mockReset removes the original implementation of a spied on function instead of just mockImplementations

🐛 Bug Report

Edit

turns out the problem was mockReset(), there might be a bug in mockReset()though, please read the first comment

I also changed the title, so this first post might be confusing. The title this first post refers to was “jest.spyOn does not call the original method” because this is what I thought happend. The actual issue I found though is that mockReset removed the original implementation if a method I spied on

---

I am trying to test if a method on a class has been called but I do not want to mock that function. I.e. I want my code to still call the original function, I only want to be able to see if it has been called appropriately.

To my understanding (and my previous experience), if i just use jest.spyOn like so:

const connnectSpy = jest.spyOn(sblendid.adapter, "connect");

what I was describing should already be the default behavior. I.e. I should be able to write tests like this:

expect(connnectSpy).toBeCalledTimes(1);

and the interal call to

sblendid.adapter.connect()

that is part of my implementation should have been remained untouched.

But this is not happening. When I use jest.spyOn I can see that my original implementation never gets called. When I remove jest.spyOn I can see it gets called again.

This is my code block

peripheral.ts Lines 10-35

it’s a few lines, so I will go through it step by step

I am spyingOn sblendid.adapter.connect

beforeAll(async () => {
  const sblendid = await Sblendid.powerOn();
  peripheral = await sblendid.find(name);
  connnectSpy = jest.spyOn(sblendid.adapter, "connect");
});

peripheral.ts Line 13

In line 26 I am calling the method (peripheral.connect()) that internally will call the spied on function (sblendid.adapter.connect)

it("connects to peripheral", async () => {
  await expect(peripheral.connect()).resolves.toBe(undefined);
  expect(connnectSpy).toBeCalledTimes(1);
  await peripheral.disconnect();
}, 10000);

peripheral.ts Line 26-27

Now every time I use spyOn here my tests time out, when I remove the spy my tests work, but of course I cannot check if my method has been called. So I did some very primitive low-level debugging—using console.log.

I am logging "🙃🙃🙃 orig 🙃🙃🙃" whenever my original method gets called

console.log("🙃🙃🙃 orig 🙃🙃🙃");

index.ts line 75

And even deeper in my code I have added another console.log

console.log("🎉🎉🎉 connect has been called 🎉🎉🎉");

peripheral.ts (implementation file not the file from before) line 21

I can see that these lines get called when I do not spyOn my method once I spyOn, these logs never get printed.

To Reproduce

You can clone my repo at the given commit:

git clone [e-mail]:LukasBombach/sblendid.git
git checkout 0ba503df0a27541a8ffd66aa3510cf27ac7a72bf
yarn install # you need to use yarn because this is a monorepo with yarn workspaces 
cd packages/sblendid
yarn test --watch

• you can see the logs never appear
• comment out lines 13, 17 and 22 of src/__tests__/peripheral.ts (the refernces to the spy)
• see that the logs appear

Expected behavior

The spiedOn method should be called regardless of the spy

Link to repl or repo (highly encouraged)

https://github.com/LukasBombach/sblendid/tree/0ba503df0a27541a8ffd66aa3510cf27ac7a72bf

envinfo

  System:
    OS: macOS 10.14.5
    CPU: (8) x64 Intel(R) Core(TM) i5-8259U CPU @ 2.30GHz
  Binaries:
    Node: 12.9.1 - ~/.nvm/versions/node/v12.9.1/bin/node
    Yarn: 1.17.3 - /usr/local/bin/yarn
    npm: 6.10.2 - ~/.nvm/versions/node/v12.9.1/bin/npm

[jayfunk]

Anyone still hitting this? Using 26.6.3 and when calling mockRestore the original implementation is NOT being restored. In fact it is getting set to null after calling mockRestore. I have found that calling jest.restoreAllMocks() will do what mockRestore or mockReset on the individual spy should do. Seems like it might be an issue only with spy instance.

[LukasBombach]

Nono, @SimenB gets it right. I will assemble a small repo for production, give me a little time.