mastering-typescript

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SKILL.md (first lines)

# Mastering Modern TypeScript

Build enterprise-grade, type-safe applications with TypeScript 5.9+.

> **Compatibility:** TypeScript 5.9+, Node.js 22 LTS, Vite 7, NestJS 11, React 19

## Quick Start

```bash
# Initialize TypeScript project with ESM
pnpm create vite@latest my-app --template vanilla-ts
cd my-app && pnpm install

# Configure strict TypeScript
cat > tsconfig.json << 'EOF'
{
  "compilerOptions": {
    "target": "ES2024",
    "module": "ESNext",
    "moduleResolution": "bundler",
    "strict": true,
    "noUncheckedIndexedAccess": true,
    "exactOptionalPropertyTypes": true,
    "esModuleInterop": true,
    "skipLibCheck": true
  }
}
EOF
```

## When to Use This Skill

Use when:
- Building type-safe React, NestJS, or Node.js applications
- Migrating JavaScript codebases to TypeScript
- Implementing advanced type patterns (generics, mapped types, conditional types)
- Configuring modern TypeScript toolchains (Vite, pnpm, ESLint)
- Designing type-safe API contracts with Zod validation
- Comparing TypeScript approaches with Java or Python

## Project Setup Checklist

Before starting any TypeScript project:

```
- [ ] Use pnpm for package management (faster, disk-efficient)
- [ ] Configure ESM-first (type: "module" in package.json)
- [ ] Enable strict mode in tsconfig.json
- [ ] Set up ESLint with @typescript-eslint
- [ ] Add Prettier for consistent formatting
- [ ] Configure Vitest for testing
```

## Type System Quick Reference

### Primitive Types

```typescript
const name: string = "Alice";
const age: number = 30;
const active: boolean = true;
const id: bigint = 9007199254740991n;
const key: symbol = Symbol("unique");
```

### Union and Intersection Types

```typescript
// Union: value can be one of several types
type Status = "pending" | "approved" | "rejected";

// Intersection: value must satisfy all types
type Employee = Person & { employeeId: string };

// Discriminated union for type-safe handling
type Result<T> =
  | { success: true; data: T }
  | { success: false; error: string };

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

### Type Guards

```typescript
// typeof guard
function process(value: string | number): string {
  if (typeof value === "string") {
    return value.toUpperCase();
  }
  return value.toFixed(2);
}

// Custom type guard
interface User { type: "user"; name: string }
interface Admin { type: "admin"; permissions: string[] }

function isAdmin(person: User | Admin): person is Admin {
  return person.type === "admin";
}
```

### The `satisfies` Operator (TS 5.0+)

Validate type conformance while preserving inference:

```typescript
// Problem: Type assertion loses specific type info
const colors1 = {
  red: "#ff0000",
  green: "#00ff00"
} as Record<string, string>;

From dsactivi-2/claude-settings, MIT.

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