Getting Started

Inside your cloned javascript-basics folder you should have the following folders:

src/
__tests__/

src/ is where you will be writing code in this week's exercise. The __tests__/ folder contains some code that we've written that will check your code works properly. In future weeks, you will write your own tests to ensure your code is doing what it's supposed to be.

We have written the tests in a library called Jest.

Jest is a JavaScript testing framework - it allows us to write automated tests that:

• describe how our code should behave
• assert that it does behave in the desired way

In your terminal, navigate to the javascript-basics folder that you cloned and run npm test. This command will run the test script defined in your package.json (under scripts and then test), which simply runs Jest. When we run Jest, it will execute the test code written in the __tests__ directory. For now, you should see a message saying that all of the tests were skipped.

Take a look at the files in the __tests__ directory - you will see a few files in here, all ending with .test.js. Each of these correlate to a file in the src directory.

For example __tests__/strings.test.js contains the tests for the functions defined in src/strings.js.

Take a look at the __tests__/strings.test.js. At the top, we have a require statement.

const {
  sayHello,
  uppercase,
  lowercase,
  countCharacters,
  firstCharacter,
  firstCharacters
} = require("../strings");

This imports the functions from our src/strings.js file into the test file, so that the tests can use the functions we have written in that file.

Beneath the require statements are the actual tests.

These are broken into describe blocks, which group the tests for each function together. Each describe block contains some it blocks. Inside each it block we call (or invoke) a function and then we make an assertion (have an expectation) on what the function should return. At the moment, the it blocks will actually be xit blocks... more on that shortly.

For example:

describe('sayHello', () => {
  it('returns "Hello world!" when passed "world"', () => {
    expect(sayHello('world')).toEqual('Hello, world!');
  });
});

describe('sayHello', () => { ... }); tells us that all of the code inside the block is concerned with testing the sayHello function.

it('returns "Hello world!" when passed "world"', () => { ... }); tells us what the return value of the function should be when passed a certain argument.

expect(sayHello('world')).toEqual('Hello, world!'); this line invokes the function with the string 'world as an argument, and make an assertion about the return value of this action - it should equal 'Hello, world!'.

The reason all of the tests were skipped when we ran npm test before is because we used xit instead of it. Change the xit on the first test to an it and then run npm test in your terminal again. You should now see the test run, and fail - with the following error message:

Expected value to equal:
      "Hello, world!"
    Received:
      undefined

    Difference:
      Comparing two different types of values. Expected string but received undefined.

This tells us that calling the helloWorld function with the string 'world' returned undefined, but the test was expecting it to return the string 'Hello, world!'

Now take a look at src/strings.js and in particular at the sayHello function:

const sayHello = (string) => {
  // your code here
};

Can you see why we got the test result that we did?

Your challenge will be to write some code in the sayHello function in src/strings.js that will make the test pass, and to do the same for all of the other functions in all of the other files.

We will start with numbers...

The only change to code you should make in the __tests__ folder is xit to it. All other code should be written in the src directory.