TypeScript Migration Pack

We built this so you don't have to wrestle with tsconfig hell, any-filled codebases, or broken builds while trying to modernize your JavaScript. If you are an engineer who knows that type safety reduces runtime incidents and that strict mode is the only way to ship reliable code, you already know the pain of migration. We've seen teams spend weeks fighting compiler errors, introducing regressions, and eventually reverting to // @ts-nocheck because the process felt impossible. We created the TypeScript Migration Pack to automate the heavy lifting, enforce strictness from day one, and give you a validated path from legacy JavaScript to production-grade TypeScript.

The any Trap and Why Incremental Migration Fails

Most engineers start a migration by sprinkling @ts-check at the top of files and hoping for the best. This works until your codebase hits a few thousand lines of untyped logic, and suddenly every function returns any, every object is a Record, and your type safety is a lie. You end up with a hybrid codebase that is harder to maintain than the original JavaScript because now you have two paradigms fighting each other. The any type spreads like a virus, defeating the entire purpose of the migration.

Incremental adoption requires a strategy, not just a compiler flag. You need to configure your build tools correctly, handle module resolution quirks, and manage the transition without breaking your CI/CD pipeline. A gradual migration strategy is essential for large codebases, where you can't rewrite everything overnight ^[3]. You need to set up your project structure correctly from the start, ensuring that your TypeScript configuration supports strict mode without immediately crashing on legacy patterns. If you haven't properly setting up a new TypeScript project, you are likely missing critical compiler options like strictNullChecks or exactOptionalPropertyTypes that catch entire classes of bugs.

The official TypeScript documentation emphasizes that migrating from JavaScript requires a step-by-step approach, handling type definitions and module systems carefully ^[8]. Without a structured plan, you risk creating a fragile codebase where the types are inconsistent, and developers are confused about when to use interfaces versus type aliases. We designed this pack to solve the structural problems before you write a single line of code. Instead of guessing which tsconfig options to enable, you get a production-grade configuration that enforces strictness automatically.

What a Broken Migration Costs in On-Call Hours and Technical Debt

Ignoring a proper migration strategy doesn't just mean messy code; it means real dollars burned and nights lost. When you migrate to TypeScript without strict validation, you introduce type-related regressions that slip into production. These manifest as undefined is not a function errors, null pointer exceptions, and API contract mismatches that your testing suite misses because the tests were written against the untyped JavaScript behavior.

A broken migration can cost your team hundreds of engineering hours. Every time a developer encounters a type error they can't resolve, they lose context. They switch between the TypeScript compiler output, the source code, and the documentation, trying to figure out why a simple property access is failing. This friction slows down feature development and increases the likelihood of introducing new bugs. Over time, the technical debt accumulates, and the codebase becomes a minefield that only the original authors can navigate.

Furthermore, a poorly executed migration can damage customer trust. If your new TypeScript build behaves differently than the old JavaScript build due to unvalidated changes, you risk breaking user workflows. You need comprehensive testing strategies to ensure that your migration doesn't introduce behavioral changes. Without rigorous testing, you are flying blind. The cost of fixing a type-related bug in production is exponentially higher than fixing it during the migration phase. We calculated the cost of a single production incident caused by a type mismatch: it's not just the fix; it's the on-call hours, the customer support tickets, and the reputational damage.

How a Mid-Sized Codebase Migrated Without Breaking Production

Imagine a team with a 50,000-line JavaScript codebase that needed to migrate to TypeScript to support a new microservices architecture. They started by applying @ts-check to their entry points, but within a week, they were drowning in any types and compiler errors. The migration stalled because they didn't have a clear strategy for handling third-party libraries, managing module resolution, or validating that their TypeScript build matched their JavaScript behavior.

The team turned to a phased approach, inspired by best practices that recommend configuring build tools and migrating in manageable chunks ^[2]. They used a tool like @ts-migrating to automate the initial conversion, which helped scaffold the migration and apply type annotations where possible ^[1]. This tooling reduced the manual effort significantly, allowing them to focus on the complex logic that required custom type definitions.

To ensure they didn't break production, they implemented an A/B testing workflow. They ran their test suites against both the JavaScript and TypeScript builds in parallel, comparing the outputs to ensure behavioral equivalence. This step was critical because it caught subtle regressions that unit tests alone might miss. By integrating A/B testing workflows into their migration pipeline, they gained confidence that the new codebase was functionally identical to the old one. They also used an A/B testing framework to manage the rollout, gradually shifting traffic to the TypeScript version while monitoring for errors.

