name: typescript-coder description: 'Expert 10x engineer with extensive knowledge of TypeScript fundamentals, migration strategies, and best practices. Use when asked to "add TypeScript", "migrate to TypeScript", "add type checking", "create TypeScript config", "fix TypeScript errors", or work with .ts/.tsx files. Supports JavaScript to TypeScript migration, JSDoc type annotations, tsconfig.json configuration, and type-safe code patterns.'

TypeScript Coder Skill

Master TypeScript development with expert-level knowledge of type systems, migration strategies, and modern JavaScript/TypeScript patterns. This skill transforms you into a 10x engineer capable of writing type-safe, maintainable code and migrating existing JavaScript projects to TypeScript with confidence.

When to Use This Skill

User asks to "add TypeScript", "migrate to TypeScript", or "convert to TypeScript"
Need to "add type checking" or "fix TypeScript errors" in a project
Creating or configuring tsconfig.json for a project
Working with .ts, .tsx, .mts, or .d.ts files
Adding JSDoc type annotations to JavaScript files
Debugging type errors or improving type safety
Setting up TypeScript in a Node.js or JavaScript project
Creating type definitions or ambient declarations
Implementing advanced TypeScript patterns (generics, conditional types, mapped types)

Prerequisites

Basic understanding of JavaScript (ES6+)
Node.js and npm/yarn installed (for TypeScript compilation)
Familiarity with the project structure and build tools
Access to the typescript package (can be installed if needed)

Shorthand Keywords

Keywords to trigger this skill as if using a command-line tool:

openPrompt = ["typescript-coder", "ts-coder"]

Use these shorthand commands to quickly invoke TypeScript expertise without lengthy explanations. For example:

typescript-coder --check "this code"
typescript-coder check this type guard
ts-coder migrate this file
ts-coder --migrate project-to-typescript

Role and Expertise

As a TypeScript expert, you operate with:

Deep Type System Knowledge: Understanding of TypeScript's structural type system, generics, and advanced types
Migration Expertise: Proven strategies for incremental JavaScript to TypeScript migration
Best Practices: Knowledge of TypeScript patterns, conventions, and anti-patterns
Tooling Mastery: Configuration of TypeScript compiler, build tools, and IDE integration
Problem Solving: Ability to resolve complex type errors and design type-safe architectures

Core TypeScript Concepts

The TypeScript Type System

TypeScript uses structural typing (duck typing) rather than nominal typing:

interface Point {
  x: number;
  y: number;
}

// This works because the object has the right structure
const point: Point = { x: 10, y: 20 };

// This also works - structural compatibility
const point3D = { x: 1, y: 2, z: 3 };
const point2D: Point = point3D;  // OK - has x and y

Type Inference

TypeScript infers types when possible, reducing boilerplate:

// Type inferred as string
const message = "Hello, TypeScript!";

// Type inferred as number
const count = 42;

// Type inferred as string[]
const names = ["Alice", "Bob", "Charlie"];

// Return type inferred as number
function add(a: number, b: number) {
  return a + b;  // Returns number
}

Key TypeScript Features

Feature	Purpose	When to Use
Interfaces	Define object shapes	Defining data structures, API contracts
Type Aliases	Create custom types	Union types, complex types, type utilities
Generics	Type-safe reusable components	Functions/classes that work with multiple types
Enums	Named constants	Fixed set of related values
Type Guards	Runtime type checking	Narrowing union types safely
Utility Types	Transform types	`Partial<T>`, `Pick<T, K>`, `Omit<T, K>`, etc.

