@forthtilliath/test-kit

Framework-agnostic test helpers: a fetch() mock response builder, a JSON fixture loader, an in-memory file system fake, and readers/writers for Zustand's persisted state — extracted from patterns duplicated across test files in this monorepo, none of them tied to Expo or any other framework (see @forthtilliath/expo-test-kit for the Expo/Drizzle-specific ones: a real SQLite test database and an expo-file-system fake).

Install

npm install --save-dev @forthtilliath/test-kit

Or, from within this monorepo, as a workspace dependency:

{
  "devDependencies": {
    "@forthtilliath/test-kit": "workspace:*"
  }
}

Usage

Import everything you need from the package root, or one module at a time:

import { createMockResponse, loadJsonFixture } from "@forthtilliath/test-kit";
// or
import { createMockResponse } from "@forthtilliath/test-kit/createMockResponse";

createMockResponse(body, options?)

Builds a real fetch() Response resolving body — full API (clone(), blob(), arrayBuffer()…), assignable wherever a Response is expected. A string body is sent as-is; anything else is JSON-serialized, with a content-type: application/json header unless you pass one. ok follows the real rule (2xx only) unless forced with { ok }; a 204/205/304 status has no body. Wrap it in your test runner's own mock function — this package has no opinion on Jest vs Vitest.

vi.stubGlobal(
  "fetch",
  vi.fn().mockResolvedValue(createMockResponse({ id: 1 })),
);
// an error case:
vi.fn().mockResolvedValue(createMockResponse({}, { status: 404 }));

loadJsonFixture(importMetaUrl, relativePath)

Reads and JSON-parses a fixture file relative to the calling module, resolved from import.meta.url — the ESM-safe replacement for the CommonJS __dirname trick.

const release = loadJsonFixture<GithubRelease>(
  import.meta.url,
  "./fixtures/github-release.json",
);

createInMemoryFileSystem(initialFiles?)

A minimal in-memory file system (exists, readFile, writeFile, deleteFile, reset, and the underlying files map for direct inspection) — enough to test code that reads/writes files by path (e.g. via node:fs/promises) without touching the real disk.

const fakeFs = createInMemoryFileSystem({ "/config.json": "{}" });
vi.mock("node:fs/promises", () => fakeFs);

readPersisted(storage, key) / writePersisted(storage, key, state, version)

Read and write the { state, version } envelope Zustand's persist middleware keeps under its storage key — to check what a store saves on the device, or to plant the data an earlier app version left there and test the rehydration (and its migrate). storage is anything with getItem / setItem, sync or async: React Native's AsyncStorage (its Jest mock), the web's localStorage… readPersisted throws if nothing is stored, or if the value isn't such an envelope.

import AsyncStorage from "@react-native-async-storage/async-storage";

await writePersisted(AsyncStorage, "settings", { dark: true }, 1);
// …rehydrate the store, then:
await expect(readPersisted(AsyncStorage, "settings")).resolves.toEqual({
  state: { theme: "dark" },
  version: 2,
});

Scripts

pnpm run dev            # tsc --watch -> dist/
pnpm run build          # tsc -> dist/
pnpm run check-types    # tsc --noEmit
pnpm run lint           # eslint
pnpm run test           # vitest run
pnpm run test:watch     # vitest

Built to dist/ (see the exports field in package.json), so run pnpm run build (or dev) after source changes for consumers to see them.