Your JavaScript Codebase Needs TypeScript (And You Don't Want to Spend 6 Months Doing It)

You've got a 500-file JavaScript codebase that should be TypeScript. Traditional migration advice says "just convert one file at a time" which is like suggesting you eat an elephant with a teaspoon. I've seen migration horror stories where developers rage-quit after three months of manually adding type annotations to event handlers that should've been obvious.

What Actually Works in 2025

AI tools have gotten scary good at TypeScript migration. I migrated our 200-file React app last month. Took a weekend and maybe $50 in API calls - way cheaper than hiring a consultant. That includes all the time spent figuring out why Cline kept adding any types to React event handlers. The official migration docs still assume manual conversion, but AI tools changed the game completely.

What makes this year different:

• AI tools actually understand context now - Cursor reads your entire codebase, Cline runs workflows without fucking up, and Claude Code handles complex business logic
• TypeScript 5.7 (released in late 2024) has improvements for migration projects like better error messages and smarter type inference
• Workflow automation that doesn't make you want to throw your laptop out the window
• API costs dropped - competition drove prices down, making large migrations actually affordable

What Changed

The old way of migration sucked: plan everything perfectly upfront, convert files one at a time, manually fix every TypeScript compiler error, cry internally when you realize you made architecture decisions that don't work with strict mode.

The new way sucks less: set up workflows once, let AI do the grunt work, review the output like you're code reviewing a junior dev who's really eager but occasionally thinks every function parameter should be any. Expect to spend a day fixing React event handlers because AI keeps generating onChange: (e: any) => void instead of onChange: (e: ChangeEvent<HTMLInputElement>) => void.

Our "quick migration" turned into a 3-week nightmare until I figured out the build optimizations. Build times went to shit initially - like 4x slower. Took a few days of config tweaking to get back to reasonable speeds.

Why TypeScript Structure Makes AI Less Stupid

After migrating a bunch of codebases, here's what I learned: TypeScript gives AI something to work with. When you have actual type definitions instead of // TODO: figure out what this parameter is supposed to be, AI can understand your code better than you do. The type system provides context that helps AI make better suggestions instead of just guessing.

It's a decent feedback loop:

1. AI helps migrate to TypeScript (adding structure to your chaos)
2. TypeScript structure makes AI suggestions suck less
3. Less terrible AI suggestions means you don't spend 3 hours debugging why user.id is suddenly string | undefined

Tools That Won't Make You Regret Everything

After trying every tool that claims to help with TypeScript migration, here's what doesn't completely suck:

Tools that actually work:

• Cline - VS Code extension that runs autonomously. Best for "migrate this entire folder while I get coffee". Warning: it will add any types to everything and you'll spend a day fixing them. I've seen it convert function handleClick(event) to function handleClick(event: any) instead of the obvious MouseEvent<HTMLButtonElement>. Gets it wrong every time.
• Cursor - AI-first IDE that's surprisingly decent. Their TypeScript migration suggestions are better than mine, which hurts my ego but saves time.
• Claude Code - For when you need to explain why your 500-line function should stay a 500-line function and the AI should just add types, not refactor everything.

Old-school but useful:

• ts-migrate - Airbnb's tool from before AI was good. Still useful for "make everything compile first, fix types later". It adds any everywhere, but at least your build won't fail. Expect errors like TS7006: Parameter 'props' implicitly has an 'any' type on literally every React component.
• ts-morph - For when you need to write your own migration scripts because your codebase has 47 different ways to handle API responses.

Don't just pick one tool and pray. The successful migrations I've seen use 2-3 tools in sequence - start broad with ts-migrate, then let AI fix the obvious stuff, then cry over the edge cases.

Recent TypeScript Improvements for Migration

TypeScript 5.7 (released late 2024) has some decent improvements for migration projects:

Better Project Initialization:

## Recent TypeScript creates sane defaults instead of 50 lines of commented garbage
npx tsc --init

## Creates a tsconfig.json that actually works instead of breaking everything
## No more "strict mode will make you hate TypeScript" surprises

Smarter Error Messages:

// Error messages are slightly less cryptic now
// Still not great, but better than "Type 'never' is not assignable to type 'string'"
function getUser(userId: string) { /* ... */ }
// getUser(userID); // Now gives better context about the typo

Improved Type Inference:

