• Practice using the const and let variables in JavaScript

We've covered a lot of JavaScript concepts, but now it's time to put the concepts in to practice. We'll start with variables.

You might notice a few new things in this lesson that you haven't encountered before. We'll walk you through them.

When we want to run an experiment, we need to develop a hypothesis and we need to test it. In programming, we run tests to verify that programs behave the way we think they do. Tests help us identify bugs and judge how healthy our applications are.

We use tests to describe the program's behavior, just as you would in a professional coding environment, and we also use them as teaching tools. You are in charge of getting the tests to pass.

The structure of this lab — where its files and folders are located — looks roughly like the following:

├── CONTRIBUTING.md
├── LICENSE.md
├── README.md
├── index.js
├── node_modules/
├── package.json
└── test
    └── indexTest.js

All labs will more or less have the same structure. (And non-lab lessons, for that matter, will still have CONTRIBUTING.md, LICENSE.md, and README.md files.)

Open up index.js in your text editor. If you're using the Learn IDE, click the blue "Open" button in the top right hand corner of the lesson. If you open up that js-basics-variables-lab/ directory, you'll see a list of files (along with a test/ directory). Click index.js, and it will open in the editor.

In index.js, you should see, well, nothing. We'll fix that soon.

Now open up test/indexTest.js. Hey, there's something! What's all of this stuff doing?

Note: The test/indexTest.js has great info that we want to look at, but do not edit this file otherwise you may have extra difficulty passing this lab.

A few lines down in the test/indexTest.js file you will see:

describe('index.js', function () {
  // there's stuff in here, too
});

describe is a function provided by our test library, Mocha, and it's used to hold our tests. After the word describe is information about our tests. Tests are used as a way to document the behavior of a function to developers. For example, the next word describe is followed by the word companyName name. Here the test is telling us that the tests that come afterwords will be about companyName. Then comes the word it, where you see the following:

it('is set as Scuber', function () {
  // tests are here
});

This is telling us that the companyName should be set to Scuber. Finally, filling in the missing part of the it code, we see:

it('is set as Scuber', function () {
  expect(companyName).to.equal('Scuber');
});

This example shows that the test expects companyName to equal Scuber. That expect and to.equal are essentially doing the same thing as companyName == 'Scuber'. In other words, expect(companyName).to.equal('Scuber') is running code that will have this first test pass if companyName equals Scuber and fail if it does not.

Don't worry too much yet if it's hard to understand what is happening inside of the test/indexTest.js file. But it's a good idea to open up the file, and gather the information that you can. We will also provide instructions in the README.md file that will allow you to complete the lab.

To run the tests, type learn in the terminal part of the Learn IDE. (The terminal is the part below where you've been coding.)

Running the learn command will open up a new tab on your browser, showing the current status of the tests. For the moment, all of the tests fail. Let's figure out how to get one of them passing! (The rest will be up to you.)

To get our first test to pass, we can open up our index.js file, and write the following:

let companyName = 'Scuber';

Great, our first test is now passing. However, the second test that is also about companyName is not yet passing. It's not passing because, it expects a change to companyName to throw a TypeError. It sounds like it wants companyName to be declared using a different keyword than the let keyword - it needs a keyword that is used for variables that can't be changed...

Continue to work through the problems below. Keep in mind the general workflow for a lab:

1. Run learn.
2. Read the errors; vocalize what they're asking you to do.
3. Write code; repeat steps 1 and 2 often until a test passes.
4. Repeat as needed for further tests.
5. Run learn submit when finished!

If you open up test/indexTest.js, you will see the tasks in front of you:

• companyName - Inside the test/indexTest.js file, look inside of the word describe where the tests are trying to indicate that this test is describing the companyName variable. The it word that comes afterwards, tells us the features of companyName. In the first it function call, it says that it (companyName) is set as Scuber. In the next line, you can see that the test checks to make sure this occurs by seeing if companyName equals Scuber. So this means that you need to go to your index.js file and declare a variable named companyName and set it equal to Scuber. Once you do that, if learn is running, you will see the first test in this lab as passing.
• In the next it function call, we are still describing companyName. This time, it says it raises error if the companyName is changed. The next line of code tests this. It's ok if some of the code in that line is confusing. Just know that the code attempts to change companyName to a different value, and that this reassignment should throw an error. So you need to make sure that you are using the correct type of variable declaration such that attempting to reassign the variable throws an error.
• mostProfitableNeighborhood - Here we need to declare another variable, mostProfitableNeighborhood and assign to it the string 'Chelsea'. In the next it function call, you can see that our tests ensure that mostProfitableNeighborhood does not throw an error when reassigned. So you need to make sure that you are using the correct type of variable declaration such that assigning a new value to mostProfitableNeighborhood doesn't throw an error.
• companyCeo - Here, we are getting more practice with declaring variables. Once again, a reassignment should not throw an error.

• MDN: Let
• MDN: Const