Install Jest

npm install --save-dev jest

You may get some dependency warnings for the following packages - canvas, bufferutil, and utf-8-validate. If so use the following commands to install the updates

npm install --global canvas
npm install --global bufferutil
npm install --global utf-8-validate

Add the following json to the package.json file

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

Creating a test

Jest tests should be in a subfolder called _test_ of the from where package.json is located. If you want to use a folder with a different name - see below.

Each test file should be prefixed with either test.js or spec.js e.g. maths.test.js.

The jest test file cannot use TypeScript but the CUT (Class Under Test) may be written using TypeScript - see below.

maths.test.js

Create the CUT

maths.ts

Now run the test using

Because we are using TypeScript you will see the following error

The failure is because Jest does not directly support TypeScript. Babel must be installed using yarn.

Begin by install yarn via npm

Setting up Babel

Configure Babel to target your current version of Node by creating a babel.config.js

Install babel dependencies via yarn

Enabling TypeScipt

Install the @babel/preset-typescript via yarn:

Now modify the babel.config.js so that it looks like this

Configuring file structures

Add a file called jest.config.js to the root of the project

Dependencies errors

The most common errors I have found are

I have found that by rerunning these two commands the problems are resolved

You should now be able to test TypeSctipt files.

MAN - Software Development

Jest for Unit Testing