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charlie
charlie

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Stop Writing API Docs Twice: Introducing @restdocs for Next.js

#nextjs #typescript #testing #documentation

TL;DR: I built a test-driven API documentation tool for Next.js that generates OpenAPI specs, TypeScript types, and Swagger UI directly from your tests. One test, complete documentation. No duplicate work.

The Problem I Couldn't Ignore

Last month, I was onboarding a new frontend developer to our Next.js project. They asked a simple question:

"Where's the API documentation?"

I pointed them to our docs/api folder. It was 6 weeks out of date. Half the endpoints had changed. The TypeScript types didn't match the actual responses. The new developer spent 3 days reading code instead of building features.

This is broken.

Every Next.js team I know faces the same problem:

  1. Write tests to ensure APIs work
  2. Separately document those same APIs in OpenAPI/Swagger
  3. Watch them drift within 2-3 sprints
  4. Burn time keeping both in sync

It's like writing the same essay twice in different languages—except one version is always wrong.

The "Aha" Moment

I'd used Spring REST Docs in Java projects before. The concept was brilliant:

Document your API in your tests. Generate specs automatically.

But Node.js had nothing like it. Tools like Swagger JSDoc require maintaining docs separately. NestJS has decorators but locks you into a framework. I wanted something that:

  • Works with Next.js 15 App Router (the modern stack)
  • Zero framework lock-in (just Jest + your existing tests)
  • Generates everything (OpenAPI + TypeScript + Markdown)
  • 100% accuracy (docs are test-driven, not manually maintained)

So I built it. In a weekend. Using Vitest.

Introducing @restdocs

One test. Complete documentation. Always in sync.

Before @restdocs (The Old Way) 

// Step 1: Write your test
it('creates user', async () => {
  const res = await POST(request);
  expect(res.status).toBe(201);
});

// Step 2: Separately write OpenAPI docs (50+ lines)
// (Imagine 50+ lines of YAML/JSON here...)

// Step 3: Manually create TypeScript types (again)
interface CreateUserRequest {
  email: string;
  name: string;
}

// Step 4: Keep all 3 in sync forever (impossible)
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Result: 100+ lines of duplicate code. Docs drift within weeks.

After @restdocs (The New Way) 

import { api, field } from '@restdocs/core';

it('creates user', async () => {
  const res = await POST(request);

  // This ONE call generates OpenAPI + TypeScript + Swagger UI
  api.document('POST /api/users', {
    description: 'Create a new user',
    request: {
      email: field.email().required(),
      name: field.string().required().minLength(2).maxLength(50),
    },
    response: {
      id: field.uuid(),
      email: field.email(),
      name: field.string(),
      createdAt: field.datetime(),
    },
    statusCode: 201,
  });

  expect(res.status).toBe(201);
});
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Run npm test. Get:

  • openapi.json - Full OpenAPI 3.0 spec
  • types.ts - Auto-generated TypeScript interfaces
  • api.md - Human-readable Markdown docs
  • Swagger UI at /api/__restdocs

70% less code. 100% accuracy. Zero drift.

How It Works

Architecture 

Your Tests → @restdocs DSL → Schema Compiler → Generators
                                                    ↓
                                      OpenAPI + TypeScript + Markdown
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  1. Fluent DSL: Type-safe schema builder with chainable methods
  2. Schema Compiler: Normalizes schemas to OpenAPI 3.0 format
  3. Test Collector: Gathers documented endpoints during test runs
  4. Multi-Format Generators: Outputs OpenAPI, TypeScript, Markdown
  5. Dev UI: Built-in Swagger UI for Next.js

Why This Approach Works

Test-driven documentation isn't new (Spring REST Docs proved it). But Node.js lacked a modern implementation:

  • Single source of truth: Tests define behavior AND documentation
  • Type inference: Auto-detect schemas from test data
  • Built-in validation: Assert responses match schemas
  • Zero runtime cost: Documentation generated at test-time
  • Framework agnostic: Works with Express, Fastify, Next.js

Real-World Impact

For Solo Developers

  • Save 5+ hours/week: No more dual maintenance
  • Better onboarding: New contributors explore APIs via Swagger UI
  • Faster iteration: Change API → update test → docs auto-sync

For Teams

  • Frontend-backend contracts: TypeScript types auto-generated
  • Reduced miscommunication: Swagger UI shows exact formats
  • Quality gates: Built-in validation catches mismatches
  • Faster code reviews: See API changes in generated docs

Metrics from Early Adopters

  • 70% reduction in documentation code
  • 10x faster API exploration for new team members
  • Zero doc drift (tests always match docs)
  • 50% faster onboarding time

Current Status: Experimental Beta (v0.3.0)

Let's be transparent: @restdocs is young.

