Unit testing: how to stub a wrapper function

Question

I'm new to unit testing and trying to figure out how to stub a wrapper function. I'm using Sinon/Mocha.

If I have a function like:

const user = await User.findOne({ email: email });

I've been able to stub it like so:

const userFindOneStub = sandbox.stub(User, 'findOne')
  .returns({
    _id: 'userId1234',
    companies: [
    {
      _id: 'companyId1234'
    }
  ]
});

But I've had to create a wrapper for my function to reorder the params for a specific function, using Lodash:

const userWrapper = _.rearg(UserFunction, [0, 1, 2, 3, 5, 4]);
const res = await userWrapper(someargs);

I can stub the UserFunction call, but how would I stub the userWrapper call in a unit test?

Answer 1

By save userWrapper as a module and follow Sinon How to Stub Dependency.

For example you can create userWrapper.js to like this.

// File: userWrapper.js
// This is just sample
const userWrapper = () => {
  // In your case is: _.rearg(UserFunction, [0, 1, 2, 3, 5, 4]);
  console.log('real');
}
module.exports = { userWrapper };

Then you can use it in your main js to like this.

// File: main.js
const wrapper = require('./userWrapper.js');

module.exports = async function main () {
  // In your case: const res = await userWrapper();
  wrapper.userWrapper();
}

And finally the test file.

// File: test.js
const sinon = require('sinon');
const wrapper = require('./userWrapper.js');
const main = require('./main.js');

it('Stub userWrapper', () => {
  const stub = sinon.stub(wrapper, 'userWrapper').callsFake(() => {
    console.log('fake');
  });

  main();

  sinon.assert.calledOnce(stub);
});

When you run it using mocha from terminal:

$ npx mocha test.js


fake
  ✓ Stub userWrapper

  1 passing (3ms)