1. Overview

Jest, is a batteries included testing framework

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2. Set up Jest

We can install jest using npm:

npm install --save-dev jest

We save jest as a dev dependency since it will test our code and will not be used in production.

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3. Creating a Simple Test

3.1. Create a function

Firstly, we will need something to test. For this, we will create a new file called add.js and add the following code:

// add.js
export default function add(a, b) {
  return a + b
}

this code exports a default function of add, which combines two values and returns the result.

3.2. Create a test

We can now create a test file called add.test.js and add the following code:

// add.test.js
import add from './add.js'

it('adds 1 + 2 to equal 3', () => expect(add(1, 2)).toBe(3))

3.3. Running the test

We can now run all tests using the following command:

# 👇 need this flag to run tests with ES6 syntax
$ NODE_OPTIONS=--experimental-vm-modules npx jest
# 👇 output
(node:15429) ExperimentalWarning: VM Modules is an experimental feature. This feature could change at any time
(Use `node --trace-warnings ...` to show where the warning was created)
 PASS  ./add.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:        0.248 s
Ran all test suites.

or we can just run the test file directly:

# passing in the file name as a parameter to jest    👇
$ NODE_OPTIONS=--experimental-vm-modules npx jest add.test.js
# 👇 output
(node:15989) ExperimentalWarning: VM Modules is an experimental feature. This feature could change at any time
(Use `node --trace-warnings ...` to show where the warning was created)
 PASS  ./add.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:        0.225 s, estimated 1 s
Ran all test suites matching /add.test.js/i.