Project 8: E-Commerce Shopping App

Build a conversational commerce flow with strict cart invariants, revalidation gates, and trusted checkout boundaries.

Quick Reference

Attribute Value
Difficulty Advanced
Time Estimate 3-4 weeks
Main Programming Language TypeScript
Alternative Programming Languages Python, Ruby
Coolness Level Level 5
Business Potential Very High
Prerequisites Cart logic, money precision, inventory controls
Key Topics Cart state, pricing invariants, checkout safety

1. Learning Objectives

  1. Maintain cart consistency across chat turns.
  2. Enforce server-side revalidation before checkout.
  3. Use integer minor units for all money operations.
  4. Design clear transactional confirmations.

2. All Theory Needed (Per-Concept Breakdown)

Transaction Trust in Conversational Commerce

Fundamentals Commerce workflows require strong invariants: totals must be correct, stock must be current, and user consent must be explicit before side effects.

Deep Dive into the concept Conversational commerce introduces hidden risk: users can jump steps or issue ambiguous commands. To remain safe, build an explicit state machine for cart lifecycle: browse, add/remove, review, confirm, delegated checkout. Every mutation should revalidate stock and recompute totals server-side. Never trust cached client totals for final actions.

Represent monetary values in integer minor units (for example cents) to avoid floating-point drift. Include pricing version or timestamp in envelopes to identify stale calculations.

Use clear checkout boundaries. Assistant workflows can guide, but final payment authorization must use approved payment pathways with transparent handoff.

Minimal concrete example

cart summary:
{ item_count:2, subtotal_cents:24900, tax_cents:1875, total_cents:26775, pricing_version:"pv_20260211_01" }

3. Project Specification

3.1 What You Will Build

A product discovery + cart management widget with checkout handoff.

3.2 Functional Requirements

  1. Search and filter products.
  2. Add/remove/update cart quantities.
  3. Revalidate inventory and pricing on each mutation.
  4. Confirm checkout with transparent summary and handoff.

3.3 Real World Outcome

User selects product and adds to cart.
Cart summary updates with deterministic totals.
Checkout step shows explicit confirmation and handoff.

4. Solution Architecture

catalog tools -> cart reducer -> pricing revalidator -> checkout boundary tool -> confirmation renderer

5. Implementation Guide

5.1 The Core Question You’re Answering

“How do I ensure transactional trust in a chat-driven shopping experience?”

5.2 Concepts You Must Understand First

  1. Money representation as minor units.
  2. Inventory consistency under concurrent updates.
  3. Explicit transactional confirmation UX.

5.3 Questions to Guide Your Design

  1. Which cart fields are authoritative on server?
  2. What triggers revalidation?
  3. How is checkout handoff communicated?

5.4 Thinking Exercise

Define cart invariants and simulate a stock-drop race condition.

5.5 The Interview Questions They’ll Ask

  1. Why avoid floating point for prices?
  2. How do you prevent stale inventory purchases?
  3. What should be validated at checkout time?
  4. How do you make checkout intent explicit?
  5. How do you test transactional retries?

5.6 Hints in Layers

  • Hint 1: Start with read-only catalog flow.
  • Hint 2: Add cart mutation tools with revalidation.
  • Hint 3: Add confirmation and handoff boundary.
  • Hint 4: Run concurrency simulation tests.

5.7 Books That Will Help

Topic Book Chapter
Data consistency “Designing Data-Intensive Applications” Ch. 7
Domain modeling “Domain-Driven Design” Value objects
Reliable workflows “The Pragmatic Programmer” Automation/testing

6. Testing Strategy

  • Pricing math tests.
  • Inventory race-condition tests.
  • Checkout boundary tests.

7. Common Pitfalls & Debugging

Pitfall Symptom Solution
Float-based pricing Rounding mismatches Use integer minor units
No revalidation Checkout failures or bad orders Revalidate at each mutation and commit
Ambiguous checkout step User mistrust Explicit confirmation summary

8. Extensions & Challenges

  • Add coupon and discount engine.
  • Add personalized recommendations.
  • Add post-purchase order management.

9. Real-World Connections

  • Retail assistants
  • B2B procurement workflows
  • Subscription management fronts

10. Resources

  • OpenAI monetization docs
  • OpenAI submission guidelines

11. Self-Assessment Checklist

  • I can enforce cart/pricing invariants.
  • I can explain checkout trust boundaries.
  • I can recover safely from mutation retries.

12. Submission / Completion Criteria

Minimum Viable Completion

  • Search -> cart -> confirmation -> handoff flow.

Full Completion

  • Includes race-condition tests, pricing invariants, and trust-focused UX states.