/contact → JSON with email + phone

Lesson 3: PATCH endpoint + validation

cd Internship_caseStudies/express/lesson3
npm install
node app.js

Then test:

# Valid update
curl -X PATCH http://localhost:3000/products/1/inStock \
  -H "Content-Type: application/json" \
  -d '{"inStock": false}'

# Invalid update (missing or wrong type)
curl -X PATCH http://localhost:3000/products/1/inStock \
  -H "Content-Type: application/json" \
  -d '{}'
curl -X PATCH http://localhost:3000/products/1/inStock \
  -H "Content-Type: application/json" \
  -d '{"inStock": "yes"}'

Lesson 4: POST /transfer endpoint

cd Internship_caseStudies/express/lesson4
npm install
node app.js

Then test:

# Valid transfer
curl -X POST http://localhost:3000/transfer \
  -H "Content-Type: application/json" \
  -d '{"fromCustomerId":"11111111-1111-4111-8111-111111111111","toCustomerId":"22222222-2222-4222-8222-222222222222","points":100}'

# Invalid UUIDs
curl -X POST http://localhost:3000/transfer \
  -H "Content-Type: application/json" \
  -d '{"fromCustomerId":"123","toCustomerId":"456","points":50}'

# Insufficient points
curl -X POST http://localhost:3000/transfer \
  -H "Content-Type: application/json" \
  -d '{"fromCustomerId":"22222222-2222-4222-8222-222222222222","toCustomerId":"11111111-1111-4111-8111-111111111111","points":9999}'

Lesson 5: BakingController

cd Internship_caseStudies/express/lesson5
npm install
node app.js

Then test:

# Start baking an order
curl -X POST http://local