TypeScript Language Expert

Expert assistant for TypeScript type system mastery including generics, conditional types, mapped types, type inference, and advanced patterns.

Thinking Process

When activated, follow this structured thinking approach to solve TypeScript type challenges:

Step 1: Problem Classification

Goal: Understand what type of TypeScript challenge this is.

Key Questions to Ask:

• Is this a type design problem? (creating new types, modeling data)
• Is this a type error problem? (debugging, fixing type mismatches)
• Is this a type inference problem? (generics, conditional types)
• Is this a type safety problem? (runtime validation, narrowing)
• Is this a library typing problem? (third-party types, declaration files)

Decision Point: Classify to select appropriate approach:

• Type Design → Focus on modeling domain correctly
• Type Error → Trace the type mismatch source
• Type Inference → Design generics and constraints
• Type Safety → Add runtime validation with type guards
• Library Typing → Check @types packages, create declarations

Step 2: Context Analysis

Goal: Understand the TypeScript environment and constraints.

Key Questions to Ask:

• What TypeScript version is in use? (5.0+ has new features)
• What is the strictness level? (strict, noUncheckedIndexedAccess, etc.)
• What is the module system? (ESM, CommonJS, bundler)
• Are there existing type patterns to follow?

Actions:

1. Check tsconfig.json for compiler options
2. Identify TypeScript version in package.json
3. Review existing type patterns in the codebase

Version-Specific Features:

Step 3: Type Design Principles

Goal: Apply TypeScript best practices to the solution.

Thinking Framework - Core Principles:

1. Prefer Inference Over Annotation

   • Let TypeScript infer when possible
   • Annotate function parameters, return types for public APIs
   • Use satisfies to check without widening

2. Use unknown Over any

   • unknown forces type checking before use
   • any disables all type checking
   • Exception: truly dynamic scenarios (rare)

3. Discriminated Unions for Variants

   • Use literal type discriminators
   • Enables exhaustive checking
   • Self-documenting code

4. Narrow Types, Don't Widen

   • Prefer as const for literal types
   • Use type guards to narrow
   • Avoid unnecessary type assertions

Type Design Questions:

• "What is the minimal type that describes this?"
• "Can TypeScript infer this, or must I annotate?"
• "Is this union exhaustive?"

Step 4: Error Diagnosis

Goal: Systematically diagnose type errors.

Thinking Framework:

• "What does TypeScript expect vs what it received?"
• "Where did the unexpected type originate?"
• "Is this a structural or nominal mismatch?"

Common Error Patterns:

Debugging Strategy:

1. Hover over variables to see inferred types
2. Trace back to where the type was assigned
3. Check for implicit any (enable noImplicitAny)
4. Use // @ts-expect-error temporarily to isolate issues

Step 5: Generic Type Design

Goal: Design reusable, type-safe generic functions and types.

Thinking Framework:

• "What type information flows through this function?"
• "What constraints are needed on the type parameter?"
• "Can I infer more than I'm currently inferring?"

Generic Design Patterns:

Generic Best Practices:

• Name parameters meaningfully (TItem not just T)
• Add constraints that document intent
• Provide defaults when sensible (<T = string>)
• Test with edge cases (empty arrays, undefined, etc.)

Step 6: Type Narrowing Strategy

Goal: Safely narrow types for runtime operations.

Thinking Framework:

• "How do I know this is type X at runtime?"
• "What is the most reliable way to check?"
• "Does this narrowing work for TypeScript?"

Narrowing Techniques:

Type Guard Pattern:

function isUser(obj: unknown): obj is User {
  return (
    typeof obj === 'object' &&
    obj !== null &&
    'id' in obj &&
    'name' in obj &&
    typeof (obj as User).id === 'string'
  );
}

Step 7: Advanced Type Patterns

Goal: Apply advanced patterns for complex scenarios.

Thinking Framework:

• "Is there a utility type that does this?"
• "Can I compose existing types?"
• "Is the type readable and maintainable?"

Advanced Patterns:

Branded Type Example:

type UserId = string & { readonly __brand: unique symbol };

function createUserId(id: string): UserId {
  return id as UserId;
}

function fetchUser(id: UserId) { /* ... */ }

// Type error: string is not UserId
fetchUser("abc"); // Error!
fetchUser(createUserId("abc")); // OK

Step 8: Validation and Testing

Goal: Ensure types are correct and maintainable.

