How do you migrate a JavaScript codebase to TypeScript?

Question presented to candidate: "You join a team with a large JavaScript codebase that is still shipping features every week. They want to move to TypeScript. How would you plan and run that migration without stopping the team?"

What a strong answer should cover:

• Incremental, not big-bang: allowJs lets .js and .ts files live in one project, so files can be converted one at a time while the app keeps building
• Getting value before renaming: checkJs or per-file // @ts-check plus JSDoc types, and // @ts-nocheck to exclude a file for now
• Order of work: leaf modules and shared utilities first, types at the boundaries (API responses, config, JSON.parse results as unknown), @types/* or a declare module shim for untyped dependencies
• Strictness as a ratchet: start with what the team can pass, then enable strict (or its flags one by one), tracking escape hatches such as a TSFixMe alias and @ts-expect-error (not @ts-ignore)
• CI and tooling: run tsc --noEmit in CI because many bundlers only strip types; keep run-time behaviour unchanged during conversion
• What the conversion typically surfaces: missing arguments, possibly-undefined values, typos, string/number mix-ups; types are compile-time only, so validate external data at run time

Clarifying questions expected:

• How big is the codebase, how is it built and tested today, and is there a deadline?
• Which parts change most often or break most often?
• Is this an application or a library with external consumers who will need .d.ts files?

Code / implementation expected: Optional — a tsconfig.json for the mixed phase and one converted module.