# Common Patterns

## Background Tasks (waitUntil)

Non-blocking tasks after response sent (analytics, cleanup, webhooks):

```typescript
export async function onRequest(ctx: EventContext<Env>) {
  const res = Response.json({ success: true })

  ctx.waitUntil(ctx.env.KV.put('last-visit', new Date().toISOString()))
  ctx.waitUntil(
    Promise.all([
      ctx.env.ANALYTICS.writeDataPoint({ event: 'view' }),
      fetch('https://webhook.site/...', { method: 'POST' }),
    ])
  )

  return res // Returned immediately
}
```

## Middleware & Auth

```typescript
// functions/_middleware.js (global) or functions/users/_middleware.js (scoped)
export async function onRequest(ctx) {
  try {
    return await ctx.next()
  } catch (err) {
    return new Response(err.message, { status: 500 })
  }
}

// Chained: export const onRequest = [errorHandler, auth, logger];

// Auth
async function auth(ctx: EventContext<Env>) {
  const token = ctx.request.headers.get('authorization')?.replace('Bearer ', '')
  if (!token) return new Response('Unauthorized', { status: 401 })
  const session = await ctx.env.KV.get(`session:${token}`)
  if (!session) return new Response('Invalid', { status: 401 })
  ctx.data.user = JSON.parse(session)
  return ctx.next()
}
```

## CORS & Rate Limiting

```typescript
// CORS middleware
const cors = { 'Access-Control-Allow-Origin': '*', 'Access-Control-Allow-Methods': 'GET, POST' }
export async function onRequestOptions() {
  return new Response(null, { headers: cors })
}
export async function onRequest(ctx) {
  const res = await ctx.next()
  Object.entries(cors).forEach(([k, v]) => res.headers.set(k, v))
  return res
}

// Rate limiting (KV-based)
async function rateLimit(ctx: EventContext<Env>) {
  const ip = ctx.request.headers.get('CF-Connecting-IP') || 'unknown'
  const count = parseInt((await ctx.env.KV.get(`rate:${ip}`)) || '0')
  if (count >= 100) return new Response('Rate limited', { status: 429 })
  await ctx.env.KV.put(`rate:${ip}`, (count + 1).toString(), { expirationTtl: 3600 })
  return ctx.next()
}
```

## Forms, Caching, Redirects

```typescript
// JSON & file upload
export async function onRequestPost(ctx) {
  const ct = ctx.request.headers.get('content-type') || ''
  if (ct.includes('application/json')) return Response.json(await ctx.request.json())
  if (ct.includes('multipart/form-data')) {
    const file = (await ctx.request.formData()).get('file') as File
    await ctx.env.BUCKET.put(file.name, file.stream())
    return Response.json({ uploaded: file.name })
  }
}

// Cache API
export async function onRequest(ctx) {
  let res = await caches.default.match(ctx.request)
  if (!res) {
    res = new Response('Data')
    res.headers.set('Cache-Control', 'public, max-age=3600')
    ctx.waitUntil(caches.default.put(ctx.request, res.clone()))
  }
  return res
}

// Redirects
export async function onRequest(ctx) {
  if (new URL(ctx.request.url).pathname === '/old') {
    return Response.redirect(new URL('/new', ctx.request.url), 301)
  }
  return ctx.next()
}
```

## Testing

**Unit tests** (Vitest + cloudflare:test):

```typescript
import { env } from 'cloudflare:test'
import { it, expect } from 'vitest'
import { onRequest } from '../functions/api'

it('returns JSON', async () => {
  const req = new Request('http://localhost/api')
  const ctx = { request: req, env, params: {}, data: {} } as EventContext
  const res = await onRequest(ctx)
  expect(res.status).toBe(200)
})
```

**Integration:** `wrangler pages dev` + Playwright/Cypress

## Advanced Mode (\_worker.js)

Use `_worker.js` for complex routing (replaces `/functions`):

```typescript
interface Env {
  ASSETS: Fetcher
  KV: KVNamespace
}

export default {
  async fetch(request: Request, env: Env): Promise<Response> {
    const url = new URL(request.url)
    if (url.pathname.startsWith('/api/')) {
      return Response.json({ data: await env.KV.get('key') })
    }
    return env.ASSETS.fetch(request) // Static files
  },
} satisfies ExportedHandler<Env>
```

**When:** Existing Worker, framework-generated (Next.js/SvelteKit), custom routing logic

**See also:** [api.md](./api.md) for `env.ASSETS.fetch()` | [gotchas.md](./gotchas.md) for debugging