The team also followed a step-by-step guide to handle the migration, ensuring that they addressed type definitions, module systems, and compiler options systematically ^[4]. They didn't just copy-paste configurations; they understood why each option was necessary. For example, they enabled strictNullChecks to prevent null pointer exceptions and exactOptionalPropertyTypes to catch accidental property assignments. This disciplined approach allowed them to complete the migration in three months, with zero production incidents related to type errors.

Strict Mode, Zero any, and Automated Validation

Once the TypeScript Migration Pack is installed, your codebase transforms. You no longer worry about missing type definitions or inconsistent configurations. The pack provides a tsconfig.strict.json that enforces strict mode, strictNullChecks, exactOptionalPropertyTypes, and modern module resolution. This configuration is production-grade, meaning it's designed to catch bugs without being overly restrictive.

The pack includes a migrate.sh script that automates the scaffolding process. It applies @ts-check to your JavaScript files, prepares the codebase for ts-migrate, and sets up the necessary tooling. You don't have to manually edit every file; the script does the heavy lifting, allowing you to focus on the logic that requires human intervention.

Validation is built into the workflow. The validate.sh script runs the TypeScript compiler in strict mode and exits with a non-zero status if any type errors remain. This ensures that your migration is complete and that no any types have slipped through the cracks. You can integrate this script into your CI/CD pipeline to prevent untyped code from being merged.

For teams that need to validate behavioral equivalence, the pack includes an ab-test-runner.sh script. This script runs parallel test suites against the JavaScript and TypeScript builds, ensuring that the migration hasn't introduced any regressions. This is especially useful for large codebases where manual testing is impractical. By automating the validation process, you can migrate with confidence, knowing that your codebase is type-safe and functionally correct.

The pack also includes references to strict mode rules, compiler options, and ts-migrate tooling, so you always have the documentation you need at your fingertips. If you are also implementing GitOps workflows to manage your deployments, the pack integrates seamlessly, allowing you to version control your TypeScript configuration and migration scripts alongside your code.

What's in the TypeScript Migration Pack

We don't believe in vague promises. Here is exactly what you get when you install the TypeScript Migration Pack. Every file is designed to solve a specific problem in the migration process.

• skill.md — Orchestrator skill that defines the migration strategy, references all templates, scripts, validators, and references, and guides the agent through the TypeScript migration workflow.
• templates/tsconfig.strict.json — Production-grade TypeScript configuration enforcing strict mode, strictNullChecks, exactOptionalPropertyTypes, and modern module resolution.
• templates/tsconfig.base.json — Base TypeScript configuration for extending strict settings across multiple project directories.
• scripts/migrate.sh — Executable automation script that scaffolds the migration, applies @ts-check to JS files, and prepares the codebase for ts-migrate.
• scripts/validate.sh — Validator script that runs the TypeScript compiler in strict mode and exits non-zero if any type errors remain, ensuring migration quality.
• scripts/ab-test-runner.sh — Executable script for A/B testing integration that runs parallel test suites against JS and TS builds to validate behavioral equivalence.
• references/strict-mode-rules.md — Canonical reference documenting TypeScript strict mode rules, reserved keywords, use strict implications, and common compilation errors.
• references/compiler-options.md — Authoritative reference for compiler options including strictNullChecks, exactOptionalPropertyTypes, moduleResolution, and target compatibility.
• references/ts-migrate-tooling.md — Reference guide for ts-migrate tooling, @ts-check directives, incremental validation strategies, and common migration patterns.
• examples/worked-example.js — Production-grade JavaScript example demonstrating common patterns that require migration, including module usage and type-unsafe operations.
• examples/worked-example.ts — Migrated TypeScript version of the example, showcasing strict types, proper interfaces, and strict mode compliance.
• templates/.eslintrc.js — ESLint configuration tailored for TypeScript migration, enforcing strict linting rules and type-aware linting.

If you are also shipping browser extensions or other specialized tooling, you can pair this with the browser extension pack to ensure your entire development ecosystem is type-safe.

Stop Guessing. Ship a Type-Safe Codebase.

You don't need to spend weeks fighting compiler errors or risking production incidents with a half-baked migration. The TypeScript Migration Pack gives you a proven, automated path to strict mode, zero any, and validated builds. Upgrade to Pro to install the pack and start migrating with confidence.