Miniflare - Cloudflare Workers SDK

Miniflare is a fully-featured, local simulator for developing and testing Cloudflare Workers. Built on top of workerd (the same runtime that powers Cloudflare Workers in production), Miniflare provides high-fidelity local development.

Current Version: 4.20260305.0Miniflare requires Node.js 18.0.0 or higher. It’s designed as a lower-level API for tool creators. For most development workflows, use Wrangler or the Cloudflare Vite plugin.

Installation

Install Miniflare as a development dependency:

npm install miniflare --save-dev

Quick Start

Create a simple Worker instance:

import { Miniflare } from "miniflare";

// Create a new Miniflare instance, starting a workerd server
const mf = new Miniflare({
  script: `
    addEventListener("fetch", (event) => {
      event.respondWith(new Response("Hello Miniflare!"));
    })
  `,
});

// Send a request to the workerd server
const response = await mf.dispatchFetch("http://localhost:8787/");
console.log(await response.text()); // "Hello Miniflare!"

// Cleanup Miniflare, shutting down the workerd server
await mf.dispose();

Core Features

workerd Runtime

Uses the same runtime as production Workers for accurate simulation

Full API Support

Supports KV, R2, D1, Durable Objects, Queues, and more

DevTools Integration

Debug with Chrome DevTools and sourcemap support

Flexible Persistence

Store data in-memory or persist to disk

Basic Configuration

Module Workers

import { Miniflare } from "miniflare";

const mf = new Miniflare({
  modules: true,
  scriptPath: "./src/index.ts",
  compatibilityDate: "2024-03-01",
});

Service Worker Format

const mf = new Miniflare({
  script: `
    addEventListener("fetch", (event) => {
      event.respondWith(handleRequest(event.request));
    });
    
    async function handleRequest(request) {
      return new Response("Hello World!");
    }
  `,
});

With TypeScript

import { Miniflare } from "miniflare";

const mf = new Miniflare({
  modules: true,
  scriptPath: "./src/index.ts",
  modulesRules: [
    { type: "ESModule", include: ["**/*.ts"], fallthrough: false },
  ],
});

Bindings

KV Namespaces

const mf = new Miniflare({
  modules: true,
  script: `
    export default {
      async fetch(request, env) {
        await env.MY_KV.put("key", "value");
        const value = await env.MY_KV.get("key");
        return new Response(value);
      }
    }
  `,
  kvNamespaces: ["MY_KV"],
});

// Access KV namespace from Node.js
const kv = await mf.getKVNamespace("MY_KV");
await kv.put("external-key", "external-value");

R2 Buckets

const mf = new Miniflare({
  modules: true,
  script: `
    export default {
      async fetch(request, env) {
        await env.MY_BUCKET.put("file.txt", "Hello R2!");
        const object = await env.MY_BUCKET.get("file.txt");
        return new Response(await object.text());
      }
    }
  `,
  r2Buckets: ["MY_BUCKET"],
});

// Access R2 bucket from Node.js
const bucket = await mf.getR2Bucket("MY_BUCKET");
await bucket.put("file.txt", "Content from Node.js");

D1 Databases

const mf = new Miniflare({
  modules: true,
  script: `
    export default {
      async fetch(request, env) {
        const result = await env.DB.prepare(
          "SELECT * FROM users WHERE id = ?"
        ).bind(1).first();
        return Response.json(result);
      }
    }
  `,
  d1Databases: ["DB"],
});

// Access D1 database from Node.js
const db = await mf.getD1Database("DB");
await db.exec(`
  CREATE TABLE users (id INTEGER PRIMARY KEY, name TEXT);
  INSERT INTO users (id, name) VALUES (1, 'Alice');
`);

Durable Objects

const mf = new Miniflare({
  modules: true,
  script: `
    export class Counter {
      constructor(state, env) {
        this.state = state;
      }
      
      async fetch(request) {
        let count = (await this.state.storage.get("count")) || 0;
        count++;
        await this.state.storage.put("count", count);
        return new Response(count.toString());
      }
    }
    
    export default {
      async fetch(request, env) {
        const id = env.COUNTER.idFromName("global");
        const stub = env.COUNTER.get(id);
        return stub.fetch(request);
      }
    }
  `,
  durableObjects: {
    COUNTER: "Counter",
  },
});

// Access Durable Object from Node.js
const ns = await mf.getDurableObjectNamespace("COUNTER");
const id = ns.idFromName("global");
const stub = ns.get(id);
const response = await stub.fetch("http://fake-host/");

