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migrate-js-to-modern-typescript

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Migrating a JavaScript codebase to TypeScript — converting .js files to .ts, adding types to existing JS, or tightening a loosely-typed TS project toward strict mode. Covers tsconfig and allowJs strategy, incremental strict-flag ratcheting (noImplicitAny, strictNullChecks, noUncheckedIndexedAccess), typing public surfaces, replacing `any` and unsafe casts with `unknown` and narrowing, validating runtime boundaries (JSON, env, API responses), converting CommonJS to ESM and prototypes to classes, and the build/CI changes a migration needs. Trigger even when the user only says "add types", "turn on strict mode", or "convert this file to TypeScript", and especially on a mixed JS/TS repo. Distinct from general TypeScript refactoring — this is the migration act itself, performed file by file while keeping the build green.

skills/.experimental/migrate-js-to-modern-typescript/SKILL.md

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# JavaScript to TypeScript Migration Best Practices

Guide for taking a JavaScript codebase to strict, modern TypeScript without a big-bang rewrite. Contains 42 rules across 7 categories, prioritized by impact to drive an incremental, file-by-file migration that keeps the build compiling at every step.

## When to Apply

Reference these guidelines when:
- Converting a `.js` codebase to `.ts` (whole project or one module at a time)
- Adding types to existing JavaScript via JSDoc or annotations
- Choosing a `tsconfig` and `allowJs` strategy for a mixed JS/TS repo
- Turning on `strict` mode or individual strict flags on a large codebase
- Replacing `any`, `as` casts, and `!` assertions left over from a quick conversion
- Validating external data (JSON, env, API responses) so the types you wrote are true at runtime
- Converting CommonJS to ESM, prototypes to classes, and other JS idioms to TS
- Updating the build, runner, and CI to type-check and publish TypeScript