module-by-module-migration

Convert Module by Module Up Your Dependency Graph

Overview

When migrating JavaScript to TypeScript, follow the dependency graph: start with leaf modules (those with no dependencies) and work upward. This ensures that when you convert a module, all its dependencies are already typed. This minimizes type errors and makes migration manageable.

When to Use This Skill

• Migrating large codebases to TypeScript
• Converting JavaScript to TypeScript
• Managing dependencies during migration
• Planning migration order
• Teams adopting TypeScript

The Iron Rule

Migrate modules starting from leaves (no dependencies) and work up the dependency graph. This ensures dependencies are typed before dependents.

Migration Order

Leaf modules (no dependencies):
  - utils.js → utils.ts
  - constants.js → constants.ts
  - helpers.js → helpers.ts

Middle modules (depend on leaves):
  - api.js → api.ts
  - components.js → components.ts

Entry points (most dependencies):
  - app.js → app.ts
  - index.js → index.ts

Example

# 1. Convert utilities first (no dependencies)
# utils.js → utils.ts
# constants.js → constants.ts

# 2. Then services (depend on utilities)
# api.js → api.ts
# store.js → store.ts

# 3. Finally entry points (depend on everything)
# app.js → app.ts
# main.js → main.ts

Reference

• Effective TypeScript, 2nd Edition by Dan Vanderkam
• Item 82: Convert Module by Module Up Your Dependency Graph