59 build and typecheck review

Build and Typecheck Review

Purpose

Verify the project builds cleanly with strict static analysis, catch suppressed or ignored TypeScript errors and compiler warnings, review tsconfig.json strictness, assess bundle size and tree-shaking, confirm source maps are generated correctly, and identify build-tooling anti-patterns that cause silent failures or non-reproducible builds. The review runs read-only static checks (tsc --noEmit, grep) and never emits build output or mutates config without instruction.

When to use

• tsc --noEmit is failing in CI and you need to triage and fix the errors.
• A PR introduces // @ts-ignore or @ts-expect-error without justification.
• tsconfig.json was modified and you want to confirm strict mode is preserved.
• Bundle size has grown unexpectedly and you need to identify the cause.
• The build passes locally but fails in CI (environment-dependent build issues).
• A new package is added and may not be tree-shakeable.

When not to use

• The project is JavaScript only (no TypeScript) — skip the typecheck steps.
• The task is to implement a feature, not to review the build configuration.
• Build issues are in a Docker layer or deployment step — use 60-deployment-preflight-review.

Procedure

1. Run tsc --noEmit and capture output. Type-check without emitting files for the full error list. For project references, run tsc --build --noEmit. Count errors by file and by error code.
2. Review tsconfig.json strict settings. Check "strict": true; if absent, check noImplicitAny, strictNullChecks, strictFunctionTypes, noUncheckedIndexedAccess. Note skipLibCheck: true if it hides errors in .d.ts files the project owns. Confirm noEmitOnError: true.
3. Audit @ts-ignore / @ts-expect-error. For each: is there a comment explaining why? Is it hiding a real error that should be fixed? Is @ts-expect-error used (preferred — fails if the error disappears)?
4. Check any usage. Flag explicit any in non-test code, especially in function signatures and return types — it is a type-safety hole.
5. Verify build reproducibility. Confirm the output hash is stable across two consecutive clean builds. Instability suggests timestamp injection, random IDs, or non-deterministic module ordering.
6. Review bundle size and tree-shaking. Check sideEffects: false for library packages; dynamic import() for large sections; whole-library imports (import _ from 'lodash') that should be path imports.
7. Check source map configuration. Production maps should be external (devtool: 'source-map'), not inline; not served publicly; uploaded to an error tracker.
8. Verify build command consistency. package.json scripts, the CI workflow, and the Dockerfile must call the same build command.
9. Check warnings treated as errors. --noUnusedLocals and --noUnusedParameters enabled; bundler warnings (circular dependency, missing exports) surfaced, not suppressed.
10. Review path aliases. tsconfig.json paths must match the bundler config and Jest moduleNameMapper; mismatches cause "module not found" in only one context.

Concrete checks

• tsc --noEmit exits with zero errors.
• "strict": true (or equivalent individual flags) is set in tsconfig.json.
• "noEmitOnError": true is set.
• All @ts-ignore usages have an explanatory comment; none suppress real correctness bugs.
• as any and : any usages in non-test code are minimal and justified.
• Build output is reproducible (same hash on two consecutive clean builds).
• Bundle size is within defined thresholds (or thresholds are defined if missing).
• Dynamic imports are used for large page sections.
• Source maps are external, not inline, in production builds.
• Path aliases are consistent across tsconfig.json, bundler config, and Jest config.
• --noUnusedLocals and --noUnusedParameters are enabled.

Commands or Templates

# Type gate: full error list, no emit (use --build for project references)
npx tsc --noEmit 2>&1 | tee /tmp/tsc.txt
grep -oE "error TS[0-9]+" /tmp/tsc.txt | sort | uniq -c | sort -rn   # errors by code

# Strictness flags actually in effect
npx tsc --showConfig | grep -E '"strict"|"noImplicitAny"|"strictNullChecks"|"noEmitOnError"|"noUnusedLocals"'

# Suppressions and any-holes in non-test code
grep -rn "@ts-ignore\|@ts-expect-error" src/
grep -rnE ":\s*any\b|as any|as unknown as " src/ --include="*.ts" --include="*.tsx"

# Whole-library imports that defeat tree-shaking
grep -rnE "import .* from ['\"](lodash|moment|rxjs)['\"]" src/

# Reproducibility: hash output twice from clean
rm -rf dist && npm run build >/dev/null 2>&1 && find dist -type f -exec sha256sum {} + | sort > /tmp/b1
rm -rf dist && npm run build >/dev/null 2>&1 && find dist -type f -exec sha256sum {} + | sort > /tmp/b2
diff /tmp/b1 /tmp/b2 && echo "reproducible" || echo "NON-REPRODUCIBLE"

tsconfig strictness reference

"strict": true is the umbrella; when it is off, audit these individually because each closes a real class of bug:

Flag	What it catches	Risk if off
noImplicitAny	params/vars with no inferable type	silent any everywhere
strictNullChecks	undefined/null not in the type	Cannot read properties of undefined at runtime
strictFunctionTypes	unsound callback parameter variance	wrong-shaped callbacks accepted
noUncheckedIndexedAccess	arr[i] assumed defined	out-of-bounds reads typed as present
noEmitOnError	emitting JS despite type errors	broken build artifacts ship
noUnusedLocals/Parameters	dead bindings	code rot accumulates

skipLibCheck: true is acceptable for third-party .d.ts noise, but if the project generates its own declaration files, it also silences their errors — note that explicitly.

