fetch-mock-cache

Caching mock fetch implementation for all runtimes and frameworks.

Copyright (c) 2023 by Gadi Cohen. MIT Licensed.

Introduction

Instead of individually handcrafting a mock for each and every fetch() call in your code, maybe you'd like to perform a real fetch() once, cache the result, and use that cache result for future calls. Super useful for TDD against existing APIs!! ("Test Driven Development").

Not only do you get a muuuch faster feedback loop, but you can commit the cache to your repository / source control so your entire team benefits, and for CI. Based off earlier work in yahoo-finance2 where we test 1400+ API "calls" in < 3s on every commit. Feature requests welcome!

NB: if you're still using v1 (formerly jest-fetch-mock-cache), see the old README and/or MIGRATING.md for how to upgrade.

Quick Start

Generally your code will look something like this, but, see further below for the exact code for different runtimes and testing frameworks.

import createFetchCache from "fetch-mock-cache";
// See list of possible stores, below.
import Store from "fetch-mock-cache/lib/stores/fs";

const fetchCache = createFetchCache({ Store });

describe("cachingMock", () => {
  it("works with a JSON response", async (t) => {
    const url = "http://echo.jsontest.com/key/value/one/two";
    const expectedResponse = { one: "two", key: "value" };
    t.mock.method(globalThis, "fetch", fetchCache);

    for (let i = 0; i < 2; i++) {
      const response = await fetch(url);
      const data = await response.json();
      const expectedCacheHeader = i === 0 ? "MISS" : "HIT";
      expect(response.headers.get("X-FMC-Cache")).toBe(expectedCacheHeader);
      expect(data).toEqual(expectedResponse);
    }
  });
});

• The first time this runs, a real request will be made to jsontest.com, and the result returned. But, it will also be saved to cache.

• Subsequent requests will return the cached copy without making an HTTP request.

• Commit tests/fixtures/http (default) to your repo for super fast tests in the future for all contributors and CI.

Supported runtimes and test frameworks

Click on the "Quick Start / Example" links to see a working implementation for your framework of choice.

Runtime	Framework	Status	Quick Start / Example
Node 20+	node:test		direct or with fetch-mock
	jest		direct or with jest-fetch-mock
	vitest		direct or with vitest-fetch-mock
Deno	deno test		direct
Bun	bun:test		direct, maybe soon bun-bagel

What's cached

Sample output from the Quick Start code above, when used with NodeFSStore:

$ cat tests/fixtures/http/echo.jsontest.com\!key\!value\!one\!two

{
  "request": {
    "url": "http://echo.jsontest.com/key/value/one/two"
  },
  "response": {
    "ok": true,
    "status": 200,
    "statusText": "OK",
    "headers": {
      "access-control-allow-origin": "*",
      "connection": "close",
      "content-length": "39",
      "content-type": "application/json",
      "date": "Fri, 21 Jul 2023 16:59:17 GMT",
      "server": "Google Frontend",
      "x-cloud-trace-context": "344994371e51195ae21f236e5d7650c4"
    },
    "bodyJson": {
      "one": "two",
      "key": "value"
    }
  }
}

For non-JSON bodies, a bodyText is stored as a string. We store an object as bodyJson for readability reasons.

Debugging

We use debug for debugging. E.g.:

$ DEBUG=fetch-mock-cache:* yarn test
yarn run v1.22.19
$ jest
  fetch-mock-cache:core Fetching and caching 'http://echo.jsontest.com/key/value/one/two' +0ms
  fetch-mock-cache:core Using cached copy of 'http://echo.jsontest.com/key/value/one/two' +177ms
 PASS  src/index.spec.ts
  cachingMock
    ✓ should work (180 ms)

Available Stores

• stores/fs - use your runtime's FileSystem API to store cached requests to the filesystem, for persistance. These can be committed to your projects repository / source control for faster future testing, including for CI.

• stores/memory - keep the cache in memory. The cache will not persist and will be created again from scratch each time you run your code.

Create your own Store

See also the store "root" class. Don't instantiate directly; rather extend this class overriding at least fetchContent and storeContent, and perhaps, idFromRequest, the constructor and others according to your needs. Here's an example to combine with a database:

import FMCStore from "fetch-mock-cache/store";
import type { FMCCacheContent, FMCStoreOptions } from "fetch-mock-cache/store";
import db from "./db"; // your existing db

export default class MyStore extends FMCStore {
  async fetchContent(req: FMCCacheContent["request"]) {
    const _id = await this.idFromRequest(request);
    return (await db.collection("fmc").findOne({ _id })).content;
  }
  async storeContent(content: FMCCacheContent) {
    const _id = await this.idFromRequest(content.request);
    await db.collection("jfmc").insertOne({ _id, content });
  }
}

Internal and Experimental Features

Internal and experimental features are generally prefixed by an underscore ("_"). You're welcome to use them, however, they are not part of our API contract - as such, they may change or disappear at any time, without following semantic versioning.

Often these are used for new ideas that are still in development, where, we'd like you to have easy access to them (and appreciate your feedback!), but, they're not (yet) considered stable.

Current experiments:

Passing options to be used for the next fetch call

// These will be used for the next fetch call ONCE only.  However, `_once()`
// may be called multiple times to queue options for multiple future calls.
fetchCache._once({
  /* options */
});

Manually specifying an ID

Generally we don't need to think about cache IDs, as we can reliably generate them from the Request object (e.g. based on URL and hashes of the headers, body, etc.).

But sometimes, we may want to specify this manually, e.g.

1. We'd rather use the test name as the id, vs something URL-based.
2. It can't be reliably generated, e.g. formData with a random boundary.

In this case, we can:

fetchCache._once({ id: "mytest" });
fetch(/* ... */); // or code that uses fetch();

Make sure the id is relevant for your store. e.g. if using the fs store, make sure id is a valid file name (the fs store will still append .json at the end).

TODO

• Cache request headers too and hash them in filename / key / id.
• Browser-environment support. Please open an issue if you need this, and in what cases. jsdom?
• Handle and store invalid JSON too?