Ban Type Assertions

Ban Type Assertions

Enable @typescript-eslint/consistent-type-assertions with assertionStyle: 'never' in a package and replace all as X casts with patterns the compiler can verify.

Core Philosophy

Pick the strictly correct path, not the simpler one.

Every as assertion is a spot where the developer told the compiler "trust me." The goal is to make the compiler verify instead. If you replace as Foo with a type guard that is equally unverified, you have not improved anything -- you have just moved the assertion.

Quick Reference

• Rule: @typescript-eslint/consistent-type-assertions
• Config: { assertionStyle: 'never' }
• Location: packages/<name>/.eslintrc.js

Workflow

1. Enable the Rule

Add to the package's .eslintrc.js:

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rules: {
  '@typescript-eslint/consistent-type-assertions': ['error', { assertionStyle: 'never' }],
}

2. Enumerate Violations

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cd packages/<name> && npm run lint 2>&1 | grep "consistent-type-assertions"

Group violations by file and pattern before fixing.

3. Research Before Fixing

Before writing any replacement code:

1. Check for existing zod schemas -- grep for Schema alongside the type name in @factory/common and across the repo.
2. Check if schemas exist but aren't exported -- if so, export them rather than creating new ones.
3. Check for duplicate types/interfaces across packages -- consolidate into @factory/common if found.
4. Understand the data flow -- is this a parse boundary (external data), a narrowing site (union type), or a library type gap?

4. Fix Violations Using the Pattern Hierarchy

Tier 1: Zod Parsing (for external data boundaries)

Use for any data entering the system from JSON, disk, network, IPC, etc. This gives runtime validation, not just a type annotation.

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// BAD
const data = JSON.parse(raw) as MyType;

// GOOD
const data = MySchema.parse(JSON.parse(raw));

Use safeParse when you need to handle errors gracefully (e.g., returning an error response with context like a request id):

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// BAD: throws before you can extract the request id
const request = RequestSchema.parse(JSON.parse(raw));

// GOOD: safeParse lets you return a proper error
const parsed = RequestSchema.safeParse(JSON.parse(raw));
if (!parsed.success) {
  return errorResponse(rawObj?.id ?? null, INVALID_PARAMS, parsed.error.message);
}
const request = parsed.data;

Tier 2: Control Flow Narrowing (for union types)

Use switch, in, instanceof, or discriminated unions:

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// BAD
(error as NodeJS.ErrnoException).code

// GOOD
if (error instanceof Error && 'code' in error) {
  const code = error.code;
}

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// BAD
if (METHODS.has(method as Method)) { ... }

// GOOD: switch narrows exhaustively
switch (method) {
  case 'foo':
  case 'bar':
    return handle(method); // narrowed
}

Tier 3: eslint-disable with Justification (last resort)

Only for genuinely unavoidable cases (library type gaps, generic parameters that can't be inferred). Always explain why:

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// eslint-disable-next-line @typescript-eslint/consistent-type-assertions -- ws library types require generic parameter
ws.on('message', handler);

Anti-Pattern: Type Guards That Are Disguised Assertions

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// NOT an improvement -- checks shape but not content
function isDaemonRequest(x: unknown): x is DaemonRequest {
  return typeof x === 'object' && x !== null && 'method' in x;
}

A zod schema validates values. A type guard like this is an unverified assertion with extra steps. Only use type guards when the narrowing logic is truly sufficient.

5. Use Strict Schemas, Not Permissive Ones

When a schema exists (e.g., SessionSettingsSchema), use it strictly rather than z.record(z.unknown()). This ensures forward compatibility -- if fields are removed in a migration, stale data gets cleaned on read.

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// BAD: accepts anything
const settings = z.record(z.unknown()).parse(raw);

// GOOD: validates against the real shape
const settings = SessionSettingsSchema.parse(raw);

6. Promote Shared Schemas to @factory/common

If you find duplicate interfaces, types, or schemas across packages, consolidate them:

1. Create the schema in @factory/common/<domain>/<subdomain>/schema.ts
2. Put any enums in a sibling enums.ts (required by factory/enum-file-organization)
3. Export via a subpath (e.g., @factory/common/session/summary), not the barrel index.ts
4. Delete all local duplicates
5. Update all consumers to import from the common subpath
6. Run npm run knip at repo root to catch unused barrel re-exports

7. Fix Test Mocks to Match Schemas

Once you replace as X with .parse(), test mocks that relied on the assertion will fail validation. Fix the mocks -- do not disable the rule in tests.

Create helper functions to centralize valid test fixtures:

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function mockSessionSummary(
  overrides?: Partial<SessionSummaryEvent>,
): SessionSummaryEvent {
  return {
    type: 'session_start',
    id: 'test-id',
    title: 'Test Session',
    owner: 'test-owner',
    ...overrides,
  };
}

8. Parse at the Boundary, Inside Error Handling

Make sure parsing happens where failures produce proper error responses, not unhandled exceptions:

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// BAD: parse outside try/catch -- if it throws, you lose context
const request = RequestSchema.parse(data);
try { handle(request); } catch { ... }

// GOOD: safeParse before try, handle error with context
const parsed = RequestSchema.safeParse(data);
if (!parsed.success) {
  return errorResponse(rawData?.id ?? null, INVALID_PARAMS, parsed.error.message);
}
try { handle(parsed.data); } catch { ... }

Verification

Run for all affected packages (a change in @factory/common can break downstream lint):

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# Lint (all affected packages)
cd packages/<name> && npm run lint

# Typecheck
npm run typecheck

# Tests
npm run test

# Unused exports (repo root)
npm run knip

Reminders

• factory/enum-file-organization requires TypeScript enums to live in files named enums.ts
• no-barrel-files prevents re-exporting types from barrel files -- consumers must import from the subpath directly
• When promoting types to common, add a package.json exports entry for the new subpath if one doesn't exist
• Test overrides for the rule in .eslintrc.js may be needed if test files use assertion syntax in mock setup -- but prefer fixing mocks over disabling the rule