PRD Example 7: Expense Tracker CLI

Difficulty: Beginner | Project Type: CLI Tool | Time: 2-3 hours

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Overview

What	A command-line expense tracker for logging and viewing spending
Who	Anyone who wants to track daily expenses without a complex app
Why	Provides quick expense logging and simple monthly summaries from the terminal

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Core Features (MVP)

1. Add Expense: expense add 25.50 "Lunch" food → Logs expense with amount, description, category
2. List Expenses: expense list → Shows all expenses for current month
3. Summary: expense summary → Shows spending by category for current month
4. Delete: expense delete 5 → Removes expense by ID

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Non-Goals

Will NOT build:

• Income tracking or budgeting
• Multiple currencies or conversion
• Recurring expenses
• Receipt photos or attachments
• Export to CSV/Excel
• Charts or visualizations
• Date range filtering
• Multi-user or shared accounts

Will NOT use:

• External APIs
• Database (use JSON file)
• Web or GUI interface
• Financial libraries

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Technical Constraints

Language	Node.js 20+
Framework	None (stdlib only)
Dependencies	None — stdlib only
Data Storage	JSON file at ~/.expenses/data.json
Testing	Node.js built-in test runner
Code Style	ESLint, JSDoc comments

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Success Criteria

• [ ] expense add creates entry with amount, description, category, timestamp
• [ ] expense list shows current month's expenses with running total
• [ ] expense summary groups by category with subtotals
• [ ] expense delete removes by ID with confirmation
• [ ] Amounts are stored as cents (integers) to avoid float issues
• [ ] All tests pass

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Implementation Phases

Phase 1: Data Model & Add Command

Goal: Set up expense storage and creation

Tasks:

1. Create project with expense.js and package.json
2. Define expense schema: \{id, amount, description, category, date\}
3. Implement add command with argument parsing
4. Store amounts as cents (multiply by 100)

Verification:

node expense.js add 25.50 "Lunch" food
# Output: "Expense #1 added: $25.50 for Lunch (food)"

cat ~/.expenses/data.json
# Shows: {"expenses": [{"id": 1, "amount": 2550, "description": "Lunch", ...}]}

Deliverables: expense.js with add functionality

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Phase 2: List & Summary

Goal: Display expenses and summaries

Tasks:

1. Implement list command filtering by current month
2. Format amounts as currency ($XX.XX)
3. Implement summary command grouping by category
4. Show running totals

Verification:

node expense.js list
# Output:
# February 2025 Expenses:
# #1  02/12  $25.50  Lunch (food)
# #2  02/12  $4.50   Coffee (food)
# #3  02/11  $50.00  Gas (transport)
# Total: $80.00

node expense.js summary
# Output:
# February 2025 Summary:
# food:      $30.00 (2 items)
# transport: $50.00 (1 item)
# Total:     $80.00

Deliverables: List and summary commands

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Phase 3: Delete & Testing

Goal: Complete CRUD and add tests

Tasks:

1. Implement delete command with ID validation
2. Add confirmation prompt before delete
3. Write tests using Node.js test runner
4. Handle edge cases (empty data, invalid ID)

Verification:

node expense.js delete 1
# Output: "Delete Expense #1: $25.50 Lunch? (y/n): "
# User types: y
# Output: "Expense #1 deleted"

node --test
# All tests pass

Deliverables: Complete CLI with tests

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Agent Rules

Always	Ask First	Never
Store amounts as integers (cents)	Before changing JSON schema	Use floating point for money
Validate expense exists before delete	Before adding new categories	Store data outside ~/.expenses/
Confirm before destructive actions	Before changing date filtering	Add budget or income tracking
Format currency consistently ($XX.XX)		Delete without confirmation