Devonair
Use CasesguideNovember 18, 20256 min read

AI-Powered JavaScript to TypeScript Migration

Migrate your JavaScript codebase to TypeScript with AI. Learn how AI agents handle type inference, file conversion, and error resolution automatically.

Migrating from JavaScript to TypeScript is one of those projects that everyone agrees should happen but nobody wants to do. The benefits are clear: better tooling, fewer runtime errors, improved refactoring. The cost is also clear: touching every file in your codebase, adding types everywhere, and fixing a cascade of type errors.

AI agents change the economics of this migration. What used to take weeks of tedious manual work can now happen incrementally, automatically, with you reviewing the results rather than doing the typing.

Why TypeScript Migrations Stall

Most JavaScript-to-TypeScript migrations follow the same pattern:

  1. Team agrees TypeScript would be valuable
  2. Someone sets up the TypeScript config
  3. A few files get converted
  4. Other priorities take over
  5. The migration sits at 15% complete for months
  6. New JavaScript files keep getting added
  7. The migration becomes increasingly daunting

The problem isn't that TypeScript is hard. It's that manually adding types to hundreds of files is tedious enough that it never wins against feature work. Every hour spent adding types is an hour not spent shipping.

The AI-Assisted Approach

Instead of converting files one by one, describe what you want:

@devonair convert all files in /src/utils from JavaScript to TypeScript, inferring types where possible

The agent analyzes your JavaScript code, infers types from usage patterns, and converts files to TypeScript. It handles:

  • Renaming .js files to .ts
  • Adding type annotations to function parameters and return values
  • Converting JSDoc comments to TypeScript types
  • Handling module imports and exports
  • Inferring types from variable assignments and usage

Migration Strategies

The Incremental Approach

Don't convert everything at once. Start with isolated modules:

@devonair convert all files in /src/utils to TypeScript

Review, merge, verify. Then move to the next module:

@devonair convert all files in /src/api to TypeScript

This approach lets you catch issues early and keeps PRs reviewable.

The Strict-Later Approach

Start with loose TypeScript settings, then tighten:

@devonair convert /src to TypeScript with noImplicitAny disabled

Once everything compiles, gradually enable strict settings:

@devonair fix all noImplicitAny errors in /src/utils

@devonair add explicit types to all function parameters that are currently 'any'

The Types-First Approach

Generate type definitions before converting code:

@devonair create TypeScript interfaces for all data structures used in /src/api

@devonair add type definitions for all API response shapes in /src/types

Then convert the implementation:

@devonair convert /src/api to TypeScript using the interfaces in /src/types

Handling Common Migration Challenges

Type Inference Limitations

AI can infer many types from usage, but some require human input:

@devonair convert /src/utils to TypeScript and flag any places where types couldn't be inferred

The agent will mark uncertain types with any or comments, so you know where to add explicit types.

Third-Party Libraries

Some libraries don't have type definitions:

@devonair convert /src to TypeScript and add @ts-ignore comments for untyped library calls

Or better:

@devonair convert /src to TypeScript and create declaration files for untyped dependencies

Dynamic Code Patterns

JavaScript's dynamic nature doesn't always translate cleanly:

// This pattern needs human judgment
function getValue(obj, key) {
  return obj[key];
}

The agent will make reasonable choices, but review carefully where dynamic patterns appear.

Test Files

Don't forget tests:

@devonair convert all test files in /src/__tests__ to TypeScript

@devonair update test mocks to use proper TypeScript types

A Complete Migration Workflow

Here's how a full migration might look:

Phase 1: Setup and utilities

@devonair set up tsconfig.json with recommended settings for a gradual migration

@devonair convert /src/utils to TypeScript

Phase 2: Types and interfaces

@devonair analyze /src and create TypeScript interfaces for all data structures

@devonair create type definitions for all API endpoints

Phase 3: Core conversion

@devonair convert /src/api to TypeScript using existing type definitions

@devonair convert /src/components to TypeScript

@devonair convert /src/hooks to TypeScript

Phase 4: Tests

@devonair convert /src/__tests__ to TypeScript

@devonair fix any type errors in test files

Phase 5: Strictness

@devonair enable strict mode in tsconfig and fix all resulting errors

@devonair replace all 'any' types with proper types where possible

Maintaining Type Quality

Migration is just the beginning. Keep types healthy:

@devonair schedule weekly: report any new 'any' types added to the codebase

@devonair on PR: if new files are added, verify they have proper TypeScript types

@devonair schedule monthly: identify functions missing return type annotations

When to Migrate Manually

AI handles mechanical conversion well, but some situations need human judgment:

  • Complex generics: Generic types with multiple constraints often need manual design
  • API boundaries: External-facing types should be deliberately designed, not inferred
  • Branded types: Type-safe IDs and similar patterns need intentional implementation
  • Overloaded functions: Multiple function signatures benefit from human clarity

Use AI for the bulk conversion, then refine the tricky parts manually.

Measuring Migration Progress

Track your migration:

@devonair report on TypeScript migration status: percentage of files converted, count of 'any' types, strict mode compliance

Set goals:

  • 100% of files converted
  • Strict mode enabled
  • Less than N any types remaining
  • All public APIs have explicit types

Getting Started

Pick a module to start with. Something self-contained, well-tested, and representative of your codebase:

@devonair convert /src/utils to TypeScript, inferring types where possible

Review the PR. Check that types make sense, that tests still pass, and that the code is cleaner than before.

Then pick the next module. Before you know it, you'll have a TypeScript codebase - without spending weeks on manual conversion.

FAQ

Will the migration break my existing code?

The conversion preserves behavior - it adds types without changing logic. However, TypeScript may catch bugs your JavaScript tests missed. That's a feature, not a bug.

How do I handle files that mix JavaScript patterns?

Start with loose settings, convert everything, then tighten. Strict mode can wait until the basics are in place.

What about JSDoc comments?

Devonair can convert JSDoc type annotations to TypeScript types. If you've been using JSDoc for type hints, the migration will preserve that information.

Can I migrate a monorepo?

Yes. Convert packages one at a time, starting with leaf packages (no internal dependencies) and working up to packages that depend on others.

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