This skill should be used when the user asks about "Workers API", "fetch handler", "Workers runtime", "request handling", "response handling", "Workers bindings", "environment variables in Workers", "Workers context", or discusses implementing Workers code, routing patterns, or using Cloudflare bindings like KV, D1, R2, Durable Objects in Workers.
This skill provides comprehensive guidance for developing Cloudflare Workers, including runtime APIs, fetch event handlers, request/response handling, bindings usage, and common development patterns. Use this skill when implementing Workers code, designing Workers architecture, or working with the Workers runtime environment.
Cloudflare Workers run on the V8 JavaScript engine at the edge. Workers use the Service Worker API with extensions specific to the Cloudflare platform.
Workers execute on incoming requests:
1. Request arrives at Cloudflare edge
2. Worker isolate spawns (or reuses existing)
3. Fetch event handler executes
4. Response returns to client
5. Isolate may persist for subsequent requests
The fetch event handler is the entry point for Workers:
```javascript
export default {
async fetch(request, env, ctx) {
// request: Request object
// env: Bindings and environment variables
// ctx: Execution context with waitUntil() and passThroughOnException()
return new Response('Hello World');
}
};
```
**request**: Incoming `Request` object (Web API standard)
**env**: Environment object containing bindings
**ctx**: Execution context
Must return a `Response` object or a Promise that resolves to a Response:
```javascript
// Direct return
return new Response('Hello', { status: 200 });
// Async return
return await fetch('https://api.example.com');
// With headers
return new Response(JSON.stringify({ ok: true }), {
status: 200,
headers: {
'Content-Type': 'application/json'
}
});
```
Common routing patterns for multi-route Workers:
```javascript
export default {
async fetch(request, env, ctx) {
const url = new URL(request.url);
// Path-based routing
if (url.pathname === '/api/users') {
return handleUsers(request, env);
}
if (url.pathname.startsWith('/api/')) {
return handleAPI(request, env);
}
// Method-based routing
if (request.method === 'POST') {
return handlePost(request, env);
}
// Default
return new Response('Not Found', { status: 404 });
}
};
```
See `examples/fetch-handler-patterns.js` for complete routing examples.
Access request properties:
```javascript
const url = new URL(request.url);
const method = request.method;
const headers = request.headers.get('Authorization');
const cookies = request.headers.get('Cookie');
// Cloudflare-specific properties
const country = request.cf?.country;
const colo = request.cf?.colo; // Data center code
```
Parse request bodies based on content type:
```javascript
// JSON
const data = await request.json();
// Form data
const formData = await request.formData();
const field = formData.get('fieldName');
// Text
const text = await request.text();
// Array buffer
const buffer = await request.arrayBuffer();
```
```javascript
// Text response
return new Response('Hello World');
// JSON response
return new Response(JSON.stringify({ message: 'Success' }), {
headers: { 'Content-Type': 'application/json' }
});
// HTML response
return new Response('<h1>Hello</h1>', {
headers: { 'Content-Type': 'text/html' }
});
// Status codes
return new Response('Not Found', { status: 404 });
return new Response('Created', { status: 201 });
```
```javascript
// Set headers
const headers = new Headers({
'Content-Type': 'application/json',
'Cache-Control': 'max-age=3600',
'X-Custom-Header': 'value'
});
return new Response(body, { headers });
// CORS headers
const corsHeaders = {
'Access-Control-Allow-Origin': '*',
'Access-Control-Allow-Methods': 'GET, POST, OPTIONS',
'Access-Control-Allow-Headers': 'Content-Type'
};
```
```javascript
// 301 permanent redirect
return Response.redirect('https://example.com', 301);
// 302 temporary redirect
return Response.redirect('https://example.com', 302);
```
Bindings provide access to Cloudflare resources and are configured in `wrangler.toml` or `wrangler.jsonc`.
See `references/bindings-guide.md` for complete binding configuration and usage patterns.
**KV (Key-Value):**
```javascript
// Read
const value = await env.MY_KV.get('key');
const json = await env.MY_KV.get('key', 'json');
// Write
await env.MY_KV.put('key', 'value');
await env.MY_KV.put('key', JSON.stringify(data), {
expirationTtl: 3600 // Expire in 1 hour
});
```
**D1 (Database):**
```javascript
// Query
const result = await env.MY_DB.prepare(
'SELECT * FROM users WHERE id = ?'
).bind(userId).first();
// Insert
await env.MY_DB.prepare(
'INSERT INTO users (name, email) VALUES (?, ?)'
).bind(name, email).run();
```
**R2 (Object Storage):**
```javascript
// Read
const object = await env.MY_BUCKET.get('file.txt');
const text = await object.text();
// Write
await env.MY_BUCKET.put('file.txt', 'content');
```
```javascript
export default {
async fetch(request, env, ctx) {
try {
// Your logic here
const result = await processRequest(request, env);
return new Response(JSON.stringify(result), {
headers: { 'Content-Type': 'application/json' }
});
} catch (error) {
console.error('Error:', error);
return new Response(JSON.stringify({
error: error.message
}), {
status: 500,
headers: { 'Content-Type': 'application/json' }
});
}
}
};
```
```javascript
function errorResponse(message, status = 500) {
return new Response(JSON.stringify({ error: message }), {
status,
headers: { 'Content-Type': 'application/json' }
});
}
// Usage
if (!apiKey) {
return errorResponse('API key required', 401);
}
```
See `examples/error-handling.js` for comprehensive error handling patterns.
Use `ctx.waitUntil()` to perform background tasks that extend beyond the response:
```javascript
export default {
async fetch(request, env, ctx) {
// Respond immediately
const response = new Response('Request received');
// Continue processing in background
ctx.waitUntil(
logRequest(request, env)
);
return response;
}
};
async function logRequest(request, env) {
await env.MY_DB.prepare(
'INSERT INTO logs (url, timestamp) VALUES (?, ?)'
).bind(request.url, Date.now()).run();
}
```
**Important**: `waitUntil` extends execution but doesn't guarantee completion. Use for non-critical tasks like logging, analytics, or cache warming.
Workers support standard Web APIs and Cloudflare-specific extensions.
See `references/runtime-apis.md` for complete API documentation and examples.
Workers fully support TypeScript with official type definitions:
```typescript
export interface Env {
MY_KV: KVNamespace;
MY_DB: D1Database;
MY_BUCKET: R2Bucket;
MY_SECRET: string;
}
export default {
async fetch(
request: Request,
env: Env,
ctx: ExecutionContext
): Promise<Response> {
// Type-safe access to bindings
const value = await env.MY_KV.get('key');
return new Response(value);
}
};
```
Install types: `npm install -D @cloudflare/workers-types`
For detailed information, consult:
Working examples in `examples/`:
For the latest documentation:
Use the cloudflare-docs-specialist agent to search documentation and fetch the latest information.
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