Environment Variables

const mf = new Miniflare({
  modules: true,
  script: `
    export default {
      async fetch(request, env) {
        return new Response(env.API_URL);
      }
    }
  `,
  bindings: {
    API_URL: "https://api.example.com",
    DEBUG: true,
  },
});

Queues

const mf = new Miniflare({
  modules: true,
  script: `
    export default {
      async fetch(request, env) {
        await env.MY_QUEUE.send({ message: "Hello Queue!" });
        return new Response("Message sent");
      },
      async queue(batch, env) {
        for (const message of batch.messages) {
          console.log("Received:", message.body);
        }
      }
    }
  `,
  queueProducers: ["MY_QUEUE"],
  queueConsumers: {
    MY_QUEUE: { maxBatchSize: 10, maxBatchTimeout: 5 },
  },
});

Service Bindings

Worker-to-Worker Communication

const mf = new Miniflare({
  workers: [
    {
      name: "main",
      modules: true,
      script: `
        export default {
          async fetch(request, env) {
            const response = await env.API.fetch(request);
            return response;
          }
        }
      `,
      serviceBindings: {
        API: "api-worker",
      },
    },
    {
      name: "api-worker",
      modules: true,
      script: `
        export default {
          async fetch(request) {
            return new Response("API Response");
          }
        }
      `,
    },
  ],
});

Custom Fetch Handler

const mf = new Miniflare({
  modules: true,
  script: `
    export default {
      async fetch(request, env) {
        return env.CUSTOM_SERVICE.fetch(request);
      }
    }
  `,
  serviceBindings: {
    CUSTOM_SERVICE: async (request, miniflare) => {
      // Custom logic in Node.js
      return new Response("Custom response from Node.js");
    },
  },
});

External HTTP Service

const mf = new Miniflare({
  modules: true,
  script: `
    export default {
      async fetch(request, env) {
        return env.EXTERNAL_API.fetch("https://api.external.com/data");
      }
    }
  `,
  serviceBindings: {
    EXTERNAL_API: {
      external: {
        address: "api.external.com:443",
        https: { certificateHost: "api.external.com" },
      },
    },
  },
});

Persistence

Miniflare supports flexible persistence options for data storage.

In-Memory Storage (Default)

const mf = new Miniflare({
  kvPersist: false,
  r2Persist: false,
  d1Persist: false,
  durableObjectsPersist: false,
});

File-System Persistence

const mf = new Miniflare({
  // Persist to default location (.mf in current directory)
  kvPersist: true,
  r2Persist: true,
  d1Persist: true,
  
  // Or specify custom paths
  // kvPersist: "./data/kv",
  // r2Persist: "./data/r2",
  // d1Persist: "./data/d1",
});

Using defaultPersistRoot

const mf = new Miniflare({
  defaultPersistRoot: "./storage",
  kvPersist: true,      // → ./storage/kv
  d1Persist: true,      // → ./storage/d1
  r2Persist: false,     // → in-memory
});

HTTP Server

Custom Host and Port

const mf = new Miniflare({
  host: "0.0.0.0",
  port: 3000,
  script: `
    export default {
      async fetch(request) {
        return new Response("Server running on custom port!");
      }
    }
  `,
  modules: true,
});

const url = await mf.ready;
console.log(`Server running at ${url}`);

HTTPS Server

import { Miniflare } from "miniflare";
import { readFileSync } from "fs";

const mf = new Miniflare({
  httpsKeyPath: "./key.pem",
  httpsCertPath: "./cert.pem",
  // Or provide as strings:
  // httpsKey: readFileSync("./key.pem", "utf8"),
  // httpsCert: readFileSync("./cert.pem", "utf8"),
  modules: true,
  script: `
    export default {
      async fetch(request) {
        return new Response("HTTPS enabled!");
      }
    }
  `,
});

Generate a self-signed certificate for local development:

openssl req -newkey rsa:2048 -nodes -keyout key.pem -x509 -days 365 -out cert.pem

Debugging

Chrome DevTools

const mf = new Miniflare({
  inspectorPort: 9229,
  modules: true,
  scriptPath: "./src/index.ts",
});

Then:

Open chrome://inspect in Chrome
Click “inspect” next to your Worker
Use breakpoints, console, and profiler