What's Stable

  • Core DSL and schema system (247 tests passing)
  • Next.js 15 App Router integration
  • Jest reporter and test collection
  • OpenAPI 3.0 + TypeScript + Markdown generation
  • Dev UI with Swagger

What's Experimental

  • Scale testing: Validated with ~50 endpoints, not 500+
  • Complex schemas: Nested objects work, edge cases may exist
  • Production readiness: Use in dev/staging first
  • Framework support: Next.js is solid, others are lighter

Roadmap

  • Vitest native support
  • Advanced schema features (discriminated unions, recursive types)
  • Performance optimization for large APIs
  • Plugin ecosystem for custom generators
  • Fastify/Hono/Express deep integration

Use it. Break it. Help make it production-ready.

Getting Started (5 Minutes)

Install 

npm install @restdocs/nextjs --save-dev
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Configure Next.js 

// next.config.ts
import { withRestDocs } from '@restdocs/nextjs/config';

export default withRestDocs({
  restdocs: {
    enabled: process.env.NODE_ENV === 'development',
    path: '/api/__restdocs',
  },
});
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Configure Jest 

// jest.config.js
const nextJest = require('next/jest');
const createJestConfig = nextJest({ dir: './' });

module.exports = createJestConfig({
  reporters: [
    'default',
    ['@restdocs/jest/reporter', {
      outputDir: './docs/api',
      formats: ['openapi', 'markdown', 'typescript'],
    }],
  ],
});
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Create Dev UI Route 

// app/api/__restdocs/route.ts
import { createDevUIHandler } from '@restdocs/nextjs/dev-ui';

const handler = createDevUIHandler();
export { handler as GET };
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Document Your First API 

// __tests__/api/users.test.ts
import { api, field } from '@restdocs/core';
import { POST } from '@/app/api/users/route';

it('creates user', async () => {
  const res = await POST(request);

  api.document('POST /api/users', {
    description: 'Create a new user',
    request: {
      email: field.email().required(),
      name: field.string().required().minLength(2),
    },
    response: {
      id: field.uuid(),
      email: field.email(),
      name: field.string(),
    },
    statusCode: 201,
  });

  expect(res.status).toBe(201);
});
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Run Tests 

npm test
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You'll get:

  • docs/api/openapi.json
  • docs/api/types.ts
  • docs/api/api.md

Start dev server:

npm run dev
open http://localhost:3000/api/__restdocs
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Interactive Swagger UI with all your APIs!

Lessons Learned

1. TypeScript Type Inference is Hard

Getting schema inference to preserve literal types required deep dives into TypeScript's type system.

const schema = api.infer({ status: 'active' as const });
// schema.status is 'active', not string
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2. OpenAPI 3.0 is Complex

Edge cases like oneOf, allOf, discriminator are tricky. We support the 80% use case well.

3. Developer Experience Matters Most

Method chaining wins:

// Verbose
field.string({ required: true, minLength: 2 })

// Fluent (better!)
field.string().required().minLength(2)
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4. Testing Documentation Tools is Meta

Writing tests for a tool that documents tests... recursion all the way down.

What's Next?

Short-term (v0.4.0)

  • Vitest native support
  • Performance benchmarks (100+ endpoints)
  • Plugin system
  • Advanced schema features

Medium-term (v0.5.0)

  • Fastify, Hono, Elysia integrations
  • React Query/tRPC codegen
  • Postman collection generator

Long-term (v1.0.0)

  • Production-ready guarantees
  • Enterprise support
  • Cloud-hosted docs service

Join the Journey

@restdocs is experimental. I need your help!

Ways to Contribute

Final Thoughts

API documentation doesn't have to suck. It shouldn't require duplicate effort. And it definitely shouldn't drift out of sync.

@restdocs is my attempt to fix this for Next.js. It's experimental, imperfect, and needs your feedback.

If you hate writing docs twice, give it a try. Let me know what breaks. Help me make it better.

Let's build the Node.js equivalent of Spring REST Docs together.

Try It Now 

npm install @restdocs/nextjs --save-dev
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Full example: nextjs-app-router

Star the repo: github.com/json-choi/restdocs ⭐

Built with ❤️ for developers who believe API docs should just work.

Questions? Drop them in the comments or open a discussion on GitHub!
Twice: Introducing @restdocs for Next.js

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