Step-by-Step Workflows

Task 1: Install and Configure TypeScript

For a new or existing JavaScript project:

Install TypeScript as a dev dependency:
```
npm install --save-dev typescript
```
Install type definitions for Node.js (if using Node.js):
```
npm install --save-dev @types/node
```
Initialize TypeScript configuration:
```
npx tsc --init
```

Configure tsconfig.json for your project:

{
  "compilerOptions": {
    "target": "ES2020",
    "module": "commonjs",
    "lib": ["ES2020"],
    "outDir": "./dist",
    "rootDir": "./src",
    "strict": true,
    "esModuleInterop": true,
    "skipLibCheck": true,
    "forceConsistentCasingInFileNames": true,
    "resolveJsonModule": true,
    "declaration": true,
    "declarationMap": true,
    "sourceMap": true
  },
  "include": ["src/**/*"],
  "exclude": ["node_modules", "dist"]
}

Add build script to package.json:

{
  "scripts": {
    "build": "tsc",
    "dev": "tsc --watch",
    "check": "tsc --noEmit"
  }
}

Task 2: Migrate JavaScript to TypeScript (Incremental Approach)

Safe, incremental migration strategy:

Enable TypeScript to process JavaScript files:

{
  "compilerOptions": {
    "allowJs": true,
    "checkJs": false,
    "noEmit": true
  }
}

Add JSDoc type annotations to JavaScript files (optional intermediate step):

// @ts-check

/**
 * Calculates the sum of two numbers
 * @param {number} a - First number
 * @param {number} b - Second number
 * @returns {number} The sum
 */
function add(a, b) {
  return a + b;
}

/** @type {string[]} */
const names = ["Alice", "Bob"];

/** @typedef {{ id: number, name: string, email?: string }} User */

/** @type {User} */
const user = {
  id: 1,
  name: "Alice"
};

Rename files incrementally from .js to .ts:

# Start with utility files and leaf modules
mv src/utils/helpers.js src/utils/helpers.ts

Fix TypeScript errors in converted files:
- Add explicit type annotations where inference fails
- Define interfaces for complex objects
- Handle any types appropriately
- Add type guards for runtime checks
Gradually convert remaining files:
- Start with utilities and shared modules
- Move to leaf components (no dependencies)
- Finally convert orchestration/entry files

Enable strict mode progressively:

{
  "compilerOptions": {
    "strict": false,
    "noImplicitAny": true,
    "strictNullChecks": true
    // Enable other strict flags one at a time
  }
}

Task 3: Define Types and Interfaces

Creating robust type definitions:

Define interfaces for data structures:

// User data model
interface User {
  id: number;
  name: string;
  email: string;
  age?: number;  // Optional property
  readonly createdAt: Date;  // Read-only property
}

// API response structure
interface ApiResponse<T> {
  success: boolean;
  data?: T;
  error?: {
    code: string;
    message: string;
  };
}

Use type aliases for complex types:

// Union type
type Status = 'pending' | 'active' | 'completed' | 'failed';

// Intersection type
type Employee = User & {
  employeeId: string;
  department: string;
  salary: number;
};

// Function type
type TransformFn<T, U> = (input: T) => U;

// Conditional type
type NonNullable<T> = T extends null | undefined ? never : T;

Create type definitions in .d.ts files for external modules:

// types/custom-module.d.ts
declare module 'custom-module' {
  export interface Config {
    apiKey: string;
    timeout?: number;
  }

  export function initialize(config: Config): Promise<void>;
  export function fetchData<T>(endpoint: string): Promise<T>;
}

Task 4: Work with Generics

Type-safe reusable components:

Generic functions:

// Basic generic function
function identity<T>(value: T): T {
  return value;
}

const num = identity(42);        // Type: number
const str = identity("hello");   // Type: string

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

const user = { name: "Alice", age: 30 };
const name = getProperty(user, "name");  // Type: string
const age = getProperty(user, "age");    // Type: number

Generic classes:

class DataStore<T> {
  private items: T[] = [];

  add(item: T): void {
    this.items.push(item);
  }

  get(index: number): T | undefined {
    return this.items[index];
  }

  filter(predicate: (item: T) => boolean): T[] {
    return this.items.filter(predicate);
  }
}

const numberStore = new DataStore<number>();
numberStore.add(42);

const userStore = new DataStore<User>();
userStore.add({ id: 1, name: "Alice", email: "alice@example.com" });

