Unit Testing With Jest

Now, we're going to get setup with Jest, so that we can write unit tests.

Create a new folder to work in. You can call it whatever you want, but I'm going to call it unit-testing-algorithms.

We will be installing Jest using NPM, so we first need to run npm init to create a package.json file.

Install Jest

Then, we can install Jest with

npm install -D jest

We are installing it as a dev-dependency, because we only need it for development.

Create a .gitignore file

Next, create a .gitignore file and add node_modules to it. This will prevent the node_modules folder from being added to the repository.

Create the test script

In your package.json file, add a test script:

"scripts": {
  "test": "jest"
}

Now, when we need to run our tests, we just run npm test.

The way it works, is any file where we want to test, we make a file with the same name, but with .test.js at the end. For example, if we want to test a file called sum.js, we would create a file called sum.test.js.

Let's create a file called sum.test.js and add the following code:

const sum = require('./sum');

test('adds 1 + 2 to equal 3', () => {
  expect(sum(1, 2)).toEqual(3);
});

We are importing the sum function from the sum.js file. Then, we are using the test function to create a test. The first argument is the name of the test, and the second argument is a function that contains the actual test, which is we expect to get 3 as an output, when we have an input of 1 and 2.

Let's run the test with npm test. You should see a fail message like this

 FAIL  ./sum.test.js
  × adds 1 + 2 to equal 3

  ● adds 1 + 2 to equal 3

    TypeError: sum is not a function

      2 |
      3 | test('adds 1 + 2 to equal 3', () => {
    > 4 |   expect(sum(1, 2)).toBe(3);
        |          ^
      5 | });
      6 |

      at Object.sum (sum.test.js:4:10)

Test Suites: 1 failed, 1 total
Tests:       1 failed, 1 total
Snapshots:   0 total
Time:        0.351 s, estimated 1 s

Obviously, this will fail because we did not even create a sum() function yet.

It will show how many tests ran, passed and failed and will also give us an error message.

In your sum.js file, add the following code:

function sum(a, b) {
  return a + b;
}

module.exports = sum;

Now, when you run npm test, you should see a success message like this:

 PASS  ./sum.test.js
  √ adds 1 + 2 to equal 3 (1 ms)

Test Suites: 1 passed, 1 total
Tests:       1 passed, 1 total
Snapshots:   0 total
Time:        0.413 s, estimated 1 s

Congrats! You just wrote your first unit test!