// Better type narrowing in newer versions
const config = {
  apiUrl: "https://api.example.com",
  timeout: 5000
} satisfies Record<string, string | number>;

// TypeScript is smarter about inferring the actual types here

The Cost-Benefit Reality Check

Let's talk actual numbers based on 2025 migration experiences:

Traditional Manual Migration (1000-file project):

• Time: 3-6 months (if you don't quit first)
• Developer cost: Whatever your sanity is worth
• Risk: High (I've seen teams abandon half-migrated codebases)
• Result: You'll miss obvious stuff because you're burned out

AI-Assisted Migration (1000-file project):

• Time: 2-4 weeks (plus another week fixing the AI's mistakes)
• AI API cost: Could be a few hundred in API calls depending on usage (check current pricing)
• Developer cost: Your dignity when you realize the AI is better at this than you
• Risk: Low (iterative approach means you can always fix stuff later)
• Result: Consistently decent, though you'll still need to fix the React event handlers manually

Even if you blow money on API calls, it beats manually converting function(event) to function(event: React.FormEvent<HTMLFormElement>) thousands of times. I spent half a day just figuring out why onClick={(e) => handleSubmit(e)} was throwing TS2345: Argument of type 'MouseEvent' is not assignable to parameter of type 'FormEvent'.

When AI Migration Makes the Most Sense

Perfect candidates for AI-assisted migration:

• Codebases 20-5000+ files (sweet spot for AI efficiency)
• Projects with existing patterns (AI can learn and replicate)
• Teams comfortable with iterative approaches (not perfectionism)
• TypeScript-curious developers (motivated to learn)

When to consider traditional approaches:

• Tiny projects (<20 files - faster to do manually)
• Highly unique domain logic (AI training data lacks context)
• Teams with zero TypeScript experience (need educational value of manual work)

Setting Realistic Expectations

AI-assisted migration isn't magic. Here's what to expect:

What AI does excellently:

• Bulk file renaming and basic type annotations
• Pattern recognition and replication
• Boilerplate generation
• Consistency across large codebases

What requires human oversight:

• Complex type relationships and generics (AI loves generating T extends Record<string, unknown> for everything)
• Domain-specific business logic types (AI doesn't know your API returns userId as string sometimes, number other times)
• Performance optimization decisions (AI will happily generate 47 nested generic types that tank your build)
• Team coding standards (unless you want every interface named IWhatever)

The goal isn't to have AI do everything perfectly - it's to have AI handle 80-90% of the tedious work so humans can focus on the interesting, high-value decisions that actually matter.

This guide shows you how to set up these workflows, which tools to use when, and how to migrate without losing your shit.

The difference between "this AI stuff is magic" and "I'm going back to manual migration" comes down to workflow setup. Most people skip this step and then wonder why the AI keeps generating garbage. Here's what works after you've made the same mistakes I did.

Phase 1: Environment and Tool Preparation (30-60 minutes)

1. Install and Configure Your AI Tool

For Cline (recommended for large migrations):

## Install Cline in VS Code
## Go to Extensions → Search "Cline" → Install

## Set up API keys (choose one):
export ANTHROPIC_API_KEY="your-key-here"  # For Claude 3.5 Sonnet
export GOOGLE_API_KEY="your-key-here"     # For Gemini 2.5 Pro  
export OPENAI_API_KEY="your-key-here"     # For GPT-4o

Claude 3.5 Sonnet is best for code understanding, but the others work fine too.

2. Create a Migration-Friendly tsconfig.json

Critical: Don't copy your production TypeScript config. I learned this the hard way when our build took 45 minutes because I had "declaration": true enabled during migration. You need a migration-specific config or you'll spend more time fixing errors than migrating:

{
  "compilerOptions": {
    "target": "ES2022",
    "module": "ESNext", 
    "lib": ["ES2022", "DOM"],
    "allowJs": true,              // THIS IS CRITICAL - lets JS and TS coexist
    "checkJs": false,             // Don't type-check JS files (they'll fail)
    "declaration": false,         // Skip .d.ts generation (it's slow) 
    "outDir": "./dist",
    "rootDir": "./src",
    "strict": false,              // START FALSE - enable after migration
    "noImplicitAny": false,       // This will break everything initially
    "skipLibCheck": true,         // Ignore node_modules types (performance)
    "esModuleInterop": true,
    "forceConsistentCasingInFileNames": true,
    "moduleResolution": "node",
    "resolveJsonModule": true,
    "exactOptionalPropertyTypes": false,  // This breaks a lot of existing code
    "noUncheckedIndexedAccess": false     // Another migration killer
  },
  "include": ["src/**/*"],
  "exclude": ["node_modules", "dist", "**/*.test.js", "**/*.test.ts"]
}

Pro tip: I wasted weeks because I started with "strict": true. The moment I enabled it, every file threw TS2322: Type 'string | undefined' is not assignable to type 'string' and I couldn't even compile anything. Don't be me. Enable strict mode after your migration is complete, not before.