Verbose Logging

import { Miniflare, Log, LogLevel } from "miniflare";

const log = new Log(LogLevel.VERBOSE);

const mf = new Miniflare({
  log,
  verbose: true, // Enable workerd verbose logging
  modules: true,
  script: `
    export default {
      async fetch(request) {
        console.log("Request received");
        return new Response("OK");
      }
    }
  `,
});

Custom Logger

import { Miniflare, Log, LogLevel } from "miniflare";

class CustomLog extends Log {
  log(message) {
    // Custom logging logic
    console.log(`[CUSTOM] ${message}`);
  }
}

const mf = new Miniflare({
  log: new CustomLog(LogLevel.INFO),
  // ... other options
});

Advanced Features

Live Reload

const mf = new Miniflare({
  liveReload: true,
  modules: true,
  scriptPath: "./src/index.ts",
});

// Update configuration without restarting
await mf.setOptions({
  modules: true,
  scriptPath: "./src/index.ts",
  bindings: { NEW_VAR: "updated" },
});

Changes made via setOptions() will trigger a page reload when liveReload: true is enabled.

Workers Sites / Static Assets

const mf = new Miniflare({
  modules: true,
  script: `
    import manifest from "__STATIC_CONTENT_MANIFEST";
    
    export default {
      async fetch(request, env) {
        // Serve static files from sitePath
        return env.__STATIC_CONTENT.fetch(request);
      }
    }
  `,
  sitePath: "./public",
  siteInclude: ["**/*.html", "**/*.css", "**/*.js"],
  siteExclude: ["**/*.map"],
});

Wrapped Bindings

Create custom bindings with JavaScript:

const mf = new Miniflare({
  workers: [
    {
      modules: true,
      script: `
        export default {
          async fetch(request, env) {
            const value = await env.CUSTOM.get("key");
            return new Response(value);
          }
        }
      `,
      wrappedBindings: {
        CUSTOM: {
          scriptName: "custom-binding",
          bindings: { PREFIX: "custom:" },
        },
      },
    },
    {
      name: "custom-binding",
      modules: true,
      script: `
        class CustomBinding {
          constructor(env) {
            this.prefix = env.PREFIX;
          }
          
          async get(key) {
            return this.prefix + key;
          }
        }
        
        export default function(env) {
          return new CustomBinding(env);
        }
      `,
    },
  ],
});

Outbound Service

Intercept all outbound fetch() requests:

const mf = new Miniflare({
  modules: true,
  script: `
    export default {
      async fetch(request) {
        // This fetch will go through outboundService
        const response = await fetch("https://api.example.com/data");
        return response;
      }
    }
  `,
  outboundService: async (request) => {
    console.log("Outbound request to:", request.url);
    // Mock the response
    return new Response(JSON.stringify({ mocked: true }), {
      headers: { "Content-Type": "application/json" },
    });
  },
});

Multiple Workers

const mf = new Miniflare({
  workers: [
    {
      name: "frontend",
      routes: ["example.com/*"],
      modules: true,
      script: `
        export default {
          async fetch(request, env) {
            return env.BACKEND.fetch(request);
          }
        }
      `,
      serviceBindings: {
        BACKEND: "backend",
      },
    },
    {
      name: "backend",
      modules: true,
      script: `
        export default {
          async fetch(request) {
            return new Response("Backend response");
          }
        }
      `,
    },
  ],
});

Testing

Miniflare is perfect for testing Workers:

import { Miniflare } from "miniflare";
import { describe, it, expect, beforeAll, afterAll } from "vitest";

describe("Worker Tests", () => {
  let mf: Miniflare;

  beforeAll(() => {
    mf = new Miniflare({
      modules: true,
      scriptPath: "./src/index.ts",
      kvNamespaces: ["TEST_KV"],
    });
  });

  afterAll(() => mf.dispose());

  it("should return 200", async () => {
    const response = await mf.dispatchFetch("http://localhost/");
    expect(response.status).toBe(200);
  });

  it("should store in KV", async () => {
    const kv = await mf.getKVNamespace("TEST_KV");
    await kv.put("test-key", "test-value");
    
    const value = await kv.get("test-key");
    expect(value).toBe("test-value");
  });
});

API Reference

Miniflare Constructor

const mf = new Miniflare(options: MiniflareOptions);

Core Methods

// Wait for server to be ready
const url: URL = await mf.ready;

// Dispatch a fetch event
const response: Response = await mf.dispatchFetch(input, init?);

// Update configuration
await mf.setOptions(options: MiniflareOptions);

