Next.js Integration

The @emulators/adapter-next package embeds emulators directly into a Next.js App Router app, running them on the same origin. This is particularly useful for Vercel preview deployments where OAuth callback URLs change with every deployment.

Install

npm install @emulators/adapter-next @emulators/github @emulators/google

Only install the emulators you need. Each @emulators/* package is published independently, keeping serverless bundles small.

Route Handler

Create a catch-all route that serves emulator traffic:

// app/emulate/[...path]/route.ts
import { createEmulateHandler } from '@emulators/adapter-next'
import * as github from '@emulators/github'
import * as google from '@emulators/google'

export const { GET, POST, PUT, PATCH, DELETE } = createEmulateHandler({
  services: {
    github: {
      emulator: github,
      seed: {
        users: [{ login: 'octocat', name: 'The Octocat' }],
        repos: [{ owner: 'octocat', name: 'hello-world', auto_init: true }],
      },
    },
    google: {
      emulator: google,
      seed: {
        users: [{ email: 'test@example.com', name: 'Test User' }],
      },
    },
  },
})

This creates the following routes:

• /emulate/github/** serves the GitHub emulator
• /emulate/google/** serves the Google emulator

Auth.js / NextAuth Configuration

Point your provider at the emulator paths on the same origin:

import GitHub from 'next-auth/providers/github'

const baseUrl = process.env.VERCEL_URL
  ? `https://${process.env.VERCEL_URL}`
  : 'http://localhost:3000'

GitHub({
  clientId: 'any-value',
  clientSecret: 'any-value',
  authorization: { url: `${baseUrl}/emulate/github/login/oauth/authorize` },
  token: { url: `${baseUrl}/emulate/github/login/oauth/access_token` },
  userinfo: { url: `${baseUrl}/emulate/github/user` },
})

No oauth_apps need to be seeded. When none are configured, the emulator skips client_id, client_secret, and redirect_uri validation.

Font Tracing for Serverless

Emulator UI pages use bundled fonts. Wrap your Next.js config to include them in the serverless trace:

// next.config.mjs
import { withEmulate } from '@emulators/adapter-next'

export default withEmulate({
  // your normal Next.js config
})

If you mount the catch-all at a custom path, pass the matching prefix:

export default withEmulate(nextConfig, { routePrefix: '/api/emulate' })

Persistence

By default, emulator state is in-memory and resets on every cold start. To persist state across restarts, pass a persistence adapter.

Custom Adapter (Vercel KV, Redis, etc.)

import { createEmulateHandler } from '@emulators/adapter-next'
import * as github from '@emulators/github'

const kvAdapter = {
  async load() { return await kv.get('emulate-state') },
  async save(data: string) { await kv.set('emulate-state', data) },
}

export const { GET, POST, PUT, PATCH, DELETE } = createEmulateHandler({
  services: { github: { emulator: github } },
  persistence: kvAdapter,
})

File Persistence (Local Dev)

For local development, @emulators/core ships a file-based adapter:

import { filePersistence } from '@emulators/core'

// persists to a JSON file
persistence: filePersistence('.emulate/state.json'),

How Persistence Works

• Cold start: The adapter loads state from the persistence adapter. If found, it restores the full Store and token map (skipping seed). If not found, it seeds from config and saves the initial state.
• After mutating requests (POST, PUT, PATCH, DELETE): State is saved. Saves are serialized via an internal queue to prevent race conditions.
• No persistence configured: Falls back to pure in-memory. Seed data re-initializes on every cold start.

How It Works

1. Incoming request: /emulate/github/login/oauth/authorize?client_id=...
2. Parse: service = github, rest = /login/oauth/authorize
3. Strip prefix: A new Request is created with the stripped path and forwarded to the GitHub service app
4. Rewrite response: HTML action and href attributes, CSS url() font references, and Location headers get the service prefix prepended
5. Persist: After mutating requests, state is saved via the persistence adapter

Limitations

• Requires the Node.js runtime (not Edge) since emulators use crypto.randomBytes
• Concurrent serverless instances writing to the same persistence adapter use last-write-wins semantics (acceptable for dev/preview traffic)

Config Reference

createEmulateHandler(config)

Each EmulatorEntry:

withEmulate(nextConfig, options?)

Wraps a Next.js config to include emulator font files in the serverless output trace. Call it around your exported config in next.config.mjs or next.config.ts.

PersistenceAdapter

interface PersistenceAdapter {
  load(): Promise<string | null>
  save(data: string): Promise<void>
}

The built-in filePersistence(path) from @emulators/core provides a file-based adapter for local development.