Using an AI coding tool of your choice (e.g., Cursor), build a minimal full‑stack web application that supports simple monetary transactions and account balances. Requirements: expose a REST or GraphQL API for creating transactions (credit/debit), persist data (in‑memory or lightweight DB is acceptable), provide a simple frontend to submit and view transactions, validate inputs, prevent negative balances, and include basic tests. Provide setup instructions and a concise README.

This question evaluates full‑stack engineering competencies including API design, data persistence, input validation, transactional integrity for monetary data, frontend-backend integration, automated testing, and developer documentation.

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This is a medium difficulty System Design question, commonly asked during Technical Screen rounds at Brex.

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This question is commonly asked for Software Engineer candidates at Brex during technical interviews.

Build AI-Assisted Full-Stack Transactions App

Build a minimal full‑stack app for monetary transactions

Objective

Implement a minimal full‑stack web application that supports basic monetary transactions and account balances.

Requirements

Backend API
- Expose a REST or GraphQL API (choose one) to:
  1. Create accounts.
  2. Create transactions (credit or debit) against an account.
  3. Retrieve accounts and transactions.
Data persistence
- In‑memory store or a lightweight database is acceptable.
Frontend
- A simple page to:
  1. Create an account.
  2. Submit a credit or debit transaction.
  3. View the current balance and recent transactions.
Validation and rules
- Validate inputs (e.g., non‑negative amounts with up to 2 decimal places, required fields).
- Prevent negative balances (reject debits that would drive the balance below zero).
Tests
- Include basic automated tests covering happy paths and error cases.
Developer experience
- Provide setup instructions and a concise README.

Deliverables

Source code for backend and frontend.
Tests and instructions to run them.
README with how to run, API summary, and any assumptions.

Notes

Focus on correctness and clarity over breadth.
Handling money precisely is important; avoid floating‑point errors.
Keep the UI minimal; functionality matters more than styling.

Requirements

Backend API

Expose a REST or GraphQL API (choose one) to:
1. Create accounts.
2. Create transactions (credit or debit) against an account.
3. Retrieve accounts and transactions.

Data persistence

In‑memory store or a lightweight database is acceptable.

Frontend

A simple page to:
1. Create an account.
2. Submit a credit or debit transaction.
3. View the current balance and recent transactions.

Validation and rules

Validate inputs (e.g., non‑negative amounts with up to 2 decimal places, required fields).
Prevent negative balances (reject debits that would drive the balance below zero).

Tests

Include basic automated tests covering happy paths and error cases.

Developer experience

Provide setup instructions and a concise README.

Build AI-Assisted Full-Stack Transactions App

Quick Overview

Build a minimal full‑stack app for monetary transactions

Objective

Requirements

Deliverables

Notes

Solution

Submit Your Answer to Earn 20XP

Build AI-Assisted Full-Stack Transactions App

Quick Overview

Build a minimal full‑stack app for monetary transactions

Objective

Requirements

Deliverables

Notes

Solution

Submit Your Answer to Earn 20XP