3. Create Core Type Definitions (Don't Skip This)

This is the step that separates successful migrations from disasters. I need to manually define core types BEFORE letting AI loose. Otherwise the AI will invent types that don't match my actual data.

I create src/types/index.ts with MY actual data structures:

// Look at your actual API responses and define these
export interface User {
  id: string;                    // Check if it's actually string or number
  email: string;
  name: string;
  createdAt: string;            // API probably returns string, not Date
  updatedAt: string;            // Fix this after you see what you actually get
}

export interface ApiResponse<T = unknown> {
  data: T;
  message?: string;             // Probably optional in your API
  status: 'success' | 'error';
  timestamp: string;
}

// Utility types that actually help during migration
export type Partial<T> = { [P in keyof T]?: T[P] };  // Already exists, but good to know
export type RequireOnly<T, K extends keyof T> = Pick<T, K> & Partial<Omit<T, K>>;

// React event types that AI always gets wrong
export type FormSubmit = React.FormEvent<HTMLFormElement>;
export type ButtonClick = React.MouseEvent<HTMLButtonElement>;
export type InputChange = React.ChangeEvent<HTMLInputElement>;

// Use these like: onClick: (e: ButtonClick) => void

Critical: I base these on my ACTUAL data, not what I think my data should look like. I use browser dev tools to inspect my API responses to make sure my types match reality.

Phase 2: AI Workflow Configuration (15-30 minutes)

1. Create Cline Migration Workflow

I save this as .cline/workflows/migrate-typescript.md:

## TypeScript Migration Workflow

You are migrating JavaScript files to TypeScript following these steps:

### Context Collection
1. Read the target file to understand its purpose and patterns
2. Check for existing TypeScript files in the same directory for style consistency  
3. Look for `types/index.ts` for project-specific type definitions
4. Review `tsconfig.json` for project configuration

### Migration Process
1. **File Renaming**: Convert .js to .ts (or .jsx to .tsx for React)
2. **Type Annotations**: Add type annotations for:
   - Function parameters and return types
   - Variable declarations where type isn't obvious
   - Object properties and interfaces
   - Event handlers (use predefined types from types/index.ts)
3. **Import Updates**: 
   - Add type-only imports: `import type { User } from '../types'`
   - Update relative imports to include file extensions if needed
4. **JSDoc to TSDoc**: Convert JavaScript doc comments to TypeScript format

### Quality Standards  
- Prefer specific types over `any`
- Use `unknown` instead of `any` when type is truly unknown
- Leverage `satisfies` operator for type validation without losing inference
- Add `// @ts-expect-error` with explanation for temporary issues

### Validation
1. Run `npm run build` to check for compilation errors
2. Run `npm run lint` to verify code style
3. Provide summary of changes made

### Important Rules
- NEVER change function/variable names
- NEVER change business logic
- ALWAYS preserve existing behavior
- Use project's existing type definitions from types/index.ts

2. Configure Project-Specific Rules

I create .cline/rules.md:

## TypeScript Migration Rules

### Code Style
- Use interface for object types, type for unions/primitives
- Prefer `const assertions` over type annotations where applicable
- Use template literal types for string patterns
- Export types and interfaces from dedicated type files

### Migration Priorities
1. Core types and utilities first
2. API layer and data models second  
3. React components third
4. Test files last (optional)

### Error Handling Strategy
- Use `@ts-expect-error` with TODO comments for complex issues
- Prefer gradual improvement over perfect types initially
- Document any `any` usage with explanation and migration plan

Phase 3: Execution Strategy (The Human-in-the-Loop Process)

1. Start with Foundation Files

I begin migration with these file types (in order):

src/types/           # Already done manually
src/utils/           # Pure functions, easiest to type
src/constants/       # Simple objects and enums  
src/config/          # Configuration objects
src/api/            # HTTP clients and data fetching
src/hooks/          # Custom React hooks (if applicable)
src/components/     # React components
src/pages/          # Route components