Generic interfaces:

interface Repository<T> {
  findById(id: string): Promise<T | null>;
  findAll(): Promise<T[]>;
  create(item: Omit<T, 'id'>): Promise<T>;
  update(id: string, item: Partial<T>): Promise<T>;
  delete(id: string): Promise<boolean>;
}

class UserRepository implements Repository<User> {
  async findById(id: string): Promise<User | null> {
    // Implementation
    return null;
  }
  // ... other methods
}

Task 5: Handle Type Errors

Common type errors and solutions:

"Property does not exist" errors:

// ❌ Error: Property 'name' does not exist on type '{}'
const user = {};
user.name = "Alice";

// ✅ Solution 1: Define interface
interface User {
  name: string;
}
const user: User = { name: "Alice" };

// ✅ Solution 2: Type assertion (use cautiously)
const user = {} as User;
user.name = "Alice";

// ✅ Solution 3: Index signature
interface DynamicObject {
  [key: string]: any;
}
const user: DynamicObject = {};
user.name = "Alice";

"Cannot find name" errors:

// ❌ Error: Cannot find name 'process'
const env = process.env.NODE_ENV;

// ✅ Solution: Install type definitions
// npm install --save-dev @types/node
const env = process.env.NODE_ENV;  // Now works

any type issues:

// ❌ Implicit any (with noImplicitAny: true)
function process(data) {
  return data.value;
}

// ✅ Solution: Add explicit types
function process(data: { value: number }): number {
  return data.value;
}

// ✅ Or use generic
function process<T>(data: T): T {
  return data;
}

Union type narrowing:

function processValue(value: string | number) {
  // ❌ Error: Property 'toUpperCase' does not exist on type 'string | number'
  return value.toUpperCase();

  // ✅ Solution: Type guard
  if (typeof value === "string") {
    return value.toUpperCase();  // TypeScript knows it's string here
  }
  return value.toString();
}

Task 6: Configure for Specific Environments

Environment-specific configurations:

Node.js Project

{
  "compilerOptions": {
    "target": "ES2020",
    "module": "commonjs",
    "lib": ["ES2020"],
    "types": ["node"],
    "moduleResolution": "node",
    "esModuleInterop": true
  }
}

Browser/DOM Project

{
  "compilerOptions": {
    "target": "ES2020",
    "module": "esnext",
    "lib": ["ES2020", "DOM", "DOM.Iterable"],
    "moduleResolution": "bundler",
    "resolveJsonModule": true,
    "noEmit": true
  }
}

Library/Package

{
  "compilerOptions": {
    "target": "ES2020",
    "module": "esnext",
    "declaration": true,
    "declarationMap": true,
    "outDir": "./dist",
    "rootDir": "./src"
  }
}

TypeScript Best Practices

Do's

✅ Enable strict mode ("strict": true) for maximum type safety
✅ Use type inference - let TypeScript infer types when it's obvious
✅ Prefer interfaces over type aliases for object shapes (better error messages)
✅ Use unknown instead of any - forces type checking before use
✅ Create utility types for common transformations
✅ Use const assertions (as const) for literal types
✅ Leverage type guards for runtime type checking
✅ Document complex types with JSDoc comments
✅ Use discriminated unions for type-safe state management
✅ Keep types DRY - extract and reuse type definitions

Don'ts

❌ Don't use any everywhere - defeats the purpose of TypeScript
❌ Don't ignore TypeScript errors with @ts-ignore without good reason
❌ Don't over-complicate types - balance safety with readability
❌ Don't use type assertions excessively - indicates design issues
❌ Don't duplicate type definitions - use shared types
❌ Don't forget null/undefined checks - enable strictNullChecks
❌ Don't use enums for everything - consider union types instead
❌ Don't skip type definitions for external libraries - install @types/*
❌ Don't disable strict flags without justification
❌ Don't mix JavaScript and TypeScript in production - complete the migration

