Node.js Mocha unit testing not changing object value - node.js

Question

I'm using Mocha/Chai/Sinon to develop some unit tests on a Node.js application (Express). I made a fake request/response object to pass to the function that I'm trying to test. Inside one of my stubs, the value of the property statusCode of the response object is changed to 200. However, at the end of the test, Mocha reports it as being the default value for the property (500, in this case).
Here is the test code:
it('should return status 200 if product was saved', function(){
//fake objects
const fakeReq = {
flash(){return true},
body:{
title: '',
price: 0,
description: ''
},
file: {
path: 'path'
},
session: {
user: ''
}
}
const fakeRes = {
statusCode: 500,
redirect(){
this.statusCode = 200; //this successfully changes to 200
return this;
}
}
//fake methods
sinon.stub(expressValidator, 'validationResult').callsFake(fakeValidation);
sinon.stub(Product.prototype, 'save').callsFake(sinon.fake.resolves('ok'));
//expectations
adminController.postAddProduct(fakeReq, fakeRes, ()=>{});
sinon.restore();
expect(fakeRes.statusCode).to.equal(200); //test fails returning 500
});
Here is the method that I want to test, more specifically, the product.save() call, at the end:
exports.postAddProduct = (req, res, next) => {
const validationErrors = validationResult(req);
if (!validationErrors.isEmpty()) {
req.flash('validationErrors', validationErrors.array());
return res.status(422).redirect('/admin/add-product');
}
const title = req.body.title;
const image = req.file;
const price = req.body.price;
const description = req.body.description;
if(!image){
req.flash('validationErrors', [{param:'productImage', msg: 'Insira uma imagem válida!'}]);
return res.status(422).redirect('/admin/add-product');
}
const product = new Product({
title: title,
price: price,
imageUrl: image.path,
description: description,
userId: req.session.user
});
product.save()
.then(results => {
return res.redirect('/admin/products');
})
.catch(err => {
next(new Error(err));
});
};
EDIT:
Changed the test function to use a spy in order to check if the redirect method was being called, but it's always returning false. I'm thinking that it might be because Js doesn't pass by reference, unless the contents of the object gets changed. New testing code:
const fakeRes = {
status(code){
this.statusCode = code;
return this;
},
redirect(){return true;},
statusCode: 500
}
const redirectSpy = sinon.spy(fakeRes, 'redirect');
//fake methods
sinon.stub(expressValidator, 'validationResult').callsFake(fakeValidation);
sinon.stub(Product.prototype, 'save').callsFake(sinon.fake.resolves('ok'));
//expectations
adminController.postAddProduct(fakeReq, fakeRes, ()=>{});
sinon.restore();
expect(redirectSpy.withArgs('/admin/products').calledOnce).to.be.true;
});