TypeScript Adoption Readiness Checklist
A readiness checklist for adopting TypeScript in a JavaScript codebase. It covers compiler setup, allowJs coexistence, a strictness ramp, third-party types, an any policy, and a CI type-check gate.
When to Use This Checklist
Use this checklist before adopting TypeScript, a typed superset of JavaScript, in an existing JavaScript codebase. Teams adopt TypeScript to catch errors at compile time, improve editor tooling, and document intent through types. The migration should be incremental, so this list focuses on the setup and strategy that make that possible.
How to Use This Checklist
Start by sizing the codebase and finding high-churn, high-value modules; those are where types pay off first. Set up the compiler and integrate it into the build, then enable allowJs so JavaScript and TypeScript files coexist. That coexistence is what turns a risky rewrite into a steady, file-by-file migration.
Choose a strictness ramp deliberately. Starting with loose settings lets the codebase compile early, then you tighten options like strict null checks and no-implicit-any over time. Add type definitions for third-party libraries and decide a clear policy on the any escape hatch so types do not erode silently.
Lock in value with CI. Make type checking a required gate so types cannot regress, define a migration order starting with leaf modules, and where possible generate types from API schemas so the boundary between frontend and backend stays honest.
What Good Looks Like
The compiler is wired into the build with allowJs enabling mixed files, and a strictness ramp moves the codebase toward strict over time. Third-party types are in place, an any policy is enforced, and type checking is a required CI gate. Migration follows a sensible leaf-first order, and the team shares conventions.
Common Pitfalls
The biggest pitfall is enabling full strict mode on day one, which floods the team with errors and stalls adoption. Overusing the any type defeats the purpose and hides real bugs. Teams also forget to make type checking a CI gate, so types silently rot. Migrating high-coupling modules first, instead of leaves, creates cascading churn.
Related Resources
Pair this with continuous-integration and code-review best practices, trunk-based development for incremental merges, and a clear definition of done. See the JavaScript-to-TypeScript and TypeScript 4-to-5 migrations.