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JavaScript Basics

The repository contains test cases and empty function definitions which need to be populated to solve the different challenges set in this weeks walkthrough. 🚨You should not update the tests 🚨

Getting started

Clone the repository

First up, create a fork of this repo.

Then clone your copy of the repo:

Then cd into the repository.

You will see a few files and directory in here. Most interestingly we have:

  • src directory - this is where our code lives
  • test directory - this is where our test code lives
  • a package.json file, which contains some configuration for our project

Install the project dependencies

The npm install command (or npm i for short) will tell npm (node package manager) to download the dependencies and devDependencies for this project, which are defined in the package.json file.

Take a look in package.json - what dependencies does this project have?

Now run the npm install command. Once it has completed, you should see a new node_modules directory in the repository. This is where our dependencies live. It's important that we don't add this directory to git, since it can contain 100000's of files that we don't need to maintain, and can just be downloaded.

In the package.json file you can see that there are only devDependencies for this project.

Run the test code

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

  1. describe how our code should behave
  2. assert that is does behave in the desired way

Once you have installed the dependencies, run npm test or npm t for short. This command will run the test script defined in our package.json, which simply runs Jest. When we run Jest, it will execute the test code written in the test directory. For now, you should see a message saying that all of the tests were skipped.

Take a look at the files in the test directory - you will see a few files in here, all ending with .test.js. These map to the files in the src directory, and contain the tests for these files.

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

Take a look at the test/strings.test.js. At the top, we have a require statement. 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 requires 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, which define an individual test, along with . (At the moment, these should actually be xit blocks... more on that imminently).

The individual it blocks make some assertions about what should happen when we call the function we are testing.

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 is because we have 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?

Update the function so that the test pass

You challenge is to write some code in the sayHello function in src/strings.js that will make the test pass. 🚨You should not update the tests 🚨