How Spy/Stub or fake a functions

Testing functions attached to objects, other than a class instance, constitutes an intimidating edge case from first sight. Such objects range from object literals to modules. This blog explores some test doubles techniques to shine a light on such cases.

For context, the difference between a function and a method, is that a method is a function encapsulated into a class.

In this article we will talk about:

Even though this blog post was designed to offer complementary materials to those who bought my Testing nodejs Applications book, the content can help any software developer to tuneup working environment. You use this link to buy the book. Testing nodejs Applications Book Cover

Show me the code

var fs = require('fs');

module.exports.removeUserPhoto = function(req, res, next){
    let filepath = `/path/to/photos/${req.params.photoId}.jpg`;
    fs.unlink(filepath, (error) => {
        if(error) return next(error);
        return res.status(200).json({
            message: `Your photo is removed - Photo ID was ${req.params.photoId}`

Example: A simple controller that takes a PhotoID and deletes files associated to it

What can possibly go wrong?

Some challenges when mocking chained functions:

Show me the tests

From the How to mock chained functions article, there are three relevant to the current context avenues we leverage for our mocking strategy.

let outputMock = { ... };
sinon.stub(obj, 'func').returns(outputMock);
sinon.stub(obj, 'func').callsFake(function fake(){ return outputMock; })
let func = sinon.spy(function fake(){ return outputMock; });

We can put those approaches to test in the following test case

var sinon = require('sinon');
var assert = require('chai').assert;

// Somewhere in your code. 
it('#fs:unlink removes a file', function () {
    this.fs = require('fs');
    var func = function(fn){ return fn.apply(this, arguments); };//mocked behaviour 
    //Spy + Stubbing fs.unlink function, to avoid a real file removal
    var unlink = sinon.stub(this.fs, "unlink", func);
    assert(this.fs.unlink.called, "#unlink() has been called");

    unlink.restore(); //restoring default function 


In this article, we established the difference between stub/spy and fake concepts, how they work in concert to deliver effective test doubles, and how to leverage their drop-in-replacement capabilities when testing functions.

Testing tends to be more of art, than a science, practice makes perfect. There are additional complimentary materials in the “Testing nodejs applications” book.


tags: #snippets #code #annotations #question #discuss