legacy-modernization

Legacy Modernization

Overview

Legacy code isn't bad code — it's code that solved a real problem when it was written. Modernization is the process of adapting it to current requirements without breaking what works.

Core principle: Never rewrite from scratch. Strangle it incrementally. Each step must leave the system in a working state.

Iron Law:

THE SYSTEM MUST WORK AFTER EVERY CHANGE
There is no "temporarily broken while we modernize"

When to Use

• Migrating from CommonJS to ESM
• Upgrading major framework versions (NestJS 9 → 10)
• Replacing callback patterns with async/await
• Moving from JavaScript to TypeScript
• Replacing deprecated APIs
• Architectural migration (monolith → modules)

Assessment Phase

Inventory

Before any changes, understand what you have:

# Size and complexity
find src/ -name "*.ts" -o -name "*.js" | \
  xargs wc -l | sort -rn | head -20

# Dependency graph
npx madge --circular src/

# Test coverage (what's protected)
npx jest --coverage --coverageReporters text-summary

# Outdated packages
npm outdated

# Known patterns to modernize
grep -rn "require\(" src/ --include="*.ts" | wc -l  # CommonJS in TypeScript
grep -rn "callback\|\.then\|\.catch" src/ --include="*.ts" | wc -l  # Promises vs async
grep -rn ": any" src/ --include="*.ts" | wc -l  # Untyped code

Risk Assessment

Rate each area for modernization risk:

## Modernization Risk Register

| Module | Lines | Test Coverage | External Deps | Criticality | Risk |
|--------|-------|---------------|---------------|-------------|------|
| rules.service.ts | 120 | 85% | None | High | LOW |
| mcp.service.ts | 450 | 45% | MCP SDK | High | HIGH |
| main.ts | 60 | 0% | NestJS | Medium | MEDIUM |

Risk = (Criticality × (100 - Coverage%)) / 100

Modernization Backlog

## Modernization Items

| ID | Pattern | Count | Files | Effort |
|----|---------|-------|-------|--------|
| M-001 | `any` type → explicit types | 23 | 8 files | 2h |
| M-002 | Callbacks → async/await | 12 | 4 files | 4h |
| M-003 | CommonJS → ESM imports | 45 | 15 files | 1h |
| M-004 | NestJS 9 → NestJS 10 | 1 | package.json | 8h |