// Get bindings for a worker
const bindings = await mf.getBindings<Env>(workerName?);

// Get a worker's fetch handler
const worker: Fetcher = await mf.getWorker(workerName?);

// Get storage instances
const kv = await mf.getKVNamespace(bindingName, workerName?);
const bucket = await mf.getR2Bucket(bindingName, workerName?);
const db = await mf.getD1Database(bindingName, workerName?);
const namespace = await mf.getDurableObjectNamespace(bindingName, workerName?);
const queue = await mf.getQueueProducer(bindingName, workerName?);
const caches = await mf.getCaches();

// Get cf object
const cf = await mf.getCf();

// Cleanup
await mf.dispose();

Best Practices

Always Dispose

Always call dispose() when you’re done to clean up resources:

const mf = new Miniflare({ ... });
try {
  // Your code
} finally {
  await mf.dispose();
}

Use Persistence for Development

Persist data during development to avoid losing state:

const mf = new Miniflare({
  defaultPersistRoot: "./.mf",
  kvPersist: true,
  d1Persist: true,
});

Separate Test Data

Use in-memory storage for tests to avoid side effects:

const mf = new Miniflare({
  kvPersist: false,
  d1Persist: false,
});

Match Production Configuration

Use the same compatibilityDate and flags as production:

const mf = new Miniflare({
  compatibilityDate: "2024-03-01",
  compatibilityFlags: ["nodejs_compat"],
});

Environment Variables

MINIFLARE_WORKERD_PATH

Use a custom workerd binary:

export MINIFLARE_WORKERD_PATH="/path/to/workerd"

MINIFLARE_WORKERD_CONFIG_DEBUG

Dump the workerd configuration for debugging:

export MINIFLARE_WORKERD_CONFIG_DEBUG="./workerd-config.json"

Comparison with Wrangler Dev

Feature	Miniflare	Wrangler Dev
Use Case	Programmatic API, testing	CLI-based development
API	JavaScript/TypeScript API	Command-line interface
Configuration	JavaScript objects	wrangler.jsonc file
Integration	Embed in tools	Standalone tool
Runtime	workerd	workerd

For most development workflows, use Wrangler Dev. Use Miniflare when you need programmatic control or are building tooling.

Get Started

Core Tools

Development

Deployment

Platform Features

Advanced

​Installation

​Quick Start

​Core Features

workerd Runtime

Full API Support

DevTools Integration

Flexible Persistence

​Basic Configuration

​Module Workers

​Service Worker Format

​With TypeScript

​Bindings

​KV Namespaces

​R2 Buckets

​D1 Databases

​Durable Objects

​Environment Variables

​Queues

​Service Bindings

​Worker-to-Worker Communication

​Custom Fetch Handler

​External HTTP Service

​Persistence

​In-Memory Storage (Default)

​File-System Persistence

​Using defaultPersistRoot

​HTTP Server

​Custom Host and Port

​HTTPS Server

​Debugging

​Chrome DevTools

​Verbose Logging

​Custom Logger

​Advanced Features

​Live Reload

​Workers Sites / Static Assets

​Wrapped Bindings

​Outbound Service

​Multiple Workers

​Testing

​API Reference

​Miniflare Constructor

​Core Methods

​Best Practices

​Environment Variables

​MINIFLARE_WORKERD_PATH

​MINIFLARE_WORKERD_CONFIG_DEBUG

​Comparison with Wrangler Dev

​Additional Resources

​Related Tools

Wrangler

Vitest Pool Workers

Build docs developers (and LLMs) love

Installation

Quick Start

Core Features

Basic Configuration

Module Workers

Service Worker Format

With TypeScript

Bindings

KV Namespaces

R2 Buckets

D1 Databases

Durable Objects

Environment Variables

Queues

Service Bindings

Worker-to-Worker Communication

Custom Fetch Handler

External HTTP Service

Persistence

In-Memory Storage (Default)

File-System Persistence

Using defaultPersistRoot

HTTP Server

Custom Host and Port

HTTPS Server

Debugging

Chrome DevTools

Verbose Logging

Custom Logger

Advanced Features

Live Reload

Workers Sites / Static Assets

Wrapped Bindings

Outbound Service

Multiple Workers

Testing

API Reference

Miniflare Constructor

Core Methods

Best Practices

Environment Variables

MINIFLARE_WORKERD_PATH

MINIFLARE_WORKERD_CONFIG_DEBUG

Comparison with Wrangler Dev

Additional Resources

Related Tools