Testing With Javascript

Learning Goals

Learn about testing in JavaScript

Introduction

Many of the labs you will complete in this program use tests. Passing the tests verifies that the code you write behaves as desired and produces the expected results. But writing tests is also a way to provide specifics about exactly how the code should function. In a common development strategy known as test-driven development (or TDD), programmers first write the test for a specific function of the code then write the code to make the tests pass. TDD is considered the most reliable methodology for delivering quality code.

What this means is that the results of running the tests will be an important tool in figuring out how to get those tests passing. Code testing can feel like an abstract concept at first, but it's worth starting to learn how they work. When you're having difficulty passing a test, being able to read and understand the test output — and the tests themselves — can be an invaluable skill.

Getting Started

The lesson is set up as a code-along, so you'll first need to fork and clone it to your local environment.

Quick Review:

1. click the Fork icon in the upper right of this page. This will bring you to GitHub. If you are prompted to choose where to fork, choose your personal GitHub account.

2. Once your fork is created, click the Code button in GitHub, make sure SSH is selected, and copy the provided git URL info.

Then, in your terminal:

3. Make sure you're in Development/code (or wherever you're storing your code for the course) and clone the repo to your local machine with git clone followed by the git URL you copied.

$ git clone git@github.com:learn-co-curriculum/phase-0-pac-3-what-is-a-test.git

4. The previous command will create a folder in the location you're currently in containing your fork of this lab's repository. cd into the repository that you just cloned down in the terminal, then run code . to open the files in Visual Studio Code.

$ cd phase-0-pac-3-what-is-a-test
$ code .

Open up index.js in your code editor. You are going to see mostly familiar things:

const name = "Joe";
const height = 74;
const message = `${name} is ${height} inches tall`;

module.exports = { name, height, message };

This should all look familiar except for that last line. You don't need to worry about it for now — just know that line of code makes the variables available to the test file.

Take a look at the message variable:

const message = `${name} is ${height} inches tall`;

We can use console.log to take a look at the value of the message variable. To do that, first type console.log(message); on the last line of index.js and save the file. Next, navigate to the terminal, and type the following command in the command line and hit enter (be sure you're still in the lab's directory):

$ node index.js

The node command executes the code in whatever file you specify (in this case, index.js). You should see "Joe is 74 inches tall" logged in the terminal.

Top Tip: console.log is one of the debugging tools you can use as you're writing your code. Logging a variable and executing the code will allow you to verify that the value of the variable is what you're expecting.

In the line of code above, we are using string interpolation to inject the values of the name and height variables into the message. Recall that, for this to work, you have to wrap the entire string in backticks and wrap the variables themselves in ${}. If you'd like a refresher, try leaving out the ${}s or switching to a different type of quotes and run your code again to see what the value of message is. The backticks and the ${} tell Javascript to grab the value inside the variable, not just that variable name.

The Tests

We have our code, now let's take a look at the tests. They are located in the test folder inside a file named index-test.js. In this lesson we'll get familiar with the tests, and then run them in the next lesson.

const { name, height, message } = require("../index.js");

/*
describe('what-is-a-test', () => {
  describe('Name', () => {
    it('returns "Susan"', () => {
      expect(name).toEqual('Susan')
    })
  })


  describe('Height', () => {
    it('is less than 40', () => {
      expect(height).toBeLessThan(40)
    })
  })

  describe('Message', () => {
    it('gives the name and height', () => {
      expect(message).toInclude(name)
      expect(message).toInclude(height)
    })
  })
})
*/

In the first line, we're enabling the tests to access the variables in index.js. You don't need to worry about exactly how this works at this point — just know that the module.exports and require keywords allow us to access variables written in the index.js file from within the test file.

The next thing to notice is that the test code itself is commented out using the /* and */ block commenting syntax. This is because, as mentioned above, we don't actually want to run the tests yet. We will run the tests in the lab that follows this lesson; in the version of the files provided for the lab, you'll see that the /* and */ have been removed.

Next, note that the test code consists of three individual tests (each starting with describe) nested inside a block for the tests as a whole (also starting with describe).

The first grouping is testing our name variable:

describe("Name", () => {
  it('returns "Susan"', () => {
    expect(name).toEqual("Susan");
  });
});

Take a look at the line that begins with expect. If we read it out loud, we get "Expect name to equal Susan". That's exactly what it's saying! If we continue down to the Height section you'll see this code:

describe("Height", () => {
  it("is less than 40", () => {
    expect(height).toBeLessThan(40);
  });
});

Again, reading the line starting with expect out loud, we get "Expect height to be less than 40." Again, this is just what the test is checking. Let's look at the final one:

describe("Message", () => {
  it("gives the name and height", () => {
    expect(message).toInclude(name);
    expect(message).toInclude(height);
  });
});

This one has two expect statements. If you read them out as English you'll discover that the tests expect the value of index.message to include both index.name and index.height.

OK Great. You now understand what the tests are saying. In the next lesson we are going to cover how to run them, and then you will solve the lab!

Submitting the Assignment

To register completion of this lesson, submit the URL for your forked copy of the repo in Canvas. Even though you didn't need to write any code for this lesson, you'll need to submit a link in order for this lesson to be marked as complete in Canvas.