2. The AI Migration Command

For each file or directory:

## In Cline chat interface:
@migrate-typescript.md src/utils/formatters.js

## Or for entire directories:
@migrate-typescript.md src/utils/

3. Review and Refinement Process

After AI generates the TypeScript code:

1. Immediate Review (2-3 minutes per file):

   • Check that imports resolve correctly
   • Verify types make sense for the domain
   • Ensure no business logic changed

2. Compilation Check:

   npm run build
   # Fix any TypeScript errors iteratively
   

3. Quality Assessment:

   npx type-coverage --detail
   # Aim for >80% type coverage
   

Phase 4: Advanced AI Techniques

1. Batch Processing for Similar Files

Group similar files for consistent treatment:

## All React components at once
@migrate-typescript.md src/components/forms/

## All API clients  
@migrate-typescript.md src/api/

2. Iterative Type Improvement

Use AI for progressive type enhancement:

## After initial migration, improve type specificity
"Review src/api/userService.ts and replace any 'any' types with specific interfaces based on the API responses we actually receive"

3. Pattern Recognition and Application

Let AI learn from your best examples:

"Based on the pattern in src/components/Button.tsx, migrate all components in src/components/ui/ to follow the same TypeScript patterns for props, event handlers, and styling"

Performance Optimization During Migration

Build Speed Configuration:

// tsconfig.json additions for faster development
{
  "compilerOptions": {
    "incremental": true,
    "tsBuildInfoFile": ".tsbuildinfo",
    "skipLibCheck": true
  },
  "ts-node": {
    "transpileOnly": true,
    "files": true
  }
}

Webpack/Build Tool Configuration:

// webpack.config.js
module.exports = {
  module: {
    rules: [
      {
        test: /\.(ts|tsx)$/,
        use: [
          {
            loader: 'ts-loader',
            options: {
              transpileOnly: true  // Skip type checking in webpack
            }
          }
        ]
      }
    ]
  }
};

Run type checking separately:

## In package.json scripts
{
  "scripts": {
    "dev": "webpack serve --mode development",
    "type-check": "tsc --noEmit",
    "type-check:watch": "tsc --noEmit --watch"
  }
}

Quality Gates and Checkpoints

Weekly Migration Review:

• Type coverage percentage (aim for +10% per week)
• Build time impact (should stabilize after initial spike)
• Developer satisfaction (address pain points quickly)
• Bug report trends (should decrease over time)

Success Metrics:

• ✅ All files compile without errors
• ✅ Type coverage >85%
• ✅ No runtime behavioral changes
• ✅ Team can confidently refactor TypeScript code
• ✅ Development experience improved (better autocomplete, catch more bugs)

The key to successful AI-assisted migration is treating it as a partnership: AI handles the repetitive work, humans guide the strategic decisions. With proper workflow setup, most teams find this approach 5-10x faster than manual migration while producing higher quality results.

After migrating 5 codebases, here's what actually works for advanced patterns. This is where you stop writing garbage that compiles and start writing code your team won't curse you for.

Modern TypeScript Features for AI-Generated Code

Using satisfies for Better Type Inference

The `satisfies` operator is perfect for AI-generated code because it validates types without losing specificity:

// AI can now generate this pattern reliably
const theme = {
  colors: {
    primary: "#007acc",
    secondary: "#f39c12",
    danger: "#e74c3c"
  },
  spacing: {
    small: "8px",
    medium: "16px", 
    large: "24px"
  }
} satisfies {
  colors: Record<string, `#${string}`>;
  spacing: Record<string, `${number}px`>;
};

// TypeScript 5.6 properly narrows these types
type PrimaryColor = typeof theme.colors.primary; // "#007acc"
type SmallSpacing = typeof theme.spacing.small;   // "8px"

AI Prompt that actually works:

"Convert this config object to use satisfies operator - keep exact types but validate the structure"

Template Literal Types for Type-Safe APIs

AI excels at generating template literal types that would be tedious to write manually. Check out this guide on advanced template literals for more patterns:

