Nock on Node 18/20/22 Fails to Intercept isomorphic-unfetch/fetch Request

On Node versions 18, 20 and 22, users may encounter an issue where Nock fails to intercept requests made via isomorphic-unfetch.

What follows is a sample error you may see (timeout is reached because the request is not intercepted).

returns a valid response

    thrown: "Exceeded timeout of 5000 ms for a test.
    Add a timeout value to this test to increase the timeout, if this is a long-running test. See https://jestjs.io/docs/api#testname-fn-timeout."

      1 |
    > 2 |   it('returns a valid response', async () => {

Why does this happen?

The issue arises because Node 18+ supplies a global.fetch. In systems that use isomorphic-unfetch, the global.fetch instance is used if available, which causes the problem. Consumers that explicitly use node-fetch or cross-fetch (which relies on node-fetch) are not affected.

See the relevant source file in isomorphic-unfetch repository: packages/isomorphic-unfetch/index.js

Relevant GitHub issues discussing this problem include:

• Nock Issue #2183
• Nock Issue #2336
• Node.js Undici Issue #531

How to Fix?

There are a few short-term approaches to resolve this issue:

One option is to override global.fetch with node-fetch. This can be done by placing the following code in a jest.setup.ts file (or the equivalent Jest/test runner setup file):

// nock doesn't support native fetch, and hence we need this patch.
import fetch, { Headers, Request, Response } from 'node-fetch';
(global as any).fetch = fetch;
(global as any).Headers = Headers;
(global as any).Request = Request;
(global as any).Response = Response;

Refer to this GitHub comment for more details.

Another option is to set NODE_OPTIONS="--no-experimental-fetch" in your test running command. For example:

"scripts": {
  "test": "NODE_OPTIONS='--no-experimental-fetch' jest"
}

This will disable the native fetch, causing isomorphic-unfetch to use node-fetch, (see Node.js docs).

For a more long-term solution, consider migrating to undici’s MockAgent().get().intercept() or using the msw (mock service worker) package, which supports fetch through its own patching. See the following links:

• Test Native fetch in Node.js with Undici interception and mock utils
• Undici MockAgent Documentation
• Mock Service Worker Fetch Patch