How to test a function call inside another function? JEST

Question

In case you're interested, I solved this already using the first solution provided. I was missing babel from jest, I was using the experimental es6+ settings found in jest documentation stating I should remove the transformation, add test node --experimental-vm-modules node_modules/jest/bin/jest.js, etc. Not really sure what is that for, I thought it was for this very thing but apparently not.

I've read many posts here and tried many solutions but nothing seems to work for me, how can I test in Jest that the calling function is being called inside init?

Here's what I have:

calling.js

const calling = () => {
  return 'calling';
};

export default calling;

init.js

import calling from './calling';

export const init = () => {
  calling();
};

test.spec.js

import { jest } from '@jest/globals';
import init from './init.js';
import calling from './calling.js';

test('init calls calling', () => {
  // what to do here?
});

package.json

{
  "name": "testing",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "test": "node --experimental-vm-modules node_modules/jest/bin/jest.js"
  },
  "type": "module",
  "jest": {
    "transform": {},
    "roots": [
      "<rootDir>"
    ],
    "modulePaths": [
      "<rootDir>"
    ],
    "moduleDirectories": [
      "node_modules"
    ]
  },
  "keywords": [],
  "author": "",
  "license": "ISC",
  "devDependencies": {
    "@types/jest": "^28.1.8",
    "jest": "^28.1.3"
  }
}
};

It kind of work now based on the solution provided above BUT! I have to do a more complicated setup in order to work and I can't figure out why. Also, I had to do name exports because it doesn't work with default export and I can't figure out how to do it either.

import { jest } from '@jest/globals';

const mock = jest.fn();

jest.unstable_mockModule('./calling.js', () => ({
  calling: mock,
}));

const { calling } = await import('./calling.js');
const { init } = await import('./init.js');

test('test', async () => {
  init();
  expect(calling).toHaveBeenCalled();
});

n1md7 · Accepted Answer · 2022-08-25 21:22:00Z

2

In this case, you can simply use the Jest autoMocking function and verify whether it actually been called

// test.spec.js
import { init } from './init';
import calling from './calling';

jest.mock('./calling');

test('init calls fn calling', () => {
  init();
  expect(calling).toHaveBeenCalled();
});

I assume it did not work out for you because you have a mistake in importing

import init from './init.js'; // not default export
// should be
import {init} from './init.js';

answered Aug 25, 2022 at 21:22

n1md7

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7 Comments

Nicolas Over a year ago

Sorry to bother you but it doesn't work for me. I did exactly as you said. I get this error: expect(received).toHaveBeenCalled() Matcher error: received value must be a mock or spy function Received has type: function Received has value: [Function calling]

n1md7 Over a year ago

Are you sure u did exactly as it's in the example? import and mock values have to match. I tested it in my machine and it works just fine.

Nicolas Over a year ago

100% sure. To be sure I double checked -> New folder, npm init -y, npm install -D jest, (no globals installed, just this). Then -> package.jsonand changed test to "test": "node --experimental-vm-modules node_modules/jest/bin/jest.js", added "type": "module", and then I added "jest": { "transform": {}} because if I don't add all of that I always get an error require is not defined. Did both files the same, copied yours and added (added import {jest} from "@jest/globals" I get the same error. What's wrong? Tried adding .js to the imports but same result

Nicolas Over a year ago

I changed everything to use commonJS instead and it works, so there's something wrong when I use ES6+ but you said it worked on your machine, there must be something wrong with my config, in my previous comment I said all I did. So what could it be?

Nicolas Over a year ago

I see. I went to the Jest site and found the babel part. I installed that, did a small config file and removed all the experimental test thing I added before. Not it works perfectly! Thanks a lot for your time and help I really appreciate it Your first solution works great now!

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