Type Coverage Improver

Turn on TypeScript strictness without a big-bang break. The skill measures where you stand (explicit any, implicit any, which strict flags are already on), then enables the strict family one sub-flag at a time — noImplicitAny, strictNullChecks, and the rest — fixing the fallout from each flag before touching the next. Every step ends with tsc --noEmit passing, so you ratchet coverage up monotonically instead of staring at 600 errors and rage-casting them away.

When to use this skill

• The codebase runs with strict: false (or a partial strict config) and is littered with any, implicit any parameters, and unchecked nullables.
• You want to reach strict: true but a single flip produces an unfixable wall of errors and a stalled PR.
• any is masking real defects — undefined is not a function, missing-property crashes — that strict typing would have caught at compile time.

Instructions

1. Measure the baseline before changing anything. Read tsconfig.json and record which strict sub-flags are already set (strict implies noImplicitAny, strictNullChecks, strictFunctionTypes, strictBindCallApply, strictPropertyInitialization, noImplicitThis, useUnknownInCatchVariables, alwaysStrict). Then quantify the any surface:

   # explicit `any` annotations
   grep -rIn -E ':\s*any\b|\bas any\b|<any>|Array<any>|any\[\]' --include='*.ts' --include='*.tsx' src | wc -l
   # existing suppressions (these are debt you must not add to)
   grep -rIn -E '@ts-ignore|@ts-expect-error' --include='*.ts' --include='*.tsx' src | wc -l
   # implicit any + the full error count under the strictest config (dry run, no edits)
   npx tsc --noEmit --strict --noErrorTruncation 2>&1 | grep -c 'error TS'

   If type-coverage is available, npx type-coverage --detail gives a single percentage and a per-identifier list — capture the starting number; it is your headline metric.

2. Order the work by risk and traffic, not alphabetically. Use git log --format= --name-only --since='6 months ago' | sort | uniq -c | sort -rn to find churned files, and grep for the modules with the most any and the most importers (entry points, shared utils, API/DB boundaries). Fix these first: a precise type on a widely-imported util propagates correctness everywhere; an any at a data boundary (HTTP response, DB row, JSON parse) is where wrong-shape bugs originate.

3. Enable exactly one sub-flag at a time. Add a single flag to tsconfig.json ("noImplicitAny": true), run npx tsc --noEmit, and fix only the errors that flag produces. Recommended order, easiest-to-hardest:

   • noImplicitAny — annotate parameters/returns the compiler couldn't infer.
   • strictNullChecks — the big one; surfaces every place null/undefined was silently allowed.
   • strictFunctionTypes, strictBindCallApply, noImplicitThis — usually small fallout.
   • strictPropertyInitialization — class fields; often the last and noisiest. Once each flag is green individually, the final flip to "strict": true is a no-op verification.

4. Replace any with the real type, narrow at the boundary. For explicit any: infer the actual shape from how the value is used and from the producer, and write the interface/type. For external/untyped data (JSON.parse, fetch().json(), env vars, dynamic imports), type the boundary as unknown and narrow with a type guard or a schema parse (e.g. zod's .parse()) — unknown forces a check; any skips it. Add explicit return types to exported functions so inference errors surface at the definition, not three call sites away.

5. Keep the typecheck green at every commit. After each flag's fallout is fixed, run the project's real check (npm run typecheck / tsc --noEmit) and the test suite, then commit that flag as its own commit. Never enable the next flag on a red tree — you lose the ability to attribute a new error to a specific flag, and the diff becomes unreviewable.

6. Re-measure and report the delta. Re-run the baseline commands from step 1. Report the before/after any count, the type-coverage percentage delta, which flags are now on, and any honest residue: spots that genuinely need unknown + a follow-up, third-party @types gaps, or generated code excluded via tsconfig exclude rather than suppressed inline.

Output

1. Baseline metrics — current tsconfig strict flags, explicit-any count, suppression count, total error count under --strict, and type-coverage percentage if available.

2. An ordered flag-by-flag plan — the sub-flags to enable in sequence, each with its estimated fallout count and the highest-priority files to fix first, e.g.:

   Step	Flag	Errors introduced	Fix-first files
   1	noImplicitAny	38	src/lib/api/client.ts, src/utils/parse.ts
   2	strictNullChecks	142	src/db/repository.ts, src/lib/session.ts
   3	strictPropertyInitialization	21	src/services/*.ts

3. Concrete type changes for the first file — the actual diff: any → named types, added return annotations, and unknown-at-the-boundary guards, with tsc --noEmit shown passing afterward. For example:

   - export function parseUser(raw: any) {
   -   return { id: raw.id, name: raw.name };
   - }
   + interface User { id: string; name: string }
   + export function parseUser(raw: unknown): User {
   +   if (typeof raw !== "object" || raw === null) throw new Error("invalid user");
   +   const r = raw as Record<string, unknown>;
   +   if (typeof r.id !== "string" || typeof r.name !== "string") throw new Error("invalid user");
   +   return { id: r.id, name: r.name };
   + }

   $ npx tsc --noEmit
   $   # exit 0 — clean