products.js

const boom = require('boom')
const productProvider = require('./services/product-provider')

function getProducts(request, reply) {
  productProvider.findAll((err, products) => {
    if (err) {
      return reply(boom.internal("Something happened!", err))
    }

    reply(products)
  }
}

module.exports = {
  getProducts: getProducts 
}

test_product.js

const productProvider = require('../../services/product_v1/provider-mongo')
const productController = require('../../services/product_v1/controllers') // <-- SUT

describe('productController', function() {
    afterEach(function() {
        // Why: we must absolutely restore the stubbed function here, in order not to disturb other tests.
        productProvider.findAll.restore()
    })

    it('should stub a provider function.', function() {
        // Given
        sinon.stub(productProvider, 'findAll').yields(null, aProduct())
        // When
        productController.getProductList(stubRequest(), result => {
            // Then
            assert.deepEqual(aProduct(), result)
        })
    })
})