avoiding-any-types

• Code contains any type annotations
• Working with external data (APIs, JSON, user input)
• Designing generic functions or types
• User mentions type safety, type checking, or avoiding any
• Files contain patterns like : any, <any>, as any

The unknown type is any's type-safe counterpart. It accepts any value like any, but requires type narrowing before use.

Key Pattern: any → unknown → type guard → safe access

Impact: Prevents runtime errors while maintaining flexibility for truly dynamic data.

Step 1: Identify any Usage

When you see any, ask:

1. Is this truly unknowable at compile time? (external data, plugin systems)
2. Can I use a more specific type? (union types, generics with constraints)
3. Is this laziness or necessity?

Step 2: Choose Replacement Strategy

Strategy A: Use Specific Types (preferred)

• Known structure → Use interfaces/types
• Multiple possible types → Use union types
• Shared shape → Use generics with constraints

Strategy B: Use unknown (when truly dynamic)

• External data with unknown structure
• Plugin/extension systems
• Gradual migration from JavaScript

Strategy C: Keep any (rare, justified cases)

• Interop with poorly-typed libraries
• Complex type manipulation that TypeScript can't express
• Performance-critical code where type checks are prohibitive

Step 3: Implement Type Guards

When using unknown, implement type guards:

1. Runtime validation (Zod, io-ts, custom validators)
2. Type predicates for custom guards
3. Built-in guards (typeof, instanceof, in)

For detailed patterns and examples:

• Type Guard Patterns: Use the using-type-guards skill for comprehensive type guard implementation
• Runtime Validation: Use the using-runtime-checks skill for validating unknown data
• Generic Constraints: Use the using-generics skill for constraining generic types

❌ Using any (unsafe)

async function fetchUser(id: string): Promise<any> {
  const response = await fetch(`/api/users/${id}`);
  return response.json();
}

const user = await fetchUser("123");
console.log(user.name.toUpperCase());

Problem: If API returns { username: string } instead of { name: string }, this crashes at runtime. TypeScript provides no protection.

✅ Using unknown + validation (safe)

async function fetchUser(id: string): Promise<unknown> {
  const response = await fetch(`/api/users/${id}`);
  return response.json();
}

function isUser(value: unknown): value is { name: string } {
  return (
    typeof value === "object" &&
    value !== null &&
    "name" in value &&
    typeof value.name === "string"
  );
}

const userData = await fetchUser("123");
if (isUser(userData)) {
  console.log(userData.name.toUpperCase());
} else {
  throw new Error("Invalid user data");
}

Better: Use Zod for complex validation (use the using-runtime-checks skill)

Example 2: Generic Function Defaults

❌ Using any in generic default (unsafe)

interface ApiResponse<T = any> {
  data: T;
  status: number;
}

const response: ApiResponse = { data: "anything", status: 200 };
response.data.nonexistent.property;

Problem: Generic defaults to any, losing all type safety.

✅ Using unknown default (safe)

interface ApiResponse<T = unknown> {
  data: T;
  status: number;
}

const response: ApiResponse = { data: "anything", status: 200 };

if (typeof response.data === "string") {
  console.log(response.data.toUpperCase());
}

Even Better: Require explicit type parameter

interface ApiResponse<T> {
  data: T;
  status: number;
}

const response: ApiResponse<User> = await fetchUser();

Example 3: Error Handling

❌ Using any for caught errors (unsafe)

try {
  await riskyOperation();
} catch (error: any) {
  console.log(error.message);
}

Problem: Not all thrown values are Error objects. This crashes if someone throws a string or number.

✅ Using unknown + type guard (safe)

try {
  await riskyOperation();
} catch (error: unknown) {
  if (error instanceof Error) {
    console.log(error.message);
  } else {
    console.log("Unknown error:", String(error));
  }
}

Example 4: Validation Function

❌ Using any parameter (unsafe)

function validate(data: any): boolean {
  return data.email && data.password;
}

Problem: Typos like data.emial are not caught. No autocomplete support.

✅ Using unknown + type guard (safe)

interface LoginData {
  email: string;
  password: string;
}

function isLoginData(data: unknown): data is LoginData {
  return (
    typeof data === "object" &&
    data !== null &&
    "email" in data &&
    "password" in data &&
    typeof data.email === "string" &&
    typeof data.password === "string"
  );
}

function validate(data: unknown): data is LoginData {
  if (!isLoginData(data)) {
    return false;
  }

  return data.email.includes("@") && data.password.length >= 8;
}

• Use unknown for external data (APIs, JSON.parse, user input)
• Use unknown for generic defaults when type is truly dynamic
• Implement type guards before accessing unknown values
• Use runtime validation libraries (Zod, io-ts) for complex structures

SHOULD:

• Prefer specific types (interfaces, unions) over unknown when structure is known
• Use type predicates (value is Type) for reusable type guards
• Narrow unknown progressively (check object → check properties → check types)

NEVER:

• Use any for external data
• Use as any to silence TypeScript errors
• Use any in public API surfaces
• Default generics to any
• Cast unknown to specific types without validation

After replacing any with unknown:

1. Type Guard Exists:

   • Every unknown value has a corresponding type guard
   • Type guards check structure AND types
   • Guards return early on invalid data

2. Safe Access Only:

   • No property access before type narrowing
   • No method calls before type narrowing
   • IDE autocomplete works after narrowing

3. Error Handling:

   • Invalid data handled gracefully
   • Clear error messages
   • No silent failures

4. Compilation:

   bashCopy

   npx tsc --noEmit
   

   Should pass without any suppressions.

1. Library Interop (temporary)

import poorlyTypedLib from "poorly-typed-lib";
const result = poorlyTypedLib.method() as any;

Consider contributing types to DefinitelyTyped or wrapping in typed facade.

2. Type Manipulation Edge Cases

type DeepPartial<T> = {
  [P in keyof T]?: T[P] extends object ? DeepPartial<T[P]> : T[P];
};

Sometimes TypeScript's type system can't express complex patterns. Document why.

3. Explicit Opt-Out

const configSchema: any = generateFromSpec();

Explicitly choosing to skip type checking for this value. Document decision.

Phase 1: Audit

grep -rn ": any" src/
grep -rn "<any>" src/
grep -rn "= any" src/

Phase 2: Classify

• External data → unknown + validation
• Known structure → specific types
• Generic defaults → remove default or use unknown
• Justified any → document with comment explaining why

Phase 3: Replace

• Start with external boundaries (API layer, JSON parsing)
• Work inward toward core logic
• Add tests to verify runtime behavior unchanged

Phase 4: Prevent

• Enable noImplicitAny in tsconfig.json
• Add lint rules forbidding any
• Use hooks to catch new any introduction