Type Testing Strategies:

1. Use @ts-expect-error to test that invalid code fails
2. Create type test files with assertions
3. Use Expect<Equal<A, B>> utility for type assertions

Type Test Pattern:

// Type tests (no runtime code)
type cases = [
  Expect<Equal<ReturnType<typeof fn>, string>>,
  Expect<Equal<Parameters<typeof fn>, [number, string]>>,
];

// Compile-time assertion utility
type Expect<T extends true> = T;
type Equal<X, Y> = (<T>() => T extends X ? 1 : 2) extends
  (<T>() => T extends Y ? 1 : 2) ? true : false;

Maintainability Checklist:

• Types are documented with JSDoc
• Complex types are broken into smaller pieces
• Type names are descriptive
• Exported types have explicit documentation

Usage

Run Type Check

bash /mnt/skills/user/typescript-master/scripts/type-check.sh [project-dir] [strict-mode]

Arguments:

• project-dir - Project directory (default: current directory)
• strict-mode - Enable strict checks: true/false (default: true)

Examples:

bash /mnt/skills/user/typescript-master/scripts/type-check.sh
bash /mnt/skills/user/typescript-master/scripts/type-check.sh ./my-project
bash /mnt/skills/user/typescript-master/scripts/type-check.sh ./my-project false

Checks:

• TypeScript compilation (noEmit)
• Strict type checking
• Unused variables/imports
• tsconfig recommendations

Documentation Resources

Official Documentation:

• TypeScript Handbook: https://www.typescriptlang.org/docs/handbook/
• Type Challenges: https://github.com/type-challenges/type-challenges

Type System Fundamentals

Utility Types

// Built-in utilities
type Partial<T> = { [P in keyof T]?: T[P] };
type Required<T> = { [P in keyof T]-?: T[P] };
type Readonly<T> = { readonly [P in keyof T]: T[P] };
type Pick<T, K extends keyof T> = { [P in K]: T[P] };
type Omit<T, K extends keyof any> = Pick<T, Exclude<keyof T, K>>;
type Record<K extends keyof any, T> = { [P in K]: T };

Custom Utility Types

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

// Strict Omit (only known keys)
type StrictOmit<T, K extends keyof T> = Pick<T, Exclude<keyof T, K>>;

// Make specific keys required
type RequireKeys<T, K extends keyof T> = T & Required<Pick<T, K>>;

// Nullable
type Nullable<T> = T | null;

Generics Patterns

Constrained Generics

// Key constraint
function getProperty<T, K extends keyof T>(obj: T, key: K): T[K] {
  return obj[key];
}

// Type constraint
function merge<T extends object, U extends object>(a: T, b: U): T & U {
  return { ...a, ...b };
}

Inference with infer

// Extract return type
type ReturnType<T> = T extends (...args: any[]) => infer R ? R : never;

// Extract promise value
type Awaited<T> = T extends Promise<infer U> ? Awaited<U> : T;

// Extract array element
type ArrayElement<T> = T extends (infer E)[] ? E : never;

// Extract function parameters
type Parameters<T> = T extends (...args: infer P) => any ? P : never;

Conditional Types

Basic Conditional

type IsString<T> = T extends string ? true : false;
type IsArray<T> = T extends any[] ? true : false;

Distributive Conditional

// Distributes over union
type ToArray<T> = T extends any ? T[] : never;
type Result = ToArray<string | number>; // string[] | number[]

// Prevent distribution
type ToArrayNonDist<T> = [T] extends [any] ? T[] : never;
type Result2 = ToArrayNonDist<string | number>; // (string | number)[]

Discriminated Unions

type Result<T, E = Error> =
  | { success: true; data: T }
  | { success: false; error: E };

function handleResult<T>(result: Result<T>): T | null {
  if (result.success) {
    return result.data; // TypeScript knows data exists
  } else {
    console.error(result.error); // TypeScript knows error exists
    return null;
  }
}

Type Guards

// Custom type guard
function isUser(obj: unknown): obj is User {
  return (
    typeof obj === 'object' &&
    obj !== null &&
    'id' in obj &&
    'name' in obj &&
    typeof (obj as User).id === 'string'
  );
}

// Assertion function
function assertNonNull<T>(
  value: T,
  message?: string
): asserts value is NonNullable<T> {
  if (value === null || value === undefined) {
    throw new Error(message ?? 'Value is null or undefined');
  }
}

Template Literal Types

type EventName = 'click' | 'focus' | 'blur';
type Handler = `on${Capitalize<EventName>}`;
// "onClick" | "onFocus" | "onBlur"

type PropKey<T extends string> = `get${Capitalize<T>}` | `set${Capitalize<T>}`;
type NameProps = PropKey<'name'>; // "getName" | "setName"

Recommended tsconfig

{
  "compilerOptions": {
    "strict": true,
    "noUncheckedIndexedAccess": true,
    "exactOptionalPropertyTypes": true,
    "noImplicitReturns": true,
    "noFallthroughCasesInSwitch": true,
    "noPropertyAccessFromIndexSignature": true,
    "moduleResolution": "bundler",
    "verbatimModuleSyntax": true
  }
}

Present Results to User

When providing TypeScript solutions:

• Prefer type inference over explicit annotations
• Use unknown instead of any
• Leverage discriminated unions
• Provide type test examples
• Note TypeScript version features (5.0+: const type parameters)

Troubleshooting

"Type 'X' is not assignable to type 'Y'"

• Check for missing properties
• Verify nullability handling
• Look for literal vs widened types

"Property does not exist on type"

• Add type guard before access
• Check if property is optional
• Verify union type narrowing

"Excessive type complexity"

• Break into smaller types
• Use intermediate type aliases
• Consider simplifying generics