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!!
Note: This README refer to v3 which is in active development. v2
(CommonJS & Node/Jest only, see
old README)
is more stable but is no longer being worked on.
See MIGRATING.md for how to upgrade. v3 works but you
may want to wait a bit before any serious use. Feature requests welcome!
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";
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.
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";
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
fetchCache._once({
});
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.
- We'd rather use the test name as the id, vs something URL-based.
- It can't be reliably generated, e.g. formData with a random boundary.
In this case, we can:
fetchCache._once({ id: "mytest" });
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