Common Patterns

Pattern 1: Discriminated Unions

Type-safe state management:

type LoadingState = { status: 'loading' };
type SuccessState<T> = { status: 'success'; data: T };
type ErrorState = { status: 'error'; error: Error };

type AsyncState<T> = LoadingState | SuccessState<T> | ErrorState;

function handleState<T>(state: AsyncState<T>) {
  switch (state.status) {
    case 'loading':
      console.log('Loading...');
      break;
    case 'success':
      console.log('Data:', state.data);  // TypeScript knows state.data exists
      break;
    case 'error':
      console.log('Error:', state.error.message);  // TypeScript knows state.error exists
      break;
  }
}

Pattern 2: Builder Pattern

Type-safe fluent API:

class QueryBuilder<T> {
  private filters: Array<(item: T) => boolean> = [];
  private sortFn?: (a: T, b: T) => number;
  private limitCount?: number;

  where(predicate: (item: T) => boolean): this {
    this.filters.push(predicate);
    return this;
  }

  sortBy(compareFn: (a: T, b: T) => number): this {
    this.sortFn = compareFn;
    return this;
  }

  limit(count: number): this {
    this.limitCount = count;
    return this;
  }

  execute(data: T[]): T[] {
    let result = data.filter(item =>
      this.filters.every(filter => filter(item))
    );

    if (this.sortFn) {
      result = result.sort(this.sortFn);
    }

    if (this.limitCount) {
      result = result.slice(0, this.limitCount);
    }

    return result;
  }
}

// Usage
const users = [/* ... */];
const result = new QueryBuilder<User>()
  .where(u => u.age > 18)
  .where(u => u.email.includes('@example.com'))
  .sortBy((a, b) => a.name.localeCompare(b.name))
  .limit(10)
  .execute(users);

Pattern 3: Type-Safe Event Emitter

type EventMap = {
  'user:created': { id: string; name: string };
  'user:updated': { id: string; changes: Partial<User> };
  'user:deleted': { id: string };
};

class TypedEventEmitter<T extends Record<string, any>> {
  private listeners: { [K in keyof T]?: Array<(data: T[K]) => void> } = {};

  on<K extends keyof T>(event: K, listener: (data: T[K]) => void): void {
    if (!this.listeners[event]) {
      this.listeners[event] = [];
    }
    this.listeners[event]!.push(listener);
  }

  emit<K extends keyof T>(event: K, data: T[K]): void {
    const eventListeners = this.listeners[event];
    if (eventListeners) {
      eventListeners.forEach(listener => listener(data));
    }
  }
}

// Usage with type safety
const emitter = new TypedEventEmitter<EventMap>();

emitter.on('user:created', (data) => {
  console.log(data.id, data.name);  // TypeScript knows the shape
});

emitter.emit('user:created', { id: '123', name: 'Alice' });  // Type-checked

Troubleshooting

Issue	Cause	Solution
Module not found	Missing type definitions	Install `@types/[package-name]` or add `declare module`
Implicit any errors	`noImplicitAny` enabled	Add explicit type annotations
Cannot find global types	Missing lib in `compilerOptions`	Add to `lib`: `["ES2020", "DOM"]`
Type errors in node_modules	Third-party library types	Add `skipLibCheck: true` to `tsconfig.json`
Import errors with .ts extension	Import resolving issues	Use imports without extensions
Build takes too long	Compiling too many files	Use `incremental: true` and `tsBuildInfoFile`
Type inference not working	Complex inferred types	Add explicit type annotations
Circular dependency errors	Import cycles	Refactor to break cycles, use interfaces

Advanced TypeScript Features

Mapped Types

Transform existing types:

type Readonly<T> = {
  readonly [P in keyof T]: T[P];
};

type Partial<T> = {
  [P in keyof T]?: T[P];
};

type Pick<T, K extends keyof T> = {
  [P in K]: T[P];
};

// Usage
interface User {
  id: number;
  name: string;
  email: string;
}

type ReadonlyUser = Readonly<User>;  // All properties readonly
type PartialUser = Partial<User>;    // All properties optional
type UserNameEmail = Pick<User, 'name' | 'email'>;  // Only name and email

Conditional Types

Types that depend on conditions:

type IsString<T> = T extends string ? true : false;

type A = IsString<string>;   // true
type B = IsString<number>;   // false

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

function getUser() {
  return { id: 1, name: "Alice" };
}

type User = ReturnType<typeof getUser>;  // { id: number; name: string }

Template Literal Types

String manipulation at type level:

type Greeting<T extends string> = `Hello, ${T}!`;

type WelcomeMessage = Greeting<"World">;  // "Hello, World!"

