Jest --detectLeaks reports memory leaks on nock.cleanAll() and nock.restore()

[jaylonez]

Please avoid duplicates

• I checked all open bugs and none of them matched my problem.

Reproducible test case

https://github.com/jaylonez/nock-mem-leak

Nock Version

13.2.4

Node Version

16.13.0

TypeScript Version

4.6.4

What happened?

Running a simple test with an afterAll that runs nock.cleanAll() or nock.restore() gets reported as a memory leak when using the --detectLeaks flag in jest.

Having either of these function calls present will result in the memory leak, but commenting both of them out will result in the test passing properly.

Would you be interested in contributing a fix?

• yes

[eric-parsons]

I ran into this as well. It's not directly related to either of those functions. As long as you have a "hard" import to nock, it will cause this. For example this modified version of your sample test also triggers it for me (note the side-effect import, which is preserved in the transpiled code):

import 'nock';

describe('test', () => {
  it('can be constructed', () => {
    expect(true).toEqual(true);
  });
});

I think it has something to do with how nock monkey-patches Node's http modules which doesn't play nice with Jest's architecture. I was able to get all memory leak warnings to go away by using this workaround:

1. Create a custom Jest environment class that extends whatever built-in environment you happen to be using (in my case jest-environment-jsdom), and configure jest to use it via testEnvironment in the Jest config. Import nock in that file, and assign it to a variable in global scope, e.g. __nock. Like so:

import { TestEnvironment } from "jest-environment-jsdom";
import nock from "nock";

export default class CustomEnvironment extends TestEnvironment {
    constructor(config, context) {
        super(config, context);
        this.global.__nock = nock;
    }
}

2. In your tests and/or custom Jest setup, reference this global rather than importing nock. In TypeScript, you can get the correct typings by doing this: const nock = globalThis["__nock"] as typeof import("nock");

Now, assuming you call nock.cleanAll() and nock.restore() in afterAll() as you mentioned, the memory leak warning should go away. I have a large suite of tests and haven't noticed any negative side effects from this. In fact I think it contributed to speeding things up. Hope that helps.

[mkurapov]

Adding to @eric-parsons 's solution. I noticed

    afterAll(() => {
      nock.cleanAll()
      nock.restore()
    })

would not be enough to prevent a memory leak. Sometimes it looks like there would be pending requests as well that would lead to the memory leak warning with jest. This can be avoided by adding nock.abortPendingRequests() as such:

   afterAll(() => {
     nock.cleanAll()
     nock.abortPendingRequests()
     nock.restore()
   })

[mikicho]

@mkurapov Does this happen to you in 13.5.1? https://github.com/nock/nock/releases/tag/v13.5.1
@VladimirChuprazov fixed a memory leak.

[mkurapov]

Hi @mikicho, yes, same with 13.5.1.
I made a simple example here: https://github.com/mkurapov/nock-memory-leak

[Uzlopak]

@mkurapov

If I remove the nock code from the memory-leak.test.ts then jest will also report leaking memory.

[mkurapov]

@Uzlopak that's odd. On my end:

[VladimirChuprazov]

hey @mkurapov try to change the order of your imports from:

import nock from "nock";
import axios from "axios";

to:

import axios from "axios";
import nock from "nock";

[mkurapov]

@VladimirChuprazov changing the order of imports works for me in the test repo, but not in the repo where I am trying to address the leaks.

In any case, I think this can be added to the README maybe?

[mikicho]

Maybe related to: jestjs/jest#6814