When unit testing a class method, how do I mock a function it calls--one defined in an imported module?

Question

I am unit testing a particular method, and am having issues mocking another function that is called during the process. In my case, the method to test is defined in a class, and the function I'd like to mock is defined in a separate module. How do I mock this function? See below for my code.

In the past, I've used the Sinon package to mock/stub a dependency (example). But that doesn't work in this case. This is the first time I'm testing a method defined in a class, so perhaps that's why mocking the dependency isn't working.

---

My Code

Module Containing Test Function (myLib/myDir/combo.js)

const { externalFunction } = require('./external-function')
class Combo {
  constructor(props) {}
  async myMethod () {// The function under test.
    externalFunction()
  }
}
const myCombo = props => new Combo(props)
module.exports = { myCombo }

My Test File (test/myLib/myDir/combo.test.js); no attempt at mocking

const { myCombo } = require('../../../myLib/myDir/combo')

const comboObj = myCombo({}) // Instantiate object to expose method to test.
await comboObj.myMethod()// Call method that I want to test.  This throws type error because myMethod function calls externalFunction, which throws an error in the test environment.

My Test File (test/myLib/myDir/combo.test.js); attempt to use Sinon package to mock

const sinon = require('sinon')

const dependencyModule = require('./external-function')// Defines the method dependencyModule.methodToMock

const myStub = sinon.stub(dependencyModule, 'methodToMock').returns(555) // Stubs dependencyModule.methodToMock and ensures it always returns the value: 555.

const comboObj = myCombo({}) // Instantiate object to expose method to test.

await comboObj.myMethod()// Call method that I want to test.  This throws type error because myMethod function calls externalFunction, which throws an error in the test environment.

Answer 1

How? You need to follow "stubbed module can not be destructured." on the official guide How to stub a dependency of a module

For example I have file external-function.js, combo.js and test.js on the same directory. I choose to use console.log to show that stub works and fake function get called, because you are not expecting something returned on myMethod.

// File: external-function.js
function externalFunction () {
  console.log('Original Called');
}

module.exports = { externalFunction };

// File: combo.js
// Note: "stubbed module can not be destructured."
const externalFunction = require('./external-function')
class Combo {
  constructor(props) {}
  async myMethod () {
    externalFunction.externalFunction()
  }
}
const myCombo = props => new Combo(props)
module.exports = { myCombo };

// File: test.js
const sinon = require('sinon');
const { myCombo } = require('./combo');
const dependencyModule = require('./external-function');

describe('myCombo', () => {
  it('myMethod', async () => {
    sinon.stub(dependencyModule, 'externalFunction').callsFake(() => {
      console.log('Fake Called');
    });

    const comboObj = myCombo({});

    await comboObj.myMethod();
  });
});

When I run it using nyc and mocha on my terminal:

$ npx nyc mocha test.js


  myCombo
Fake Called
    ✓ myMethod


  1 passing (3ms)

----------------------|---------|----------|---------|---------|-------------------
File                  | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s 
----------------------|---------|----------|---------|---------|-------------------
All files             |   85.71 |      100 |      75 |   83.33 |                   
 combo.js             |     100 |      100 |     100 |     100 |                   
 external-function.js |      50 |      100 |       0 |      50 | 2                 
----------------------|---------|----------|---------|---------|-------------------