// AI can generate sophisticated API type systems
type HTTPMethod = 'GET' | 'POST' | 'PUT' | 'DELETE';
type APIVersion = 'v1' | 'v2';
type Resource = 'users' | 'posts' | 'comments' | 'notifications';

type APIEndpoint<
  Method extends HTTPMethod = HTTPMethod,
  Version extends APIVersion = APIVersion,
  Resource extends string = string
> = `${Method} /api/${Version}/${Resource}${string}`;

// Usage generates precise types
type UserEndpoints = 
  | 'GET /api/v1/users'
  | 'GET /api/v1/users/:id'  
  | 'POST /api/v1/users'
  | 'PUT /api/v1/users/:id'
  | 'DELETE /api/v1/users/:id';

AI-Powered Type Generation from Runtime Data

Using AI to Infer Types from API Responses

Paste your actual API response into your AI tool instead of guessing what the types should be:

// Paste actual API response into AI tool:
const actualUserResponse = {
  "id": "user_123",
  "profile": {
    "firstName": "John",
    "lastName": "Doe", 
    "avatar": "https://example.com/avatar.jpg",
    "preferences": {
      "theme": "dark",
      "notifications": true,
      "language": "en-US"
    }
  },
  "metadata": {
    "createdAt": "2025-09-14T10:30:00Z",
    "lastLogin": "2025-09-14T09:15:00Z",
    "isEmailVerified": true
  }
};

// AI generates this interface structure:
interface UserProfile {
  id: `user_${string}`;
  profile: {
    firstName: string;
    lastName: string;
    avatar: `https://${string}`;
    preferences: {
      theme: 'light' | 'dark';
      notifications: boolean;
      language: `${string}-${string}`;
    };
  };
  metadata: {
    createdAt: string; // Could be refined to ISO date format
    lastLogin: string;
    isEmailVerified: boolean;
  };
}

Advanced Migration Patterns for Complex Codebases

Discriminated Unions for State Management

AI tools can convert loose state patterns into proper discriminated unions:

// Before: Loose state management (JavaScript pattern)
const asyncState = {
  loading: false,
  data: null,
  error: null,
  hasData: false
};

// After: AI-generated discriminated union (TypeScript best practice)
type AsyncState<TData> = 
  | { status: 'idle' }
  | { status: 'loading' }
  | { status: 'success'; data: TData }
  | { status: 'error'; error: string };

// AI can generate reducer patterns too
function useAsyncState<TData>(): [
  AsyncState<TData>,
  {
    setLoading: () => void;
    setSuccess: (data: TData) => void;
    setError: (error: string) => void;
    reset: () => void;
  }
] {
  // Implementation generated by AI
}

Generic Type Inference for Utility Functions

AI excels at creating properly constrained generic functions:

// AI can generate sophisticated utility types
type DeepKeyOf<T> = T extends object 
  ? { [K in keyof T]: K extends string | number 
      ? `${K}` | (T[K] extends object ? `${K}.${DeepKeyOf<T[K]>}` : never)
      : never 
    }[keyof T] 
  : never;

type DeepValueOf<T, K extends DeepKeyOf<T>> = 
  K extends keyof T 
    ? T[K]
    : K extends `${infer P}.${infer S}`
      ? P extends keyof T
        ? DeepValueOf<T[P], S extends DeepKeyOf<T[P]> ? S : never>
        : never
      : never;

// AI generates type-safe property access
function getNestedProperty<T extends object, K extends DeepKeyOf<T>>(
  obj: T,
  path: K
): DeepValueOf<T, K> {
  return path
    .split('.')
    .reduce((current: any, key: string) => current?.[key], obj);
}

// Usage with full type safety and autocomplete
const user = { profile: { settings: { theme: 'dark' } } };
const theme = getNestedProperty(user, 'profile.settings.theme'); // Type: 'dark'

Monorepo and Large-Scale Architecture Patterns

Project References for Performance

AI can help set up TypeScript project references for faster builds:

// Root tsconfig.json generated by AI
{
  "files": [],
  "references": [
    { "path": "./packages/types" },
    { "path": "./packages/api-client" },  
    { "path": "./packages/ui-components" },
    { "path": "./packages/business-logic" },
    { "path": "./apps/web" },
    { "path": "./apps/mobile" }
  ]
}

// packages/types/tsconfig.json
{
  "extends": "../../tsconfig.base.json",
  "compilerOptions": {
    "composite": true,
    "declaration": true,
    "declarationMap": true,
    "outDir": "./dist"
  },
  "include": ["src/**/*"]
}

