Cloudflare Workers Runtime APIs

Master the Workers runtime APIs: Fetch, Streams, Crypto, Cache, WebSockets, and text encoding.

Quick Reference

API	Purpose	Common Use
Fetch	HTTP requests	External APIs, proxying
Streams	Data streaming	Large files, real-time
Crypto	Cryptography	Hashing, signing, encryption
Cache	Response caching	Performance optimization
WebSockets	Real-time connections	Chat, live updates
Encoding	Text encoding	UTF-8, Base64

Quick Start: Fetch API

export default {
  async fetch(request: Request, env: Env): Promise<Response> {
    // Basic fetch
    const response = await fetch('https://api.example.com/data');

    // With options
    const postResponse = await fetch('https://api.example.com/users', {
      method: 'POST',
      headers: {
        'Content-Type': 'application/json',
        'Authorization': `Bearer ${env.API_KEY}`,
      },
      body: JSON.stringify({ name: 'John' }),
    });

    // Clone for multiple reads
    const clone = response.clone();
    const json = await response.json();
    const text = await clone.text();

    return Response.json(json);
  }
};

Critical Rules

1. Always set timeouts for external requests - Workers have a 30s limit, external APIs can hang
2. Clone responses before reading body - Body can only be read once
3. Use streaming for large payloads - Don't buffer entire response in memory
4. Cache external API responses - Reduce latency and API costs
5. Handle Crypto operations in try/catch - Invalid inputs throw errors
6. WebSocket hibernation for cost - Use Durable Objects with hibernation

Top 10 Errors Prevented

Error	Symptom	Prevention
Body already read	TypeError: Body has already been consumed	Clone response before reading
Fetch timeout	Request hangs, worker times out	Use AbortController with timeout
Invalid JSON	SyntaxError: Unexpected token	Check content-type before parsing
Stream locked	TypeError: ReadableStream is locked	Don't read stream multiple times
Crypto key error	DOMException: Invalid keyData	Validate key format and algorithm
Cache miss	Returns undefined instead of response	Check cache before returning
WebSocket close	Connection drops unexpectedly	Handle close event, implement reconnect
Encoding error	TypeError: Invalid code point	Use TextEncoder/TextDecoder properly
CORS blocked	Browser rejects response	Add proper CORS headers
Request size	413 Request Entity Too Large	Stream large uploads

Fetch API Patterns

With Timeout

async function fetchWithTimeout(url: string, timeout: number = 5000): Promise<Response> {
  const controller = new AbortController();
  const timeoutId = setTimeout(() => controller.abort(), timeout);

  try {
    const response = await fetch(url, { signal: controller.signal });
    return response;
  } finally {
    clearTimeout(timeoutId);
  }
}

With Retry

async function fetchWithRetry(
  url: string,
  options: RequestInit = {},
  retries: number = 3
): Promise<Response> {
  for (let i = 0; i < retries; i++) {
    try {
      const response = await fetch(url, options);
      if (response.ok) return response;

      // Retry on 5xx errors
      if (response.status >= 500 && i < retries - 1) {
        await new Promise(r => setTimeout(r, Math.pow(2, i) * 1000));
        continue;
      }

      return response;
    } catch (error) {
      if (i === retries - 1) throw error;
      await new Promise(r => setTimeout(r, Math.pow(2, i) * 1000));
    }
  }
  throw new Error('Max retries exceeded');
}

Streams API

Transform Stream

function createUppercaseStream(): TransformStream<string, string> {
  return new TransformStream({
    transform(chunk, controller) {
      controller.enqueue(chunk.toUpperCase());
    }
  });
}

// Usage
const response = await fetch('https://example.com/text');
const transformed = response.body!
  .pipeThrough(new TextDecoderStream())
  .pipeThrough(createUppercaseStream())
  .pipeThrough(new TextEncoderStream());

return new Response(transformed);

Stream Large Response

async function streamLargeFile(url: string): Promise<Response> {
  const response = await fetch(url);

  // Stream directly without buffering
  return new Response(response.body, {
    headers: {
      'Content-Type': response.headers.get('Content-Type') || 'application/octet-stream',
    },
  });
}