Worked error triage

tsc --noEmit reports 38 errors. Grouping by code (the command in the templates) shows: 31× TS2532 Object is possibly 'undefined', 5× TS2345, 2× TS7006. The 31 TS2532 errors all trace to one source: strictNullChecks was just enabled and array/map lookups are no longer assumed defined. The fix is not 31 as casts — it is a few guard patterns at the lookup sites (const u = users.get(id); if (!u) return;). Triaging by error code rather than by file reveals that most of the count is a single root cause, so the report should say "31 errors, one cause (missing null guards after enabling strictNullChecks), ~6 guard sites to fix" — not "38 unrelated errors".

Common issues & anti-patterns

• strict: false inherited from a template. Teams disable strict mode to suppress initial errors and never re-enable it; type safety degrades.
• skipLibCheck: true hiding owned .d.ts errors. Errors in generated declaration files the project ships are also skipped.
• Build succeeds but emits with errors. noEmitOnError is false; the build produces broken JS from files with type errors.
• as unknown as Foo cast chains. Defeat the type checker entirely — equivalent to any without the grep signal for any.
• Full lodash import. import _ from 'lodash' pulls ~73KB minified; import debounce from 'lodash/debounce' pulls ~2KB.
• Date-stamped bundle filename without content hash. bundle.2026-05-31.js invalidates CDN cache by date, not content; use bundle.[contenthash].js.
• Mismatched target and lib. target: "es5" with lib: ["esnext"] emits ES5 that calls ES2022 methods absent at runtime.
• Circular dependency. Bundles work until an edge case triggers an undefined reference at runtime; surface the warning.
• Incremental build cache hiding errors. A stale tsBuildInfo or bundler cache can report success on code that no longer compiles clean; verify from a clean state when in doubt.
• transpileOnly / Babel type-stripping treated as a typecheck. ts-node --transpileOnly, esbuild, and swc strip types without checking them; the only real type gate is tsc --noEmit. A green dev server says nothing about type correctness.

Verify the typecheck is real

A frequent false signal is "the app runs, so types are fine". Dev servers and bundlers transpile by stripping types, not checking them — so type errors pass straight through to runtime. Always run a dedicated tsc --noEmit (or tsc --build --noEmit for references) as the gate, separate from the build/run step, and confirm it is wired into CI. If the project's "typecheck" script secretly runs the bundler, it is not a type gate; flag that as a finding.

"Passes locally, fails in CI"

This recurring class almost always traces to an environment difference, not a real code change. Check, in order:

• Filesystem case sensitivity. macOS/Windows are case-insensitive; Linux CI is not. import './Utils' for a file named utils.ts passes locally and fails in CI with "module not found". Grep imports against actual filenames.
• Dependency drift. Local node_modules was built from a stale lockfile; CI does a clean npm ci. A version that exists locally but not in the lockfile resolves differently. Reproduce with a clean install.
• tsconfig resolution differences. A paths alias resolved by the IDE/ts-node locally but absent from the bundler or tsc build config used in CI.
• Node/TypeScript version mismatch. Local Node 22 vs CI Node 18; a syntax or lib feature differs. Confirm engines and the CI matrix agree.
• Case-only env differences. A type that depends on process.env.NODE_ENV being set narrows differently when the var is unset in CI.

The fix is to make the local environment match CI (clean install, same Node, Linux-equivalent case behavior) and reproduce the failure — not to add a suppression so the local build stays green while CI stays red.

Required output

## Build and Typecheck Review
### TypeScript errors
- Total: N | Top files: path — N | Codes by frequency: TS2345 (N), TS2322 (N)
### tsconfig.json strictness
| Option | Value | Status |
### Type suppression audit
- @ts-ignore: N (N without comment) | as any in non-test: N | files with most suppressions
### Bundle analysis (if bundler configured)
- Total (gzip): X KB | Largest chunks | Whole-library imports | Tree-shaking enabled/disabled
### Source maps
- Format: external/inline/none | CI upload to error tracker: yes/no
### Path alias consistency
- tsconfig paths | Bundler match: yes/no | Jest moduleNameMapper match: yes/no
### Recommended fixes (priority order)
1. ...

Safety

• Run only tsc --noEmit, grep, and read-only file inspection. Do not run tsc with emit, npm run build, or bundler commands unless the user asks (the reproducibility check is opt-in).
• Do not modify tsconfig.json, package.json, or bundler config without explicit user instruction.
• Do not weaken strictness flags to make errors disappear; report them instead.

Completion criteria

Done means tsc --noEmit was run and its errors counted by file and code, strictness flags were audited, suppressions and any holes were listed with file:line, bundle and source-map config were assessed, alias consistency was checked, and fixes are ordered by priority.