Convert JavaScript to TypeScript Incrementally with Claude

Migrating a JavaScript project to TypeScript doesn't have to be a big bang rewrite. Claude can convert files one at a time while keeping everything working.

Start by asking Claude to set up the TypeScript config for incremental adoption:

Set up TypeScript in this JavaScript project for incremental migration.
Configure tsconfig.json with allowJs: true and strict: false.
Add a build script that compiles both .js and .ts files.
Don't convert any files yet — just make the infrastructure ready.

Then convert files starting from the leaves of your dependency tree:

Convert src/utils/formatters.js to TypeScript.
Add proper types for all function parameters and return values.
Replace any JSDoc type annotations with TypeScript syntax.
Make sure all files that import from this module still work.
Run the existing tests to verify nothing broke.

For bulk conversion, work directory by directory:

Convert all files in src/utils/ from .js to .ts.
Add types based on the actual usage patterns in the codebase.
Create a src/types/ directory for any shared interfaces.
Keep it practical — use explicit types for public APIs and let
TypeScript infer the rest.

Once everything is converted, ask Claude to tighten the config:

Enable strict mode in tsconfig.json and fix every resulting type error.
Don't use 'any' as an escape hatch — find the correct types.

Incremental TypeScript migration keeps your app shipping while the types catch up — Claude handles the tedious file-by-file conversion.