fetch-mock-cache

Caching mock fetch implementation for all runtimes and frameworks.

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

🏁 Introduction

• 💡 Tired of writing mocks?
  Skip manually crafting fetch() mocks — just run a real fetch() once and auto cache the result.

• ⚡ Fast feedback loop
  Cached responses make tests run much faster — ideal for TDD (Test Driven Development) against real APIs.

• 👥 Team & CI friendly
  Commit the cache to your repo so everyone (and your CI) benefits from consistent, speedy tests.

• 🔧 Proven approach
  Used by yahoo-finance2 to test 1400+ API calls in < 1s on every commit.

• 💬 Open to ideas
  Feature requests and contributions 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 .

// Default for node runtime, see grid below for bun/deno.
import createFetchCache from "fetch-mock-cache";
// See list of possible stores, below.
import Store from "fetch-mock-cache/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"
    }
  }
}

JSON bodies are stored as a decoded object in bodyJSON for increased readability. Text bodies are stored as a string in bodyText, and binary bodies are stored as a Base64 encoded string in bodyBase64.

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 });
  }
}

Overriding the Default Caching Behaviour

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).

Manually controlling cache behaviour

You can also pass the following to fetchCache.once:

{
  id?: string; // as above
  /** True (default): use cached response if available; false: always fetch from network.
   * You can also provide a promise or function that returns a boolean or promise.
   */
  readCache?:
    | boolean
    | Promise<boolean>
    | ((...args: Parameters<FMCStore["fetchContent"]>) => Promise<boolean>);
  /** If a fetch was performed, should we write it to the cache?  Can be a boolean, a
   * promise, or a function that returns a boolean or promise.  In the case of a promise,
   * the write will open occur when the promise resolves, and AFTER the response is
   * returned.  This allows for more complex patterns, where e.g. you could rely on the
   * further processing of the response in other functions before deciding whether to
   * cache it or not, but does require some extra care.
   */
  writeCache?:
    | boolean
    | Promise<boolean>
    | ((...args: Parameters<FMCStore["storeContent"]>) => Promise<boolean>);
}

See below in Tips & Tricks on how you can leverage this to conditionally replace the cache for failing tests.

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:

None.

Previous experiments:

• _once() has been promoted to _once() after a long, succesful testing period.

Tips & Tricks

Rewrite the cache only for failing tests.

1. Let's wrap the it() function to allow a callback after the test

   import { it as _it } from "..."; // your fave library
   
   export const it = function wrappedIt(
     suite: string,
     fn: (
       t: Parameters<typeof _it>[0],
       onFinish: (cb: (error?: unknown) => void | Promise<void>) => void,
     ) => void | Promise<void>,
   ) {
     const finishCallbacks = [] as Array<(error?: unknown) => void>;
   
     const onFinish = (cb: (error?: unknown) => void) => {
       finishCallbacks.push(cb);
     };
   
     const finish = (error?: unknown) => {
       for (const cb of finishCallbacks) {
         cb(error);
       }
     };
   
     const wrappedFn = (t: Deno.TestContext) => {
       let result: ReturnType<typeof fn>;
       try {
         result = fn(t, onFinish);
       } catch (error) {
         finish(error);
         throw error;
       }
   
       if (result === undefined) {
         finish();
         return undefined;
       }
   
       if (result instanceof Promise) {
         return result.then(finish).catch((error) => {
           finish(error);
           throw error;
         });
       }
   
       throw new Error(
         `Test "${name}" failed with unexpected result: ${result}`,
       );
     };
   
     _it(suite, wrappedFn);
   };

2. Now do something like this:

   import { it } from "above";
   import fetchCache from "wherever-you-set-it-up";
   
   function conditionalCache(onFinish) {
     if (process.env.FETCH_CACHE === "recache")
       // i.e. if the test fails and has an error, then `writeCache` resolves to "true"
       fetchCache.once({ readCache: false, writeCache: onFinish.then(error => !!error)})
   }
   
   it("rewrites the cache on fail", async(_t, onFinish) => {
     conditionalCache(onFinish);
     const response = await fetch(...);
     const result = await response.json();
     expect(result).toMatch({
       success: true
     });
   })

3. Now, by default, we'll use the cache as normal. However, if you set FETCH_CACHE="recache", we won't use the cache, and if the test fails, we'll replace the cached result.

   This is super useful for testing if API response (and not your code) has changed, commit the new responses, and then adapt your code as necessary. You'd only do this after first making sure all your existing tests are passing.

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?