TypeScript 5 New Features — Practical Use of the satisfies Operator and Decorators

// ❌ Korean term Korean term Korean term
type Color = "red" | "green" | "blue";
type ColorMap = Record<string, string | number[]>;

// as Korean term: Korean term Korean term O, Korean term X
const palette = {  red: [255, 0, 0],  green: "#00ff00",  blue: [0, 0, 255]
} as ColorMap;

// Korean term palette.redKorean term string | number[] Korean term Korean term
// .map()Korean term Korean term Korean term Korean term Korean term
palette.red.map(x => x); // Korean term! string | number[]Korean term map Korean term

// ✅ satisfies Korean term
type Color = "red" | "green" | "blue";
type ColorMap = Record<Color, string | number[]>;

const palette = {  red: [255, 0, 0],  green: "#00ff00",  blue: [0, 0, 255]
} satisfies ColorMap;

// Korean term Korean term Korean term O, Korean term Korean term O
palette.red.map(x => x);  // ✅ number[] Korean term
palette.green.toUpperCase();  // ✅ string Korean term
const wrong = { purple: "purple" } satisfies ColorMap; // ❌ Korean term: "purple"Korean term Color Korean term

interface ApiConfig {  baseUrl: string;  timeout: number;  retries: number;
}

// satisfiesKorean term Korean term Korean term + Korean term Korean term Korean term Korean term
const config = {  baseUrl: "https://api.example.com",  timeout: 5000,  retries: 3,
} satisfies ApiConfig;

// config.timeoutKorean term numberKorean term Korean term 5000 (Korean term Korean term)
type TimeoutType = typeof config.timeout; // 5000

type Route = {  path: string;  component: React.ComponentType;  auth?: boolean;
};

const routes = [  { path: "/", component: Home },  { path: "/dashboard", component: Dashboard, auth: true },
] satisfies Route[];

// routes[0].pathKorean term "/" (Korean term) Korean term, Route Korean term Korean term

type TranslationKey = "hello" | "goodbye" | "welcome";
type Translations = Record<TranslationKey, string>;

const ko = {  hello: "Korean term",  goodbye: "Korean term Korean term",  welcome: "Korean term",
} satisfies Translations;

const en = {  hello: "Hello",  goodbye: "Goodbye",  // welcome Korean term → Korean term Korean term Korean term! ✅
} satisfies Translations;

// ✅ TypeScript 5 Korean term Korean term
function sealed(target: typeof SealedClass) {  Object.seal(target);  Object.seal(target.prototype);
}

@sealed
class SealedClass {  greet() { return "Hello!"; }
}

function log(target: any, context: ClassMethodDecoratorContext) {  const methodName = String(context.name);  return function (this: any, ...args: any[]) {  console.log(`[${methodName}] Korean term, Korean termwater:`, args);  const result = target.call(this, ...args);  console.log(`[${methodName}] Korean term:`, result);  return result;  };
}

class Calculator {  @log  add(a: number, b: number): number {  return a + b;  }
}

const calc = new Calculator();
calc.add(1, 2);
// [add] Korean term, Korean termwater: [1, 2]
// [add] Korean term: 3

function readonly(target: any, context: ClassAccessorDecoratorContext) {  return {  set() {  throw new Error(`${String(context.name)}Korean term Korean term Korean term.`);  }  };
}

class Config {  @readonly  accessor version = "1.0.0";
}

const cfg = new Config();
console.log(cfg.version); // "1.0.0"
cfg.version = "2.0.0";  // ❌ Korean term Korean term

// Korean term API Korean term
function validate(target: any, context: ClassFieldDecoratorContext) {  context.metadata[context.name] = { required: true };
}

class UserForm {  @validate  name!: string;  @validate  email!: string;
}

// Korean term Korean term
const metadata = UserForm[Symbol.metadata];
console.log(metadata); // { name: { required: true }, email: { required: true } }

// ❌ Korean term Korean term
function identity<T>(value: T): T {  return value;
}

const result = identity(["a", "b", "c"]);
// result: string[] (Korean term Korean term)

// ✅ const Korean term Korean term
function identityConst<const T>(value: T): T {  return value;
}

const result2 = identityConst(["a", "b", "c"]);
// result2: readonly ["a", "b", "c"] (Korean term Korean term!)

function createRoute<const Path extends string>(path: Path) {  return { path, url: () => `https://app.example.com${path}` };
}

const homeRoute = createRoute("/");
// homeRoute.path: "/" (Korean term Korean term)

const dashRoute = createRoute("/dashboard");
// dashRoute.path: "/dashboard" (Korean term Korean term)

// tsconfig.json
{  "extends": [  "@tsconfig/strictest/tsconfig.json",  "@tsconfig/node18/tsconfig.json",  "./custom.tsconfig.json"  ],  "compilerOptions": {  "outDir": "dist"  }
}

// Symbol.disposeKorean term Korean term Korean term
cla

---

**Reference:** [Bank of Korea Economic Statistics](https://ecos.bok.or.kr)  <script type="application/ld+json">
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}
</script>  ## Frequently Asked Questions (FAQ)

### Q1. What is the most important new feature in TypeScript 5?
A: The satisfies operator, standard decorators, and const type parameters have the greatest practical impact.

### Q2. When should I use the satisfies operator?
A: Use it when you want to validate that an object meets specific type requirements while preserving its precise inference.

### Q3. What is the difference between satisfies and as?
A: as is a type assertion, while satisfies is type validation, so it catches incorrect structures more safely.

### Q4. Are TypeScript 5 decorators different from the old ones?
A: Standard decorators follow the TC39 process and behave differently from the existing experimentalDecorators.

### Q5. Why are const type parameters useful?
A: They preserve literal type inference in generic functions, allowing you to create more precise types.

### Q6. What should I check before upgrading to TypeScript 5?
A: Check tsconfig, decorator usage, build tools, ESLint, and library type compatibility.