TypeScript · Types that pay for themselves

How would you migrate a JavaScript codebase to TypeScript?

Mediummigrationjavascriptstrict-modeUpdated Oct 2026

Short answer

Migrate incrementally: enable allowJs, add checkJs and strictness gradually, type module boundaries first, and fix errors by narrowing rather than casting. Keep the build green and ship in small steps.

A big-bang rewrite creates a long-lived branch and a large review. An incremental migration lets the team keep shipping while the type coverage grows. The compiler can check JavaScript files alongside TypeScript, so the two can coexist during the transition.

json

{
  "compilerOptions": {
    "allowJs": true,
    "checkJs": true,
    "noEmit": true,
    "strict": false
  },
  "include": ["src"]
}

A practical sequence

Add TypeScript and a tsconfig with allowJs and noEmit
Rename leaf modules or new files first, keeping the build green
Type public module boundaries and shared data shapes
Enable strict flags one at a time, starting with noImplicitAny
Replace casts with narrowing and runtime validation at trust boundaries
Add type tests for generic helpers and keep tsc in CI

Prioritise code that changes often and code with unclear data shapes. A module that is stable and isolated can stay JavaScript longer. The goal is not 100 percent TypeScript immediately; it is a codebase where new work is typed and the riskiest boundaries are checked.

Avoid a migration strategy based on adding any to every error. That produces a TypeScript codebase with no real safety and makes the next cleanup harder.

Follow-ups you should expect

Interviewers use these to check whether the first answer was understanding or memorisation.

1Why migrate incrementally instead of rewriting?
2Which files would you convert first?
3How do you handle untyped third-party packages?
4How do you introduce strict mode without blocking delivery?

Say it in the interview

I migrate incrementally rather than rewriting everything. I enable allowJs and checkJs, add types at module boundaries first, and turn on strictness in stages. Each step keeps the build green and ships independently. Casts are a last resort; I prefer narrowing and validating so the migration improves real safety rather than just silencing errors.

Use this as a model, not a script. Saying it in your own words is what makes it survive follow-up pressure.

Now say it out loud.

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A practical sequence

Follow-ups you should expect

Related questions

Now say it out loud.