Bill Tracker - Backend Refactoring Learnings

2026-05-11 - Phase 1 Business Logic Extraction

Extract business logic from routes/bills.js into a dedicated service layer (services/billsService.js).

getDefaultCycleDay(cycleType) - Returns default cycle day based on cycle type
validateCycleDay(cycleType, cycleDay) - Validates cycle_day based on cycle_type rules
parseDueDay(value) - Parses and validates due_day (must be 1-31 integer)
parseInterestRate(value) - Parses and validates interest_rate (0-100 range)
getValidCycleTypes() - Returns array of valid cycle types
validateBillData(data, existingBill) - Comprehensive validation and normalization for bill create/update
validateCycleDayOnly(cycleType, cycleDay) - Convenience wrapper for cycle_day validation

Route handlers only - parse request, call service, send response
DB queries remain in routes (tightly coupled to HTTP flow, not pure business logic)
Error formatting with standardizeError (HTTP-layer concern)

validateBillData - Centralized validation function that handles both create and update scenarios
- Takes optional existingBill parameter to support partial updates
- Returns { errors, normalized } structure
- Validates all bill fields including category_id, history_visibility, cycle_type/cycle_day
getValidCycleTypes() - Exported constant array for consistency across files
VALID_VISIBILITY - Exported from service for reuse in other files if needed

Business logic is now testable in isolation without mocking Express request/response
Route handlers are thinner and easier to read
Validation rules are centralized in one place
Easier to add new bill-related operations without touching routes

routes/bills.js - Removed ~80 lines of business logic, replaced with service imports and calls
services/billsService.js - New file created with extracted business logic