Crypto API

Hashing

async function sha256(data: string): Promise<string> {
  const encoder = new TextEncoder();
  const dataBuffer = encoder.encode(data);
  const hashBuffer = await crypto.subtle.digest('SHA-256', dataBuffer);
  const hashArray = Array.from(new Uint8Array(hashBuffer));
  return hashArray.map(b => b.toString(16).padStart(2, '0')).join('');
}

HMAC Signing

async function signHMAC(key: string, data: string): Promise<string> {
  const encoder = new TextEncoder();
  const keyData = encoder.encode(key);
  const dataBuffer = encoder.encode(data);

  const cryptoKey = await crypto.subtle.importKey(
    'raw',
    keyData,
    { name: 'HMAC', hash: 'SHA-256' },
    false,
    ['sign']
  );

  const signature = await crypto.subtle.sign('HMAC', cryptoKey, dataBuffer);
  return btoa(String.fromCharCode(...new Uint8Array(signature)));
}

Cache API

export default {
  async fetch(request: Request, env: Env, ctx: ExecutionContext): Promise<Response> {
    const cache = caches.default;
    const cacheKey = new Request(request.url, { method: 'GET' });

    // Check cache
    let response = await cache.match(cacheKey);
    if (response) {
      return response;
    }

    // Fetch and cache
    response = await fetch(request);
    response = new Response(response.body, response);
    response.headers.set('Cache-Control', 'public, max-age=3600');

    // Store in cache (don't await)
    ctx.waitUntil(cache.put(cacheKey, response.clone()));

    return response;
  }
};

WebSockets (Durable Objects)

// Durable Object with WebSocket hibernation
export class WebSocketRoom {
  state: DurableObjectState;

  constructor(state: DurableObjectState) {
    this.state = state;
  }

  async fetch(request: Request): Promise<Response> {
    const upgradeHeader = request.headers.get('Upgrade');
    if (upgradeHeader !== 'websocket') {
      return new Response('Expected websocket', { status: 426 });
    }

    const pair = new WebSocketPair();
    const [client, server] = Object.values(pair);

    // Accept with hibernation
    this.state.acceptWebSocket(server);

    return new Response(null, { status: 101, webSocket: client });
  }

  async webSocketMessage(ws: WebSocket, message: string | ArrayBuffer) {
    // Handle incoming message
    const data = typeof message === 'string' ? message : new TextDecoder().decode(message);

    // Broadcast to all connected clients
    for (const client of this.state.getWebSockets()) {
      client.send(data);
    }
  }

  async webSocketClose(ws: WebSocket, code: number, reason: string) {
    ws.close(code, reason);
  }
}

When to Load References

Load specific references based on the task:

• Making HTTP requests? → Load references/fetch-api.md for timeout, retry, proxy patterns
• Processing large data? → Load references/streams-api.md for TransformStream, chunking
• Encryption/signing? → Load references/crypto-api.md for AES, RSA, JWT verification
• Caching responses? → Load references/cache-api.md for Cache API patterns, TTL strategies
• Real-time features? → Load references/websockets.md for WebSocket patterns, Durable Objects
• Text encoding? → Load references/encoding-api.md for TextEncoder, Base64, Unicode

Templates

Template	Purpose	Use When
templates/fetch-patterns.ts	HTTP request utilities	Building API clients
templates/stream-processing.ts	Stream transformation	Processing large files
templates/crypto-operations.ts	Crypto utilities	Signing, hashing, encryption
templates/websocket-handler.ts	WebSocket DO	Real-time applications

Resources

• Runtime APIs: https://developers.cloudflare.com/workers/runtime-apis/
• Fetch API: https://developers.cloudflare.com/workers/runtime-apis/fetch/
• Streams API: https://developers.cloudflare.com/workers/runtime-apis/streams/
• Web Crypto: https://developers.cloudflare.com/workers/runtime-apis/web-crypto/
• Cache API: https://developers.cloudflare.com/workers/runtime-apis/cache/
• WebSockets: https://developers.cloudflare.com/workers/runtime-apis/websockets/