Unit Test Numbers

The goal of this assignment is to display and test a complete set of numbers from 1 to 9, each of which has state and can be set with a button click.

This assignment builds on the React Jest Unit Tests assignment. Also some reference materials here.

Complete that assignment first. You should turn in only one project, which will contain both the React Jest Unit Tests and the Unit Tests Numbers assignments. You can turn in the React Jest Unit Tests assignment first, if you want. In other words, your project need not include both completed assignments before you turn in the first assignment.


Since we are often working on a single project that has multiple phases, I suggest creating a git tag marking your current status:

$ git tag -a v3.0.0 -m "Start Week03"
$ git push origin v3.0.0
$ git tag -n1

The first command creates a tag that has a message associated with it. The message works much like the message in a commit.

The second command pushes the tag from your local machine to the cloud.

The last command lists your tags and their message on one line. If you have only a single tag, it is not particularly useful, but once you have multiple tags you will see how helpful this can be. Increase the value of the number after -n? to see more information about your tag. You can read about tags here:


We left things, I believe in this state:


With the tests like this:


The constructor:

constructor() {
    this.state = {
        file: 'Get Nine Result will be placed here.',
        foo: 'waiting for express server',
        nine: '0'

Methods called getFoo and getNine:

getNine = () => {
    this.setState({nine: '9'});

GetFoo is the one with the rest call. We are mocking that rest call for our tests. Here is the mocked mock object that we were using by the end of class Thursday:

var fetch = function(parentThis) {
        foo: 'bar',
        file: 'api.js'

    return {
        then: function() {
            return {
                then: function() {
                    return {
                        catch: function() {


module.exports.fetch = fetch;

I’ll develop that further, but this is a start.

Add Test First

The correct way to implement tests is to follow TDD, or Test Driven Development. This includes the important Test First theory of development. That means we first create a failing test, then find a way to make it pass.

Start by creating a failing test for a property called eight. I’ll show two tests here, the first a slightly modified click nine test, and the second our new test:

it('renders button click message for nine', () => {
    const wrapper = shallow(<App />);
    const nineSign = <p className="App-intro">state.nine: 9</p>;

it('renders initial value of paragraph with state.eight', () => {
    const wrapper = shallow(<App />);
    const eightSign = <p className="App-intro">state.eight: 0</p>;
    const eightp = wrapper.find('p').last().debug();

Note that I have removed the debug lines from the renders button click for nine test, and added it to the renders initial button state.eight. I also removed it for the render initial value of paragraph with state.nine. This is because that paragraph element will no longer be the last() paragraph on our form. The last paragraph on our form is now the one that displays eight.state. (We will need to make similar changes for other properties we create.)

Jest Test with Eight

Note the failing test. Your goal is to get it pass. Start by adding state for the value eight:

this.state = {
    file: 'Get Nine Result will be placed here.',
    foo: 'waiting for express server',
    nine: '0',
    eight: '0'  <====== HERE

The next step would be to add a paragraph in which to display its value. The result should look something like this at run time:

React Unit Test Eight View

Add Eight Button and Test

Now write a test to check that we can set a new value for state.eight. I’ll leave that up to you, as it is really just a block copy of a previous test. Then replace certain obvious values.

To get the test to pass, add a button for updating and retrieving the value of state.eight. It should be fairly clear that you will also need a getEight method of your App component:

getEight = () => {
    this.setState({eight: '8'});

React view for button eight

Turn it in

Go ahead and add state, tests and button for numbers 7-1. When you are done, save your work and push.

Frankly, I’ve totally lost track of which folder this should be in. Mine is in week02-rest-basics. Hopefully yours is in something similar. Probably the best course of action is to be sure to list the folder you want me to inspect when you turn the project in.

NOTE: One thing I’m trying to do here is create one project for each set of related assignments, rather than the old technique of “copy week02-react-foo into week03-react-bar. This is partially selfish, in that it makes my life simpler, but it should also make your life simpler. If you wanted to go crazy, you could use git tags or branches to delineate one assignment in a series from the next assignment in the series. But I will, when I can, just assume that if you can complete the second or third in a series of projects, then you also completed the first in the series.


I believe the key thing is learning how to see the output from our Components using the Enzyme debug methods. This class is designed to help you do this:


Let’s step back a minute and try to understand what we are doing.

  • We create a component such as SmallNumber
  • This component produces output in the form of HTML tags that we typically see on a web page.
  • We are writing tests to prove to ourselves that the HTML being produced by our component is valid HTML and has the shape we expect it to have
  • We write tests for each significant line of output created by our component. We are saying in effect, is the HTML that displays the firstName, lastName, street, etc valid HTML and does it contain the data I expect it to contain? For instance, does it contain a valid first name, last name, etc.
  • We are, quite often, trying to match very specific output. So writing the tests is an exact art. We need to know exactly what is being generated so we are sure our tests check for the exact output we expect to see.
  • Know that exact output is tricky to guess blindly, so we use the enzyme debug methods to display exactly the code that our Component is producing for the firstName, lastName, etc, fields.
  • Once we know the exact output, we write our test to prove to ourselves that we can check for that exact output.

Once we have created a valid test, from that time forward, we should be able to say with confidence two things:

  1. That changes that we make to our code does not produce unexpected changes that break our tests
  2. That we did not accidentally change the behavior of our Component due to a careless edit, a mistake with Git, or some other unexpected event.

It takes time to create good tests, but once we have them, then we can improve, edit, and add new features to our code with confidence. If we keep our tests in memory at all times, and we check them frequently, then they will tell us immediately if something we wrote caused our code to break.

There are other reasons to write tests, such as documentation, but the above perhaps describes some of the obvious and tangible benefits.

Suppose you are on a QA team and some DEV pushes new code. You run your Unit tests, or if the tests are run automatically, and if the DEV made a mistake that broke old code, you would know immediately that it happened, and furthermore, you would know exactly where the error occurred.

I think if you know why you are writing the tests, and what you are looking for, it is easier to see how to construct them.