Mid-levelMid (3–6 yrs)JavaScriptTypeScriptMicrosoftAtlassianFlipkart

TypeScript strict mode adoption

How do you migrate a 500-file JavaScript codebase to TypeScript strict mode incrementally?

Answers use simple, clear English.

Quick interview answer

Enable allowJs + checkJs on leaf modules first, or new folders ts-only. Turn on strict flags one at a time (strictNullChecks first). Introduce zod/io-ts at API boundaries.

Detailed answer

Enable allowJs + checkJs on leaf modules first, or new folders ts-only. Turn on strict flags one at a time (strictNullChecks first). Introduce zod/io-ts at API boundaries. Avoid mass any; use unknown + narrowing. CI tsc --noEmit gate on typed percentage rising weekly.

Real example & use case

payments/ folder strict TS first; shared utils typed; legacy reports stay JS until touched.

Pros & cons

Pros: bugs caught pre-prod. Cons: initial slowdown; @ts-ignore debt if rushed.

#typescript#migration