🤹 Jest Environment

Miniflare includes a custom Jest environment that allows you to run your unit tests within the Miniflare sandbox. Note that Jest 27 is required. See this repositoryOpen external link for an example using TypeScript.

​Setup

The Miniflare environment isn't installed by default, install it and Jest with:

$ npm install -D jest-environment-miniflare jest

In the following examples, we'll assume your package.json contains "type": "module", and that you're using a tool to bundle your worker. See ⚡️ Developing with esbuild for an example.

To enable the Miniflare environment, set the testEnvironment optionOpen external link in your Jest configuration:

jest.config.js
export default {
  testEnvironment: "miniflare",
  // Configuration is automatically loaded from `.env`, `package.json` and
  // `wrangler.toml` files by default, but you can pass any additional Miniflare
  // API options here:
  testEnvironmentOptions: {
    bindings: { KEY: "value" },
    kvNamespaces: ["TEST_NAMESPACE"],
  },
};

​Writing and Running Tests

The Miniflare environment lets us import our worker's functions with regular import syntax. We can write a test for the following worker like so:

src/index.js
addEventListener("fetch", (event) => {
  event.respondWith(handleRequest(event.request));
});

// Assuming you've got a build tool that removes `export`s when you actually
// deploy your worker (e.g. https://esbuild.github.io/api/#format-iife)
export async function handleRequest(request) {
  return new Response(`URL: ${request.url} KEY: ${KEY}`);
}

test/index.spec.js
import { handleRequest } from "../src/index.js";

test("responds with url", async () => {
  const req = new Request("http://localhost/");
  const res = await handleRequest(req);
  expect(await res.text()).toBe("URL: http://localhost/ KEY: value");
});

Modules support is still experimental in Jest and requires the --experimental-vm-modules flag. To run this test:

$ NODE_OPTIONS=--experimental-vm-modules npx jest

​Isolated Storage

The Miniflare environment will use isolated storage for KV namespaces, caches, and Durable Objects in each test. This essentially means any changes you make in a test or describe-block are automatically undone afterwards. The isolated storage is copied from the parent describe-block, allowing you to seed data in beforeAll hooks.

As an example, consider the following tests:

// Gets the array
async function get() {
  const jsonValue = await TEST_NAMESPACE.get("array");
  return JSON.parse(jsonValue ?? "[]");
}

// Pushes an item onto the end of the array
async function push(item) {
  const value = await get();
  value.push(item);
  await TEST_NAMESPACE.put("array", JSON.stringify(value));
}

beforeAll(async () => {
  await push("beforeAll");
});

beforeEach(async () => {
  // This runs in each tests' isolated storage environment
  await push("beforeEach");
});

test("test 1", async () => {
  // This push(1) will only mutate the isolated environment
  await push(1);
  expect(await get()).toEqual(["beforeAll", "beforeEach", 1]);
});

test("test 2", async () => {
  await push(2);
  // Note that push(1) from the previous test has been "undone"
  expect(await get()).toEqual(["beforeAll", "beforeEach", 2]);
});

describe("describe", () => {
  beforeAll(async () => {
    await push("describe: beforeAll");
  });

  beforeEach(async () => {
    await push("describe: beforeEach");
  });

  test("test 3", async () => {
    await push(3);
    expect(await get()).toEqual([
      // All beforeAll's run before beforeEach's
      "beforeAll",
      "describe: beforeAll",
      "beforeEach",
      "describe: beforeEach",
      3,
    ]);
  });

  test("test 4", async () => {
    await push(4);
    expect(await get()).toEqual([
      "beforeAll",
      "describe: beforeAll",
      "beforeEach",
      "describe: beforeEach",
      4,
    ]);
  });
});

Note that bindings (e.g. variables, KV namespaces, etc) are only included in the global scope when you're using a service-worker format worker. In modules mode, you can use the getMiniflareBindings global method:

const { TEST_NAMESPACE } = getMiniflareBindings();

Note also that storage persistence options (kvPersist, cachePersist, and durableObjectsPersist) are ignored by the Miniflare Jest environment.

​Durable Objects

When testing Durable Objects, Miniflare needs to run your script itself to extract exported Durable Object classes. Miniflare should be able to auto-detect your script from your package.json or wrangler.toml file, but you can also set it manually in Jest configuration:

src/index.mjs
export class TestObject {
  constructor(state) {
    this.storage = state.storage;
  }

  async fetch() {
    const count = (await this.storage.get("count")) + 1;
    this.storage.put("count", count);
    return new Response(count.toString());
  }
}

jest.config.js
export default {
  testEnvironment: "miniflare",
  testEnvironmentOptions: {
    modules: true,
    scriptPath: "./src/index.mjs",
    durableObjects: {
      TEST_OBJECT: "TestObject",
    },
  },
};

To access Durable Object storage in tests, use the getMiniflareDurableObjectStorage global method:

test("increments count", async () => {
  // Durable Objects requires modules mode so bindings aren't accessible via the
  // global scope
  const { TEST_OBJECT } = getMiniflareBindings();
  const id = TEST_OBJECT.newUniqueId();

  // Seed Durable Object storage (isolated storage rules from above also apply)
  const storage = await getMiniflareDurableObjectStorage(id);
  await storage.put("count", 3);

  // Increment the count
  const stub = TEST_OBJECT.get(id);
  const res = await stub.fetch("http://localhost/");
  expect(await res.text()).toBe("4");

  // Check storage updated
  expect(await storage.get("count")).toBe(4);
});