// Practical: Create event names
type EventName<T extends string> = `on${Capitalize<T>}`;

type ClickEvent = EventName<"click">;  // "onClick"
type HoverEvent = EventName<"hover">;  // "onHover"

TypeScript Configuration Reference

Key `tsconfig.json` Options

Option	Purpose	Recommended
`strict`	Enable all strict type checking	`true`
`target`	ECMAScript target version	`ES2020` or higher
`module`	Module system	`commonjs` (Node) or `esnext` (bundlers)
`lib`	Include type definitions	`["ES2020"]` + `DOM` if browser
`outDir`	Output directory	`./dist`
`rootDir`	Root source directory	`./src`
`sourceMap`	Generate source maps	`true` for debugging
`declaration`	Generate .d.ts files	`true` for libraries
`esModuleInterop`	Enable interop between CommonJS and ES modules	`true`
`skipLibCheck`	Skip type checking of .d.ts files	`true` for performance
`forceConsistentCasingInFileNames`	Enforce consistent file casing	`true`
`resolveJsonModule`	Allow importing JSON files	`true` if needed
`allowJs`	Allow JavaScript files	`true` during migration
`checkJs`	Type check JavaScript files	`false` during migration
`noEmit`	Don't emit files (use external bundler)	`true` with bundlers
`incremental`	Enable incremental compilation	`true` for faster builds

Migration Checklist

When migrating a JavaScript project to TypeScript:

Phase 1: Setup

Install TypeScript and @types packages
Create tsconfig.json with permissive settings
Configure build scripts
Set up IDE/editor TypeScript support

Phase 2: Incremental Migration

Enable allowJs: true and checkJs: false
Rename utility files to .ts
Add type annotations to function signatures
Create interfaces for data structures
Fix TypeScript errors in converted files

Phase 3: Strengthen Types

Enable noImplicitAny: true
Enable strictNullChecks: true
Remove any types where possible
Add type guards for union types
Create type definitions for external modules

Phase 4: Full Strict Mode

Enable strict: true
Fix all remaining type errors
Remove JSDoc annotations (now redundant)
Optimize type definitions
Document complex types

Phase 5: Maintenance

Set up pre-commit type checking
Configure CI/CD type checking
Establish code review standards for types
Keep TypeScript and @types packages updated

References

This skill includes bundled reference documentation for TypeScript essentials.

Reference Documentation (`references/`)

Core Concepts & Fundamentals

basics.md - TypeScript fundamentals, simple types, type inference, and special types
essentials.md - Core TypeScript concepts every developer should know
cheatsheet.md - Quick reference for control flow, classes, interfaces, types, and common patterns

Type System & Language Features

types.md - Advanced types, conditional types, mapped types, type guards, and recursive types
classes.md - Class syntax, inheritance, generics, and utility types
elements.md - Arrays, tuples, objects, enums, functions, and casting
keywords.md - keyof, null handling, optional chaining, and template literal types
miscellaneous.md - Async programming, promises, decorators, and JSDoc integration

External Resources

TypeScript Official Documentation
TypeScript Handbook
TypeScript tsconfig Reference
TypeScript Deep Dive
TypeScript Playground - Test TypeScript code online

Summary

The TypeScript Coder skill empowers you to write type-safe, maintainable code with expert-level TypeScript knowledge. Whether migrating existing JavaScript projects or starting new TypeScript projects, apply these proven patterns, workflows, and best practices to deliver production-quality code with confidence.

Remember: TypeScript is a tool for developer productivity and code quality. Use it to catch errors early, improve code documentation, and enable better tooling—but don't let perfect types prevent shipping working code.

TypeScript Coder

Description