47. TypeScript with Node

Node doesn't execute TypeScript directly (aside from newer experimental type-stripping support); typically you compile with `tsc` (or use `ts-node`/`tsx` for development) to plain JavaScript that Node then runs. `tsconfig.json` configures the target module system, output directory and strictness.

json
{
  "compilerOptions": {
    "target": "ES2022",
    "module": "NodeNext",
    "moduleResolution": "NodeNext",
    "outDir": "dist",
    "strict": true
  }
}

ts
interface Balance { address: string; amountWei: bigint; }

function formatBalance(b: Balance): string {
  return `${b.address}: ${b.amountWei.toString()}`;
}

Types exist only at compile time and are erased in the emitted JavaScript — they catch mistakes before deployment but add no runtime checking, which is why input validation at API boundaries (see topic 27) is still necessary even in a fully-typed codebase.

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