Jest: How To Mock One Specific Function When Using Module.exports

I'm trying to mock one specific function when using module.exports. How could I test the inner function B? In my worker.js module.exports = function () { this.funcA = funcA th

Solution 1:

There isn't a way to mock funcB the way the code is currently written since funcA calls funcB directly.

The easiest way to fix it is to note that worker.js returns a constructor function and funcA and funcB are almost prototype methods...

...if you make them prototype methods then funcB can be mocked:

worker.js

class Worker {
  funcA() {
    this.funcB();
  }
  funcB() {
    throw new Error('should not make it here');
  }
}

module.exports = Worker;

worker.test.js

const Worker = require('./worker');

test('test functionB', () => { /* ... */ })

test('test functionA', () => {
  const spy = jest.spyOn(Worker.prototype, 'funcB');  // <= spy on funcB
  spy.mockImplementation(() => {});  // <= mock funcB

  const work = new Worker();
  work.funcA();  // <= call funcA

  expect(spy).toHaveBeenCalledTimes(1);  // Success!
  spy.mockRestore();  // <= restore funcB
}) 

Solution 2:

I know this is an old question but I thought I would chime in as I was searching for a way to do this as well and have discovered it is in fact possible.

Rather than calling the javascript function as above, you'll need to give it the scope of this to ensure that when you mock funcB, funcA calls the mocked version rather than just the function itself.

This means worker.js becomes

module.exports = function () {
  this.funcA = funcA
  this.funcB = funcB
}

funcA () {
  this.funcB()
}

funcB() {/* Your impl */}

And worker.test.js can remain, as before:

const Worker = require('./worker')

test('test functionB', () => {...}) 

test('test functionA', () => {
  // You could even just have: const work = require('./worker')
  const work = new Worker()
  work.funcB = jest.fn()  //mock funcB
  work.funcA()  //run funcA

  expect(work.funcB).toHaveBeenCalledTimes(1)
})