2nd/10_Wiki/Topics/Frontend/백엔드-프론트엔드 데이터 변환(Data Transformation between Backend and Frontend).md

---
id: wiki-2026-0508-백엔드-프론트엔드-데이터-변환-data-transforma
title: 백엔드-프론트엔드 데이터 변환(Data Transformation between Backend and Frontend)
category: 10_Wiki/Topics
status: verified
canonical_id: self
aliases: [DTO, Data Transformation, API 응답 변환, snake_case to camelCase]
duplicate_of: none
source_trust_level: A
confidence_score: 0.9
verification_status: applied
tags: [frontend, backend, api, data-modeling, typescript]
raw_sources: []
last_reinforced: 2026-05-10
github_commit: pending
tech_stack:
  language: typescript
  framework: react
---

# 백엔드-프론트엔드 데이터 변환(Data Transformation between Backend and Frontend)

## 매 한 줄
> **"매 server payload 매 client model 의 일치 안함 — adapter 의 boundary"**. 매 DB schema (snake_case, nullable, denormalized) 매 UI model (camelCase, optional, normalized)의 mismatch 의 explicit 의 mapping. 매 2026 의 Zod + tRPC + GraphQL codegen 의 type-safe automation.

## 매 핵심

### 매 mismatch dimensions
- **Naming**: `user_name` (snake) ↔ `userName` (camel).
- **Types**: `1693497600` (Unix epoch) ↔ `Date` object, `"2026-05-10"` ↔ `Date`.
- **Shape**: nested vs flat, `{user: {profile: {...}}}` ↔ `{userId, userName, ...}`.
- **Nullability**: `null` (DB) ↔ `undefined` (TS optional).
- **Enums**: `0/1/2` ↔ `'pending' | 'active' | 'closed'`.

### 매 layers (Clean Architecture)
- **DTO** (Data Transfer Object) — wire format, snake_case, raw.
- **Domain Model** — business rules, computed props.
- **View Model** — UI-specific (formatted strings, derived flags).

### 매 응용
1. REST API 의 camelCase 변환.
2. GraphQL → TypeScript codegen.
3. Date/Decimal 의 typed objects 변환.
4. Form 의 server payload 의 transform.

## 💻 패턴

### 1. snake_case ↔ camelCase (humps library)
```typescript
import { camelizeKeys, decamelizeKeys } from 'humps';

const apiResponse = { user_name: 'Alice', created_at: '2026-05-10' };
const camel = camelizeKeys(apiResponse) as { userName: string; createdAt: string };
// → { userName: 'Alice', createdAt: '2026-05-10' }

const payload = decamelizeKeys({ userName: 'Bob' });
// → { user_name: 'Bob' }
```

### 2. Zod schema (validate + transform)
```typescript
import { z } from 'zod';

const UserDTO = z.object({
  user_id: z.number(),
  user_name: z.string(),
  created_at: z.string(),
  is_active: z.union([z.literal(0), z.literal(1)]),
}).transform(d => ({
  userId: d.user_id,
  userName: d.user_name,
  createdAt: new Date(d.created_at),
  isActive: d.is_active === 1,
}));

type User = z.infer<typeof UserDTO>;
// { userId: number; userName: string; createdAt: Date; isActive: boolean }

async function fetchUser(id: number): Promise<User> {
  const res = await fetch(`/api/users/${id}`);
  const json = await res.json();
  return UserDTO.parse(json); // validates + transforms
}
```

### 3. TanStack Query select (boundary transform)
```typescript
import { useQuery } from '@tanstack/react-query';

export function useUser(id: number) {
  return useQuery({
    queryKey: ['user', id],
    queryFn: async () => {
      const res = await fetch(`/api/users/${id}`);
      return res.json();
    },
    select: (data) => UserDTO.parse(data), // transform per render
  });
}
```

### 4. Date / Decimal serialization
```typescript
// server JSON cannot carry Date/BigInt natively
import Decimal from 'decimal.js';

const PaymentDTO = z.object({
  amount: z.string().transform(s => new Decimal(s)),
  paid_at: z.string().datetime().transform(s => new Date(s)),
});

// reverse: outgoing
function toPayload(p: { amount: Decimal; paidAt: Date }) {
  return {
    amount: p.amount.toString(),
    paid_at: p.paidAt.toISOString(),
  };
}
```

### 5. tRPC (no transform needed — type-safe RPC)
```typescript
// server (router.ts)
import { initTRPC } from '@trpc/server';
const t = initTRPC.create();

export const appRouter = t.router({
  user: t.procedure
    .input(z.object({ id: z.number() }))
    .query(async ({ input }) => {
      const u = await db.user.findUnique({ where: { id: input.id } });
      return u; // typed end-to-end
    }),
});

// client — types inferred, no manual mapping
const { data } = trpc.user.useQuery({ id: 1 });
```

### 6. GraphQL Codegen
```yaml
# codegen.yml
schema: http://localhost:4000/graphql
documents: 'src/**/*.graphql'
generates:
  src/gql/types.ts:
    plugins:
      - typescript
      - typescript-operations
      - typescript-react-query
```

### 7. View Model layer (UI formatting)
```typescript
type User = { firstName: string; lastName: string; createdAt: Date };
type UserVM = { fullName: string; joinedAgo: string };

function toVM(u: User): UserVM {
  return {
    fullName: `${u.firstName} ${u.lastName}`,
    joinedAgo: formatDistanceToNow(u.createdAt, { addSuffix: true }),
  };
}
```

## 매 결정 기준
| 상황 | Approach |
|---|---|
| REST + TypeScript | Zod schema 의 validate + transform at boundary. |
| Full-stack TypeScript monorepo | tRPC 의 zero transform overhead. |
| GraphQL backend | GraphQL Codegen 의 auto types. |
| Legacy snake_case API | humps 의 camelize at fetch boundary. |
| BigInt / Decimal | string serialization + parse on client. |

**기본값**: Zod schema 의 boundary validation + transform. tRPC 매 monorepo 의 ideal.

## 🔗 Graph
- 부모: [[Clean Architecture]]
- 변형: [[Zod]] · [[GraphQL Codegen]]
- 응용: [[React Hook Form]]
- Adjacent: [[Schema Validation]]

## 🤖 LLM 활용
**언제**: API integration, form submission, type-safe full-stack, validation at trust boundary.
**언제 X**: 매 internal tool 의 untyped — 매 still validate 매 trust boundary.

## ❌ 안티패턴
- **Trust the API blindly**: validate 매 trust boundary — server bug, version mismatch breaks UI silently.
- **Transform 의 component-level**: scatter mapping logic — centralize at fetch layer.
- **`any` types**: defeats TS — Zod infer 의 type derivation.
- **Date as string in app code**: lose type safety — parse to Date at boundary.
- **Manual mapping 의 100 fields**: codegen 의 use.

## 🧪 검증 / 중복
- Verified (Zod docs, tRPC docs, Martin Fowler "Patterns of Enterprise Application Architecture").
- 신뢰도 A.

## 🕓 Changelog
| 날짜 | 변경 |
|---|---|
| 2026-05-08 | Phase 1 |
| 2026-05-10 | Manual cleanup — full content with Zod/tRPC/Codegen patterns |