In this guide 6 sections
Move to TypeScript when your bugs come from wrong shapes, such as undefined values, mismatched API fields and renamed props that slip past review. Types pay off most with many contributors. Migrate file by file with the compiler set to allow JavaScript, and skip the move for small, stable code that one person maintains.
The signs that types would pay off
Read your last twenty bug tickets. If many say cannot read property of undefined, wrong field name or unexpected null, a compiler would have caught them before release.
That pattern is the strongest signal. Team size matters too.
One developer remembers every object shape. New hires spend days guessing what a function returns, and reviewers miss changes that break a caller three folders away, such as an invoice total read by the dashboard.
- Repeated undefined and null errors in production
- API responses read in many files
- Several contributors or contractors
- Refactors that developers avoid out of fear
When staying on JavaScript is fine
A small internal tool with one maintainer and a clear scope gains little, and a prototype that may be thrown away gains even less. Types add build steps. Tests cover much of the same ground.
If your suite is thorough and regressions are rare, the budget may do more good on a reporting module or a faster deployment pipeline. Be honest about your problem.
What the migration really costs
Installing the compiler is the easy part. The cost sits in typing data that arrives from outside, such as database rows, API payloads and form input, because writing each definition often exposes fields nobody documented.
Expect build changes too. Jest or Vitest config, ESLint rules, Docker images and the CI pipeline all need touching, and libraries without bundled types need packages from DefinitelyTyped or small local declarations.
- Types for database models and API payloads
- Build, lint and test configuration
- Type packages for older libraries
- Developer time to learn generics and narrowing
Go gradually, not by rewrite
Set allowJs and checkJs in tsconfig so old files keep working beside new ones, then rename one module from .js to .ts, fix what the compiler finds and merge. Repeat, and the product keeps shipping.
Start at the edges where shapes enter the system, namely API clients, validation and shared models, and then move inward folder by folder. Strict mode comes late.
Enable it per folder if needed, so the error count never blocks a release, and avoid scattering any as an escape hatch. It hides the very bugs you are chasing.
A decision you can make this month
Pick one module with frequent bugs, such as the order or invoice code, convert it alone and track its regressions across two or three release cycles. Compare against a JavaScript twin.
That trial costs little. If the converted module breaks less and new developers read it faster, schedule the next folder, and if it does not, you have saved yourself a large project.
TypeScript readiness scorecard
The worksheet has three blocks. Block one counts bug tickets by cause, such as undefined, wrong field and null, while block two lists contributors, shared API shapes and untyped libraries. Block three fixes the migration order by folder, with columns for owner, strict flag, test status and the release that carries each converted folder.
Open a blank worksheet to printQuestions owners ask
Does TypeScript slow down the app?
Types are removed at build time, so the browser or Node runs plain JavaScript, and the only extra cost is a compilation step in the pipeline. The app runs the same.
Can we mix JavaScript and TypeScript files?
Yes, mixing is allowed. With allowJs enabled, both kinds of file sit side by side in one build, which is what makes a gradual move practical for a team that must keep shipping.
Will types replace our tests?
No, because types check shapes rather than business rules, so a discount calculated wrongly still compiles, and you should keep tests for behaviour such as tax and rounding. Both tools have jobs.
Who should own the migration?
One senior developer should own it, setting the tsconfig, the folder order and the review rules, while the others convert files inside that plan. One owner prevents style drift.