Barrel Exports with AI Optimization

AI can generate efficient barrel export patterns:

// src/index.ts - AI-generated barrel exports
// Core types
export type { 
  User, 
  ApiResponse, 
  PaginatedResponse,
  AsyncState 
} from './types';

// Utilities (tree-shakable)
export { 
  formatDate, 
  formatCurrency, 
  debounce, 
  throttle 
} from './utils';

// Hooks (conditional exports for React projects)
export { 
  useApi, 
  useAsync, 
  useDebounce 
} from './hooks';

// Re-exports with namespace for large APIs  
export * as UserAPI from './api/users';
export * as PostAPI from './api/posts';

Runtime Type Validation Integration

Zod Integration with AI-Generated Schemas

AI can generate Zod schemas that match your TypeScript types:

// AI generates both the TypeScript interface and Zod schema
import { z } from 'zod';

const UserProfileSchema = z.object({
  id: z.string().regex(/^user_\d+$/),
  profile: z.object({
    firstName: z.string().min(1),
    lastName: z.string().min(1),
    avatar: z.string().url().optional(),
    preferences: z.object({
      theme: z.enum(['light', 'dark']),
      notifications: z.boolean(),
      language: z.string().regex(/^[a-z]{2}-[A-Z]{2}$/)
    })
  }),
  metadata: z.object({
    createdAt: z.string().datetime(),
    lastLogin: z.string().datetime(),
    isEmailVerified: z.boolean()
  })
});

// Infer TypeScript type from Zod schema  
type UserProfile = z.infer<typeof UserProfileSchema>;

// Runtime validation function
export function validateUserProfile(data: unknown): UserProfile {
  return UserProfileSchema.parse(data);
}

// API integration with runtime validation
export async function fetchUserProfile(id: string): Promise<UserProfile> {
  const response = await fetch(`/api/users/${id}`);
  const data = await response.json();
  return validateUserProfile(data); // Runtime type safety!
}

Performance Optimization for Large Codebases

Build Performance with AI-Generated Configuration

AI can optimize TypeScript build performance:

// AI-generated performance-optimized tsconfig.json
{
  "compilerOptions": {
    "incremental": true,
    "tsBuildInfoFile": ".tsbuildinfo",
    "skipLibCheck": true,
    "skipDefaultLibCheck": true,
    "assumeChangesOnlyAffectDirectDependencies": true,
    
    // Module resolution optimizations
    "moduleResolution": "bundler", // Faster than "node"
    "resolveJsonModule": true,
    "allowSyntheticDefaultImports": true,
    
    // Emit optimizations
    "removeComments": true,
    "declaration": false, // Skip .d.ts generation in development
    "sourceMap": false,   // Skip source maps in development
    
    // Strict mode for production builds only
    "strict": false,      // Enable gradually
    "noUncheckedIndexedAccess": false, // Performance impact
  },
  
  // Exclude patterns for faster compilation
  "exclude": [
    "node_modules",
    "**/*.test.ts",
    "**/*.test.tsx", 
    "**/*.spec.ts",
    "**/*.spec.tsx",
    "**/*.stories.ts",
    "**/*.stories.tsx",
    "dist",
    "build",
    "coverage"
  ]
}

Team Collaboration and Code Review

AI-Generated Code Review Guidelines

Create TypeScript-specific review standards:

// .github/PULL_REQUEST_TEMPLATE/typescript-migration.md
// AI-generated template for TypeScript migration PRs

## TypeScript Migration Checklist

### Type Quality
- [ ] No new `any` types introduced (or properly documented)
- [ ] Interfaces preferred over type aliases for object shapes
- [ ] Generic constraints used appropriately
- [ ] Return types explicit for public APIs

### Migration Standards  
- [ ] Original functionality preserved (no behavior changes)
- [ ] Import statements updated correctly
- [ ] JSDoc converted to TSDoc format
- [ ] Error handling types specified

### Performance Impact
- [ ] Build time impact acceptable (<2x increase)
- [ ] Bundle size impact measured and documented
- [ ] Type coverage improved or maintained

### Team Knowledge
- [ ] Complex type decisions documented
- [ ] Migration patterns consistent with existing codebase
- [ ] Breaking changes flagged and discussed

AI doesn't just speed up migration - it lets you build sophisticated type systems you'd never have time to write manually. After using these patterns on multiple projects, the TypeScript code ends up better than what I could have built from scratch.