Clerk Hello World Skill

Clerk Hello World

Overview

Make your first authenticated requests using Clerk across server components, client components, API routes, and server actions. Validates your Clerk integration end-to-end.

Prerequisites

Clerk SDK installed (clerk-install-auth completed)
Environment variables configured (NEXT_PUBLIC_CLERK_PUBLISHABLE_KEY, CLERK_SECRET_KEY)
ClerkProvider wrapping application root
Middleware configured at project root

Instructions

Step 1: Server Component — auth() and currentUser()

// app/dashboard/page.tsx
import { auth, currentUser } from '@clerk/nextjs/server'
import { redirect } from 'next/navigation'

export default async function DashboardPage() {
  // auth() is lightweight — reads JWT from the session cookie, no API call
  const { userId } = await auth()

  if (!userId) redirect('/sign-in')

  // currentUser() makes a Backend API call — use sparingly, counts toward rate limit
  const user = await currentUser()

  return (
    <div>
      <h1>Hello, {user?.firstName || 'User'}!</h1>
      <p>User ID: {userId}</p>
      <p>Email: {user?.emailAddresses[0]?.emailAddress}</p>
      <p>Created: {user?.createdAt ? new Date(user.createdAt).toLocaleDateString() : 'N/A'}</p>
    </div>
  )
}

Key distinction: auth() is cheap (JWT parsing, no network). currentUser() is expensive (Backend API call, rate-limited). Prefer auth() when you only need userId, orgId, or sessionClaims.

Step 2: Protected API Route

// app/api/hello/route.ts
import { auth } from '@clerk/nextjs/server'

export async function GET() {
  const { userId, orgId, sessionClaims } = await auth()

  if (!userId) {
    return Response.json({ error: 'Unauthorized' }, { status: 401 })
  }

  return Response.json({
    message: 'Hello from Clerk!',
    userId,
    orgId: orgId || null,
    sessionId: sessionClaims?.sid,
    timestamp: new Date().toISOString(),
  })
}

Step 3: Client Component with Hooks

'use client'
import { useUser, useAuth, useClerk } from '@clerk/nextjs'

export function AuthTest() {
  const { user, isLoaded, isSignedIn } = useUser()
  const { getToken, signOut } = useAuth()
  const { openUserProfile } = useClerk()

  if (!isLoaded) return <div>Loading...</div>
  if (!isSignedIn) return <div>Not signed in</div>

  const testAPI = async () => {
    // getToken() returns the session JWT — use for calling your own API routes
    // or external services with Bearer token auth
    const token = await getToken()
    const res = await fetch('/api/hello', {
      headers: { Authorization: `Bearer ${token}` },
    })
    const data = await res.json()
    console.log('API response:', data)
  }

  return (
    <div>
      <p>Signed in as: {user.primaryEmailAddress?.emailAddress}</p>
      <img src={user.imageUrl} alt="Avatar" width={48} height={48} />
      <div className="flex gap-2 mt-4">
        <button onClick={testAPI}>Test API</button>
        <button onClick={() => openUserProfile()}>Profile</button>
        <button onClick={() => signOut()}>Sign Out</button>
      </div>
    </div>
  )
}

Step 4: Server Action with Auth

// app/actions.ts
'use server'
import { auth } from '@clerk/nextjs/server'

export async function greetUser() {
  const { userId } = await auth()
  if (!userId) throw new Error('Unauthorized')

  // Perform server-side work here (DB queries, external API calls, etc.)
  return { greeting: `Hello user ${userId}!`, timestamp: new Date().toISOString() }
}

// app/dashboard/greeting.tsx
'use client'
import { greetUser } from '@/app/actions'
import { useState } from 'react'

export function GreetingButton() {
  const [msg, setMsg] = useState<string | null>(null)

  return (
    <div>
      <button onClick={async () => {
        const result = await greetUser()
        setMsg(result.greeting)
      }}>
        Get Server Greeting
      </button>
      {msg && <p>{msg}</p>}
    </div>
  )
}

Step 5: Express.js Hello World

import express from 'express'
import { clerkMiddleware, requireAuth, getAuth } from '@clerk/express'

const app = express()
app.use(clerkMiddleware())

// Public endpoint
app.get('/api/hello', (req, res) => {
  const { userId } = getAuth(req)
  res.json({
    message: userId ? `Hello user ${userId}!` : 'Hello anonymous!',
    authenticated: !!userId,
  })
})

// Protected endpoint — returns 403 if no valid session
app.get('/api/me', requireAuth(), (req, res) => {
  const { userId, orgId } = getAuth(req)
  res.json({ userId, orgId })
})

app.listen(3001)

Error Handling

| Error | Cause | Solution | |-------|-------|----------| | userId is null | User not authenticated | Redirect to /sign-in or check middleware covers route | | currentUser() returns null | Session expired or invalid | Refresh page; ensure middleware is running | | 401 from API route | Token missing or expired | Include Authorization: Bearer <token> header | | Hydration mismatch | Server/client state differs | Guard client components with isLoaded check | | auth() was called but Clerk can't detect clerkMiddleware() | Middleware not in project root | Move middleware.ts out of app/ to project root |

Enterprise Considerations

Use auth() over currentUser() in hot paths to avoid Backend API rate limits
Deduplicate currentUser() calls with React's cache() function across server components in the same request
For microservices, pass Clerk JWTs between services and verify with @clerk/backend or @clerk/express
Session token v2 (default since April 2025) uses a more compact format -- ensure downstream JWT consumers are updated
Custom session claims can be set in Dashboard > Sessions > Customize session token (limit: 1.2KB)

Resources

Next Steps

Proceed to clerk-local-dev-loop for local development workflow setup.

Agent Skills: Clerk Hello World

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