타입스크립트(TypeScript) 시작하기, Node.js 실행과 타입 검사의 차이

$ cat > hello.ts <<'TS'
interface User { name: string; age: number }
function greet(user: User): string {
  return `안녕하세요, ${user.name}님 (${user.age}살)`;
}
const u: User = { name: "Finn", age: 30 };
console.log(greet(u));
TS
node hello.ts
안녕하세요, Finn님 (30살)

$ node -e "const { stripTypeScriptTypes } = require('node:module'); console.log(stripTypeScriptTypes(require('fs').readFileSync('hello.ts', 'utf8')))" 2>/dev/null
function greet(user      )         {
  return `안녕하세요, ${user.name}님 (${user.age}살)`;
}
const u       = { name: "Finn", age: 30 };
console.log(greet(u));

$ cat > wrong.ts <<'TS'
function add(a: number, b: number): number {
  return a + b;
}
const price: number = 1000;
console.log(add(price, "500"));
const name: string = 42;
console.log(name.toUpperCase());
TS
node wrong.ts
1000500
./wrong.ts:7
console.log(name.toUpperCase());
                 ^

TypeError: name.toUpperCase is not a function
    at Object.<anonymous> (./wrong.ts:7:18)
    at Module._compile (node:internal/modules/cjs/loader:1781:14)
    at Module._extensions..js (node:internal/modules/cjs/loader:1913:10)
    at Module.load (node:internal/modules/cjs/loader:1505:32)
    at Module._load (node:internal/modules/cjs/loader:1309:12)
    at wrapModuleLoad (node:internal/modules/cjs/loader:254:19)
    at Function.executeUserEntryPoint [as runMain] (node:internal/modules/run_main:171:5)
    at node:internal/main/run_main_module:36:49

Node.js v22.23.1

$ cat > enum.ts <<'TS'
enum Level { Low, Mid, High }
console.log(Level.High, Level[2]);
TS
node enum.ts
./enum.ts:1
enum Level { Low, Mid, High }
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
console.log(Level.High, Level[2]);

SyntaxError [ERR_UNSUPPORTED_TYPESCRIPT_SYNTAX]: TypeScript enum is not supported in strip-only mode
    at parseTypeScript (node:internal/modules/typescript:63:40)
    at processTypeScriptCode (node:internal/modules/typescript:133:42)
    at stripTypeScriptModuleTypes (node:internal/modules/typescript:163:10)
    at Module._compile (node:internal/modules/cjs/loader:1733:15)
    at Module._extensions..js (node:internal/modules/cjs/loader:1913:10)
    at Module.load (node:internal/modules/cjs/loader:1505:32)
    at Module._load (node:internal/modules/cjs/loader:1309:12)
    at wrapModuleLoad (node:internal/modules/cjs/loader:254:19)
    at Function.executeUserEntryPoint [as runMain] (node:internal/modules/run_main:171:5)
    at node:internal/main/run_main_module:36:49 {
  code: 'ERR_UNSUPPORTED_TYPESCRIPT_SYNTAX'
}

Node.js v22.23.1

$ node --experimental-transform-types enum.ts 2>&1 | grep -v -e ExperimentalWarning -e trace-warnings
2 High

$ printf 'export function add(a: number, b: number): number { return a + b; }\n' > math.ts && printf 'import { add } from "./math.ts";\nconsole.log("2 + 3 =", add(2, 3));\n' > main.ts && node main.ts
2 + 3 = 5

$ printf 'import { add } from "./math";\nconsole.log(add(2, 3));\n' > main2.ts && node main2.ts 2>&1 | grep -E 'ERR_|Cannot find'
    throw new ERR_MODULE_NOT_FOUND(
Error [ERR_MODULE_NOT_FOUND]: Cannot find module './math' imported from ./main2.ts
  code: 'ERR_MODULE_NOT_FOUND',

import type { Type1, Type2 } from './module.ts';
import { fn, type FnParams } from './fn.ts';

$ cat > types.ts <<'TS'
function first<T>(items: T[]): T | undefined {
  return items[0];
}
type Status = "ready" | "done";
function label(s: Status): string {
  return s === "ready" ? "준비" : "완료";
}
console.log(first([3, 1, 2]), first(["a", "b"]), first([]));
console.log(label("ready"), label("done"));
TS
node types.ts
3 a undefined
준비 완료

$ claude --model opus -p "이 폴더의 wrong.ts에서 타입이 맞지 않는 곳을 줄 번호와 이유로 한 줄씩만 적어. 파일은 고치지 마." --allowedTools "Read"
- 5행: `add`의 두 번째 매개변수 `b`는 `number`인데 문자열 `"500"`을 넘겼습니다.
- 6행: `string`으로 선언한 `name`에 숫자 `42`를 대입했습니다.

tsc는 실행하지 못했고 파일을 직접 읽고 판단했습니다. 7행은 `name`을 `string`으로 선언했으므로 타입 오류가 아닙니다.

{
  "compilerOptions": {
     "noEmit": true, // Optional - see note below
     "target": "esnext",
     "module": "nodenext",
     "rewriteRelativeImportExtensions": true,
     "erasableSyntaxOnly": true,
     "verbatimModuleSyntax": true
  }
}

npm install --save-dev tsx
npx tsx your-file.ts