Getting started with Jest

Let's start by testing a small sum function and check that it correctly sums two numbers.

The first step would be, of course, to install Jest:

npm install jest

Create a directory test and add a file called sum.js with the following content:

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

Now add a test spec file for this function:

// sum.spec.js
import sum from './sum'

describe('sum', () => {
  it('create sum of 2 numbers', () => {
    expect(sum(15, 8)).toBe(23)
  })
})

We need a command to run tests. Add an entry "test" to the package.json file that will call a command jest:

// package.json
"scripts": {
  //...
  "test": "jest"
}

Now if you run npm test, you will see some errors:

Errors in the test output with when we run tests with Jest

This happens because our Jest is not aware we are using ES6! So, we need to add the babel-jest dependency:

npm install babel-jest --save-dev

After babel-jest is installed, we have to add a .babelrc file with...